Recent progress in silicon carbide field effect gas sensors

CHAPTER TEN

Recent progress in silicon carbide field effect gas sensors M. Andersson, A. Lloyd Spetz, D. Puglisi Link€ oping University, Link€ oping, Sweden

Contents 10.1 Introduction 10.2 Background: transduction and sensing mechanisms 10.2.1 Transducer platform 10.2.2 Transduction mechanisms 10.2.3 Sensing mechanisms

309 312 313 316 318

10.2.3.1 General 10.2.3.2 Detection of hydrogen-containing gases 10.2.3.3 Detection of nonhydrogen-containing gases

318 320 324

10.3 Sensing layer development for improved selectivity of SiC gas sensors 10.3.1 New material combinations 10.3.2 Tailor-made sensing layers for oxygen 10.3.3 Tailoring layers for CO2 and NOx 10.4 Dynamic sensor operation and advanced data evaluation 10.5 Applications 10.5.1 Sensor packaging 10.5.2 Applications and field tests 10.6 Summary Acknowledgments References

327 327 328 329 332 335 335 336 338 339 339

10.1 Introduction Chemical sensors based on silicon field effect transistors (Si-FETs) were introduced in the 1970s when, first, the ion-sensitive FET for pH measurements1 and, in 1974, the hydrogen-sensitive metal oxide semiconductor (MOS) FET2,3 were invented. After more than 40 years of research and development on chemical gas sensors, today the field effect transistor gas sensor based on silicon carbide (SiC-FET) is recognized as the most suitable for detection of a variety of different gas molecules at operating temperatures Semiconductor Gas Sensors, Second Edition ISBN: 978-0-08-102559-8 https://doi.org/10.1016/B978-0-08-102559-8.00010-0

© 2020 Elsevier Ltd. All rights reserved.

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from about 200  C to more than 600  C.4e8 Regardless of Si or SiC as the semiconductor in the field effect gas sensor, a catalytically active gate material such as palladium (Pd), platinum (Pt), or iridium (Ir) provides its gas sensitivity. Besides transistors, MOS capacitors and Schottky diodes with catalytic gate contacts have been developed for gas-sensing purposes, the basic sensing mechanism being common to all the different field effect sensor devices. On exposure of the sensors to a certain substance or gas mixture, the interaction between the gas and gate contact changes the electrical field across the MOS structure, in turn modulating the current through, or the capacitance over, the device. The introduction of the first metal insulator semiconductor (MIS) gas sensor devices based on SiC in the early 1990s9,10 opened up for new applications of field effect sensors. In 1999, at the International Conference of Silicon Carbide and Related Materials in North Carolina, USA, the first gas sensor based on a SiC-FET was presented,11 and one of the main results of this development is devices with excellent long-term stability.7 The wide bandgap of SiC (3.26 eV for the commonly used polytype 4H) permits operational temperatures beyond the limit of approximately 200  C for Si-based sensors without suffering intrinsic conduction effects. Extending the range of sensor operation temperatures allowed exploration of gas metal interactions and catalytic reactions occurring above 200  C, facilitating detection of many more compounds. SiC is also chemically inert, preventing device degradation caused by high temperature or reactions with other materials or substances. SiC-based field effect sensors have therefore been utilized in high temperature (up to 600  C) and corrosive applications such as combustion control in car exhausts and small- and medium-scale power plants,12e15 monitoring of ammonia (NH3) slip from diesel exhaust and flue gas after treatment systems,16,17, as well as for indoor air quality control.18e20 Commercial sensor systems based on SiC are available through an SME launched in 2007 (SenSiC AB, Kista, Stockholm, Sweden, www.sensic.se). Monitoring the regeneration of nitrogen oxides (NOx) storage catalysts has also been suggested as an application suitable for field effect sensors based on SiC.21 Olga Casals et al. investigated SiC-based MIS capacitors with Pt/TaOx gate metal in an atmosphere with high relative humidity, 45% RH. Detection of 1 part per million (ppm) hydrogen (H2) at 260  C, 2 ppm carbon monoxide (CO) at 240  C, and 20 ppm ethene (C2H4) at 320  C in nitrogen (N2) was possible even at this high humidity level, and variation in the humidity (15%e45%) did not influence the response. The authors conclude that the SiC sensors

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are especially suitable for monitoring exhaust gases from hydrogen or hydrocarbon-based fuel cells.22 Field effect devices based on other wide bandgap semiconductorsdsuch as diamond,23 gallium nitride (GaN), and aluminum gallium nitride (AlGaN)24,25dhave also been demonstrated for gas-sensing purposes. Chen et al. fabricated a Schottky diode based on GaN on sapphire substrate with a Pd nanoparticleemodified top layer on the Pd gate contact. The detection limit for H2 in air is less than 0.8 ppm at 25  C; however, there is an influence of the humidity level at this low temperature.26 Chou et al. fabricated two Pd/AlGaN/GaN Schottky diodes on sapphire, one with pyramid-like Pd nanoparticles on top and one without.27 The pyramid nanoparticles improved the sensitivity to H2 with a detection limit of 10 parts per billion (ppb) in air at 27  C. Guo et al. reported ultralow electrostatic detection of trinitrotoluene from 0.1 parts per trillion (ppt) to 10 ppb in buffer solution using an AlGaN/GaN high electron mobility transistor with gold nanoparticles functionalized with cysteamine.28 Offermans et al. demonstrated AlGaN/GaN two-dimensional electron gas (2DEG) devices, which are processed as suspended membranes on Si29 and operated with ultralow power. Without additional sensing material, detection of nitrogen dioxide (NO2) concentrations between 11 and 20 ppb in single ppb steps is demonstrated at 250  C with low influence of humidity, and at 275  C ammonia (1e12 ppm in humid atmosphere) is detected in the opposite direction. By adding Pt as the gate contact, H2 is detected at 150  C for a concentration range of 300e3000 ppm. When applying a sensing layer of a pH-sensitive polymer that retains water, CO2 formed charged species in the liquid phase and could be detected at 25  C in the range 1000e6000 ppm. Weng et al. fabricated MISiC capacitors with a gate contact of Pd/TiO2 on top of oxidized SiC (Pd/TiO2/SiO2/SiC). At 325  C, in a mixture of H2 and oxygen (O2), the response to H2 is lower as compared with the response to H2 in N2. On the other hand, a mixture of hydrogen sulfide (H2S) and O2 gives a larger response as compared with H2S in N2.30 This is due to the Claus reaction,31 according to which the oxygen in the presence of titanium dioxide (TiO2) reacts with the sulfur in the H2S molecule and accordingly both hydrogen atoms are released and may participate in the detection process. Nakagomi et al. compared Schottky diodes based on the polytype 4H-SiC with a low doped epilayer and b-gallium(III) oxide (b-Ga2O3) with Pt-sensing electrodes and Ni/Pt or Ti/Al/Pt/Au as the ohmic contacts on the rear side. The Pt-Ga2O3 showed a lower detection limit for hydrogen in oxygen, at 400  C of a few tens ppm of hydrogen. Nonstoichiometry conditions of

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the Ga2O3 surface or of the Pt-Ga2O3 interface is suggested as the reason for this.32 In their next paper,33 two devices in series are processed on 1 mm b-Ga2O3 on sapphire, one with Pt gate electrode, the other device without gate electrode and with ohmic contacts as above. It is demonstrated that this design allows stable hydrogen detection in oxygen atmosphere from about 40 ppm even with temperature fluctuations as large as 150  C in the temperature range 400e550  C. For lower temperatures, the resistivity of the Ga2O3 is too large and for higher temperatures the Ga2O3 itself is sensitive to hydrogen. The crystal structure b-Ga2O3 is an n-type material, with a band gap equal to 4.9 eV. Thin films of b-Ga2O3 were deposited on top of p-type nickel(II) oxide (NiO) substrates whereby an interfacial layer of g-Ga2O3 was found between the two materials.34 The last decade has seen the development of new devices, new material combinations, and new operation modes of SiC-based field effect sensors, and, with the advent of epitaxially grown graphene on SiC, as well as with the integration of a number of 2D materials for gas sensing applications,35 the field is expanding even further. Very promising possibilities for ultrasensitive detection of gaseous compounds are offered by epitaxially grown graphene on SiC-based sensor structures,36e39 as reported in the first edition of this book.40 Nanoparticle decoration of the graphene surface has considerably improved selectivity, sensitivity, and speed of response of graphene sensors, while the intrinsic properties of graphene were retained.41e43 This area now also expands into development of novel 2D materials on SiC other than graphene for gas- and liquid-phase sensing applications.44,45 Based on theoretical modeling and material research, selectivity and sensitivity toward various gases are currently also being improved by the development of new combinations of gas-sensitive layers. Recent trends, reviewed in this chapter, include simplification of device designs to reduce fabrication costs and increase stability, as well as novel designs to facilitate sensor packaging, high reusability, and thus an efficient product development. Especially high temperature applications require advanced packaging solutions and a novel approach using low temperature cofired ceramic (LTCC) is presented. Dynamic sensor operation through temperature and gate bias cycling is another recent line of development that makes use of advanced data evaluation to enhance stability, as well as selectivity toward certain substances.

10.2 Background: transduction and sensing mechanisms In this section, the basic physical principles and electrical operation of the transducer platform, the FET device, are given. Moreover, a description

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of the sensing mechanisms, when the devices are used as gas sensors, is given in general and for hydrogen- and nonhydrogen-containing gases. For this section, we also refer to Ref. 7.

10.2.1 Transducer platform The MIS capacitor represents the heart of most field effect sensor devices, and the physics of MIS capacitors has been widely studied and treated in detail in well-known semiconductor physics and other sensor books.46e48 Here, we will only give the basic physical principles regarding the metal insulator semiconductor field effect transistor (MISFET), because this is the ultimate transducer for commercial sensor devices. MISFET devices may be distinguished in normally off or enhancement type and normally on or depletion type devices. Normally off means that with zero applied gate bias no channel between drain and source is created, whereas normally on means that a channel already exists at zero applied gate bias. More details can be found in Ref. 49. A schematic of the enhancement type MISFET device under different conditions and its corresponding currentevoltage (I/V) characteristics is shown in Fig. 10.1. The channel conductance, determined by its dimensions, the mobility of the electrons, and the inversion charge density of electrons can be modulated by the gate bias, VGS. When no gate bias is applied (VGS ¼ 0), there is no conductive path from source to drain, therefore no current flows through the conducting channel (Fig. 10.1(a)). As soon as a gate bias is applied (VGS > 0), the channel (n-type inversion layer) develops allowing electrons to flow between the source and drain terminals in response to a drain bias (VDS). For a gate bias larger than the threshold voltage VT (VGS > VT) and small VDS, the device operates in the so-called linear region (Fig. 10.1(b)). As the drain-source voltage increases, the voltage drop across the insulator near the drain terminal decreases. This means that the induced inversion charge density near the drain decreases, the channel depth (i.e., the thickness of the inversion channel) near the drain terminal is reduced, and the slope of the I/V curve decreases. The point at which the channel depth at the drain is reduced to zero is called pinch-off and represents the onset of saturation (Fig. 10.1(c)). Here, the voltage drop across the insulator at the drain is equal to the threshold voltage (VDS,sat ¼ VGSeVT). Beyond the pinch-off point, the drain-source current remains constant, resulting in a flat I/V curve (Fig. 10.1(d)). This region is called saturation region.

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+VGS

(a) S n+

G Insulator p-type epilayer

+VDS

ID

VGS < VT

D n+

n-type substrate VDS +VGS

(b) S n+

G Insulator p-type epilayer

+VDS

ID

VGS < VT, small VDS

D VGS2

n+

VGS2 > VGS1

VGS1

n-type substrate

VDS +VGS

(c) S n+

G Insulator p-type epilayer

+VDS

ID

VGS > VT, VDS = VD,sat VDS,sat (VGS2)

D n+

VDS,sat (VGS1)

n-type substrate

VDS

(d)

+VGS S n+

G Insulator p-type epilayer

+VDS

ID

VGS > VT, VDS > VDS,sat

D n+

n-type substrate VDS

Figure 10.1 Enhancement type metal insulator semiconductor field effect transistor (MISFET) device under different operating conditions and corresponding I/V curves. MISFET operated (a) in equilibrium condition (VGS ¼ 0), (b) in the linear region, (c) at the onset of saturation, and (d) beyond saturation.

Transistor-based sensor devices are commonly operated in saturation mode. The drain current (ID,sat) versus gate voltage (VGS) relationship for the saturation region is described quantitatively by ID;sat ¼

W mn εins ½VGS  VT 2 2Ldins

(10.1)

Recent progress in silicon carbide field effect gas sensors

VT ¼

2dins ½eNa εs FF 1=2 Qss dins  þ Fms þ 2FF εins εins

315

(10.2)

where W and L are the channel width and length, respectively; mn is the channel electron mobility; εins and dins are the insulator permittivity and thickness, respectively; VT is the threshold voltage; e is the elementary charge; Na the bulk doping concentration; εs the semiconductor permittivity; QSS the insulator charge density; Fms the metal-to-semiconductor work function difference; and FF is the Fermi potential, which is the potential difference between the Fermi level and the intrinsic Fermi level. Regarding the transistor-based sensor devices, enhancement and depletion type MISFET transistors are both used. A detailed study of the difference between enhancement and depletion type SiC-FET gas sensors can be found in Ref. 7. However, the depletion type MISFET has the advantages of operation at zero or very low applied gate voltage, less influence of temperature fluctuations, and generally more stable operation of the sensors, therefore it is preferable as a gas sensor. Concerning the depletion type MISFET, even when no bias is applied to the gate terminal (VGS ¼ 0), there is a current flow, i.e., a conductive path from source to drain exists. The threshold voltage of this device is defined by the difference between the built-in voltage across the gate metal/insulator/SiC stack and the pinch-off voltage. The latter is the applied voltage which cuts off the conducting path between source and drain and is dependent on the thickness, ds, and the doping level, Nd, of the n-type active layer: eNd ds (10.3) 2εs Eq. (10.3) shows the pinch off voltage of the depletion type MISFET, εs is the permittivity of the semiconductor. For a more in-depth treatment of field effect device theory and operation, see, for instance Refs. 7, 46e48. The design of the device parameters influences the size of the gas response. It has been demonstrated, in the case of a SiC-FET with porous Ir as the gate material, that a decrease of the gate length from 40 to 20 mm results in a factor two increase of the sensor response to CO in 3% oxygen at an operating temperature of 200  C.7 Other studies showed that optimizing the thickness of the gate dielectric almost doubled the gas response to ammonia. In addition, optimizing the device for lower field Vp ¼

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strength between the different terminals of the device increased the longterm performance.

10.2.2 Transduction mechanisms Parameters such as device dimensions, electron mobility, permittivity, and doping concentration are inherent to the choice of materials, the design, and the processing of field effect sensor devices. Once fabricated, the values of these are fixed but the charges located in or at the surface of the insulator, QSS, the metal-to-semiconductor work function difference, Fms, and any internal gate voltage drop, VGSint, added to the externally applied gate bias, VGSext, can also have an influence on the drain current, ID. Any change in the values of one or more of these parameters will change the I/V characteristics of the FET devices. Thus, if the interactions between the gas and gate materials on exposure to a certain substance lead to the introduction of an internal gate voltage drop, a change in gate insulator charge, and/or a change in gate metal work function, the substance could be detected through a change in drain current (see Fig. 10.2). This requires the injection of charge to or charge separation at the gate contact/insulator interface, or species capable of changing the metal work function to adsorb on the inner surface of the gate contact material. Examples of changes in ID/VGS characteristics for a gas-induced internal voltage drop are given in Fig. 10.2(c). When atoms or molecules adsorb on a surface, there is most often some kind of charge transfer between the adsorbates and the surface, and thus a separation of charge, as well as a change in work function of the material. One kind of field effectebased gas sensor, the suspended gate FET (SGFET), utilizes this latter phenomenon.50 The design of SGFETs includes a very small air gap between the gate contact material and the insulator, just large enough to facilitate rapid diffusion of gas molecules to the gate contact surface facing the insulator. Any change in drain current, i.e., sensor signal, on gas exposure is directly related to the change in work function of the gate material, resulting from adsorption of one or more gaseous substances to its surface. As mentioned, a common mode of operation of transistor-based field effect sensors is to keep the drain current constant and measure the resulting drain-source voltage drop as a sensor signal. Connecting the transistor’s drain and gate (enhancement-type devices) or source and gate (depletion-type devices) terminals, when operating the device as a gas sensor, makes it a simple two-terminal device (e.g. Ref. 7). In the other

Recent progress in silicon carbide field effect gas sensors

(a) H2

Insulator

H2O CO2 NO2 – O– –O O O– –O– _ _ _ _ _ – O H H H H +H +O + + + Insulator

p-type epilayer

p-type epilayer

NO

(b)

CO

NH3 CO H2 O2 NO

O2

NH3

eVins

eVins

Ec EFi eΦs

eVGS,ext

(c)

317

Gas

EF Exposure Ev

VGS,ext = VDS

ID

_ + _ +

eVGS,int

I S Insulator Semiconductor

M Metal

Ec

M Metal

VGS = 4V VGS = 3V VGS = 2V

VGS = 3V

VGS= 1V

VGS = 2V 3

4

5 VDS [V]

Ev

VGS,ext + VGS,int = VDS

ID

VGS = 4V

2

eΦs EF

I S Insulator Semiconductor

VGS = 5V

1

EFi

1

2

3

4

5 VDS[V]

Figure 10.2 (a) Examples of reactions on the catalytic metal gates are displayed, as well as the effect of hydrogen and oxygen anion adsorption on the number of charge carriers in the channel. (b) Corresponding changes of the energy band diagram, air/ inert atmosphere to the left and hydrogen exposure to the right and (c) the change in I/V characteristics following hydrogen exposure.

mode of operation, the drain current is measured as the sensor signal at a constant drain-source voltage. The choice of the operation mode, at a constant drain current or at a constant drain-source voltage, as well as of the electrical operating point along the currentevoltage (I/V) curve of the device also influences the size of the gas response. As an example, we demonstrated that operating a SiC-FET sensor, with porous Ir on top of a dense thin film of tungsten trioxide (WO3) as the sensing layer, at a constant drainsource voltage and measuring the drain current as the sensor signal, gave a sensor response to 100 ppb benzene which was in the saturation region about twice that in the linear region, see Fig. 10.3(a).8 In Fig. 10.3(b), the same operation mode is used for a SiC-FET with a porous Ir gate, and the detection

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Figure 10.3 (a) Sensor response to 10, 50, and 100 ppb of benzene (C6H6) at 300  C, in dry air, and under operation at the linear (upper signal) and saturation (bottom signal) regions of the transistor. (b) Detection limit as a function of relative humidity for formaldehyde (CH2O), benzene (C6H6), and naphthalene (C10H8). For C10H8, the detection limit can only be stated to be below 0.5 ppb, because our gas mixing system cannot provide C10H8 concentrations below 0.5 ppb.

limits for formaldehyde, benzene, and naphthalene are shown as a function of relative humidity. For naphthalene, the performance of the gas mixing system sets the limit to 0.5 ppb.

10.2.3 Sensing mechanisms 10.2.3.1 General Work function changes and the creation of internal voltage drops are merely the general mechanisms behind the conversion of chemical interactions between the gas and the sensor device into an electrical output. Voltage drops

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can be introduced and work function changes can be achieved in a number of different ways. To be useful for specific applications, the sensors must, however, be able to distinguish between different gas mixtures and/or quantify one or more substances with good resolution. The sensitivity and selectivity toward the substance(s) of interest are important figures of merit for a specific sensor, as are detection limit, speed of response, and stability. The sensor’s sensitivity and selectivity to the analyte of interest are largely determined by the specific interactions between the various ambient gaseous substances and the gate materials exposed to the surrounding gas. These interactions include adsorption and reactions of atoms and molecules on the surfaces of the gate materials, as well as desorption from the same surfaces. In general, adsorption and desorption are dependent on, for example, the ambient temperature, the partial pressure of the substance, the desorption energy, and the sticking coefficient. The sticking coefficient gives the probability for adsorption of a molecule incident on an empty adsorption site and is dependent on temperature and activation energy for adsorption. It is therefore different for different molecules, surface compositions, and crystal orientations. Furthermore, the adsorption of molecules on the sensor surface may be direct or, via precursor states, it may be dissociative or nondissociative and there may be interactions between adsorbed species on the surface. All these details of the adsorption will affect the equilibrium state of the molecules on the sensor surface. In addition, other constituents of the surrounding gas matrix may adsorb to the surface and affect the coverage of the target substance in different ways (e.g., by reducing or blocking adsorption of this substance or removing it from the surface through chemical reactions). At the steady state, equilibrium usually develops between the adsorption, chemical surface reactions, and desorption of different substances in the surrounding gas matrix. An overview of the surface processes and examples of their influence on the device characteristics is given in Fig. 10.2. Considering the operational temperature of the sensor to be constant, and the sticking coefficients, interaction, and desorption energies to be inherent to the molecules and the surface, the steady-state condition on the surface is dependent on the partial pressures of the gas matrix constituents and, therefore, reflects the composition of the surrounding gas. Several different gas matrices may, however, give rise to the same equilibrium surface conditions for a certain surface and operational temperature. Conversely, a different surface, or a change in operational temperature, may give rise to a different equilibrium surface condition for the same gas matrix, highlighting the importance and possibilities

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regarding the choice of gate material and sensor operational temperature, as also exemplified below. 10.2.3.2 Detection of hydrogen-containing gases Hydrogen, H2, adsorbs dissociatively on catalytic gate metals such as Pd, Pt, and Ir. In the presence of hydrogen alone, the steady-state surface coverage of hydrogen atoms follows the simple Langmuir relation51,52 and is only dependent on ambient hydrogen pressure. Normally, also other substances are present in the surrounding atmosphere and may affect the equilibrium coverage of hydrogen in different ways. Notably, oxygen also adsorbs dissociatively on commonly used gate metals at sensor operating temperatures (200e600  C)ethe recombination and desorption rates, however, being very low below 300  C. At this temperature, oxygen can basically be removed at any appreciable rate only through reaction with other atoms or molecules, such as chemisorbed hydrogen in the formation of water. In normal air, the pressure-dependent hydrogen coverage for every gate material is determined by the adsorption and desorption characteristics of hydrogen and its reaction with adsorbed oxygen. Variations in hydrogen and oxygen partial pressures thus lead to a change in hydrogen coverage. The generated hydrogen atoms are, to some extent, also withdrawn from the surface by rapid diffusion through the metal contact to the metal/insulator interface. Due to the very rapid diffusion of the hydrogen atoms, the surface coverage and interface hydrogen concentration are in equilibrium. As concluded from infrared spectroscopy,53 the hydrogen atoms adsorb to oxygen atoms in the surface of oxidic insulators, forming hydroxyl groups (OH) on the oxide accompanied by substantial charge transfer. Because OH groups have a large dipole moment, the interface layer of dipoles introduces a sharp potential step at the interface, earlier referred to as an internal voltage drop, Vint. This voltage drop adds to the externally applied bias, resulting in a shift of the I/V characteristics of the sensor, DVint, as illustrated in Fig. 10.2(c), and is given by: r DVint ¼ nH $ (10.4) ε0 where r is the dipole moment of an OH group, ε0 is the permittivity of free space, and nH is the number of hydrogen atoms per unit area at the interface, which is related to the coverage of hydrogen on the metal surface.54,55 The size of the I/V shift is thus a measure of the ambient partial pressure of hydrogen, in relation to other gases such as oxygen. A corresponding energy

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band diagram illustrating the effect of this dipole layer can be found in Fig. 10.2(b). From a sensor response point of view, it has been shown that dipole formation is the dominant effect regarding hydrogen detection. Work function changes due to adsorption on the metal side of the metal/insulator interface only have a minor influence on the sensor signal, introducing a small shift in the I/V or capacitanceevoltage (C/V) characteristics in the opposite direction to that generated by dipole formation.56 Further evidence for the importance of an oxidic insulator surface has been obtained from sensors based on both SiC and GaN Schottky diodes,57,58 for which the hydrogen response considerably improved on the introduction of a thin oxide between the metal and the semiconductor. When comparing the hydrogen response from devices with different insulator materials (e.g., Al2O3, Ta2O5, SiO2), the response correlates well with the insulator surface density of oxygen atoms,59 further emphasizing the role of the oxygen as adsorption sites for the hydrogen (see Fig. 10.4(a and b)). The choice of insulator thus influences the hydrogen sensitivity of field effect devices, as well as their dynamic range. This was also studied by Roy et al. for capacitive SiC sensors employing either hafnium(IV) oxide (HfO2) or TiO2 as the dielectric and Ti/Pd as the catalytic contact. By multiple linear regression, real-time gas concentration in a mixture of different gas species could be monitored using different catalytic gate metals and different insulators in a sensor array. This work points out that also defects in the insulator surface play a role for gas detection.60 Ofrim et al. also used SiC capacitors as hydrogen sensors utilizing silicon dioxide (SiO2), TiO2, and zinc oxide (ZnO) as the gate insulator, whereby the TiO2-based capacitive sensor showed superior performance.61 In the case of other molecules containing hydrogen, the same basic principles as for hydrogen apply if free hydrogen atoms can be generated on adsorption. At temperatures of approximately 600  C or above, field effect sensors with catalytic metal gates, here Pt, exhibit a binary response to hydrocarbons, irrespective of hydrocarbon identity (see Fig. 10.4(c)).62 As long as the oxygen concentration is such that complete oxidation of the hydrocarbons can take place on the gate metal, the high reaction rates keep the surface fairly clean from hydrocarbons and, to a large extent, oxygen covered. Any hydrocarbons sticking to the surface are oxidized directly on adsorption without generation of any free hydrogen atoms. When increasing the hydrocarbon concentration

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(a)

(b) Al O Si N SiO Al O inert Si N inert SiO inert Ta O inert

Response (V)

Response (V)

Al O

SiO

Si N

Hydrogen concentration (ppm)

Time (min)

(c)

(d) Sensor response (mV)

T=600 ± 4°C

600

500

ΔV (mV)

400 butane + 0.5% propane/4%O2 butane + 0.5% propane/4%O2 butane + 1% propane/6%O2 butane/1%O2 butane/2%O2

300 200

butane/4%O2 ethane/1%O2 ethane/2%O2 ethane/4%O2

100

0 0

0 –100 –200 –300 –400 –500 –600 –700 –800 0 –100 –200 –300 –400 –500 –600 –700 –800

10% O

100 1

2

3

α

4

5

6

25 ppm 50 ppm 75 ppm 100 ppm 125 ppm 150 ppm 175 ppm 200 ppm 225 ppm 250 ppm

3% O

150

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250

300

350

400

Temperature (°C)

Figure 10.4 The figure displays, in (a) and (b), the response to hydrogen in the range 10 ppm to 1% in air or N2 (indicated as inert in (b)) at 140  C for Pd/Pt gate field effect sensors with various insulator materials. In (c) and (d), the response DV to saturated hydrocarbons at 600  C as a function of equivalence ratio a are given, as well as the response to unsaturated hydrocarbons at temperatures of 100e400  C and concentrations well below the equivalence ratio. The equivalence ratio is defined as the ratio of the actual fuel-to-oxidizer ratio to the stoichiometric fuel-to-oxidizer ratio. (a) and €m I, Briand (b) are reprinted with permission from the Eriksson M, Salomonsson A, Lundstro D, Åbom AE. The influence of the insulator surface properties on the hydrogen response of field-effect gas sensors. J Appl Phy 2005;98(3):34903e8 © 2012 American Institute of Physics. (c) is reprinted with permission from Baranzahi A, Lloyd Spetz A, Glavmo M, €ggendal B, Mårtensson P, Lundstro €m I. Carlsson C, Nytomt J, Salomonsson P, Jobson E, Ha Response of metal-oxide-silicon carbide sensors to simulated and real exhaust gases. Sensor Actuator 1997;B43:52e9. © 1997 Elsevier. (d) is reprinted with permission from the €m T, Nilsson M, Gauffin C, Svensson H. A Andersson M, Everbrand L, Lloyd Spetz A, Nystro MISiCFET based gas sensor system for combustion control in small-scale wood fired boilers. Proc IEEE Sensors 2007:962e5. Atlanta, USA Oct 28e31. © 2007 IEEE.

beyond the stoichiometric hydrocarbon to oxygen ratio, the hydrocarbons reduce the gate metal surface and effectively deplete it of oxygen. Dissociation, rather than oxidation, is the dominating process, producing free hydrogen atoms which can reach the interface and induce an internal voltage drop.

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At temperatures below 300  C, certain hydrocarbonsde.g., unsaturated hydrocarbons such as ethene (C2H4) and propene (C3H6)dmay still reduce the catalytic metal surface and produce free hydrogen even in the presence of excess amounts of oxygen.63,64 The underlying reason is the higher sticking probability of these hydrocarbons compared with oxygen and the lower rates of oxidation at lower temperatures. However, for decomposition of saturated hydrocarbons on the catalytic sensor surface in an atmosphere of excess oxygen, all decomposed hydrogen atoms end up as water molecules, which desorb from the sensor surface. Therefore, no free hydrogen atoms are generated and no sensor response is obtained from these substances for conditions of excess oxygen.64,65 Pt gate sensors operated at 200e300  C therefore also exhibit a binary switch in sensor response to unsaturated hydrocarbons, the switch point being dependent on oxygen concentration and temperature (due to the temperature dependence of the sticking coefficients) (see Fig. 10.3(d)). Binary switch behavior was also reported by Kahng et al. using a SiC capacitor with a dense Pt gate for hydrogen sensing in ultrahigh vacuum (UHV).66 The binary behavior is due to the competition between hydrogen oxidation and diffusion to the metal/oxide interface. It was also concluded that oxygen is needed to restore the sensor baseline after exposure to hydrogen. Another hydrogen-containing substance which has attracted a great deal of interest in the field of high-temperature gas sensors is ammonia (NH3). Ammonia has not been observed to dissociate on adsorption on Pt at temperatures below approximately 225  C.67 Furthermore, there is some evidence of oxygen-mediated dissociation occurring on Pd-MOS sensor devices,68 which leads to direct oxidation of adsorbed NH3. The view that no free hydrogen atoms are generated on the Pt surface accords with the observations from sensors with dense, homogeneous Pt gates, for which no NH3 response is obtained. In case of a discontinuous/porous gate metal (see Fig. 10.2), when exposing parts of the oxide to the ambient atmosphere, the field effect devices exhibit similar sensing characteristics as for hydrogen.69,70 The generally accepted view emphasizes the importance of the three phase boundaries between oxide, metal, and the gas phase as the site for ammonia dissociation to create OH groups on the surface of the oxide.71 At the metal/oxide border, hydrogen from an ammonia molecule may be directly transferred to oxygen atoms in the surface of the oxide, possibly as a proton, the charged complex being stabilized by its proximity to the metal. Fourier transform infrared spectroscopic measurements on a model system consisting of a Pt impregnated SiO2 powder revealed the formation

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of OH groups at temperatures above 225  C on exposure to NH3.72 The amount of OH groups formed correlated well with the Pt loading (coverage), which has been interpreted as the formed OH groups being located close to the metal/oxide border. Local response measurements performed on capacitive field effect sensor devices by laterally resolved photocurrent measurements provided similar results, relating the generation of OH groups to the metal/oxide border. Furthermore, these investigations also indicated the possibility for diffusion of hydrogen/protons into the metal/oxide interface underneath the metal,70,71 inducing the same kind of internal voltage drop as in the case of hydrogen exposure. Hydrogen detection sites underneath the (Pt) metal at the metal/insulator interface was systematically studied by Åbom et al. by scratch adhesion measurements, transmission electron microscopy, and atomic force microscopy studies of ripped off metal films.73 The size of the semi-inert hydrogen response increased with roughness of the Pt metal surface facing the insulator, which showed a blocking effect of Pt metal in direct contact with the insulator (SiO2). As previously mentioned, oxygen also adsorbs dissociatively on Pt and negatively charged oxygen atoms may spillover to exposed areas of the oxide surface in devices with a discontinuous (porous) gate contact. At the steady state, an equilibrium between oxygen coverage on the Pt surface and concentration of oxygen anions on the oxide surface would then develop. It has been suggested that the response of porous Pt gate sensors to reducing substances such as hydrogen, hydrocarbons, and ammonia may partly originate from the reverse spillover of oxygen anions and their removal on the Pt surface through reactions with adsorbed hydrogen, hydrocarbon, and ammonia molecules.74,75 It should be noted that the removal of negative charges from the oxide surface has the same effect on the I/V or C/V characteristics of field effect sensors as the voltage drop introduced by OH group formation. 10.2.3.3 Detection of nonhydrogen-containing gases Carbon monoxide (CO) is an example of a reducing, nonhydrogencontaining substance for which the interaction with metal (e.g., Pt) gate field effect sensors may cause a substantial change in the I/V or C/V characteristics of a device. Without being able to generate any free hydrogen on adsorption, the CO sensitivity has been stipulated to be at least partly caused by the removal of oxygen anions,74 as discussed above, and/or the reduction of a surface platinum oxide.76e78 The CO response also correlates well with the CO oxidation characteristics on silica-supported Pt.78,79 At the point

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where the oxidation rate suddenly drops when increasing the CO/O2 ratio or decreasing the temperature, the sensor signal exhibits a binary switch from a small to a large response (see also Fig. 10.5).79,80 In analogy with the previously discussed case regarding hydrocarbons, the higher sticking probability of CO compared with oxygen at lower temperatures leads to the Pt (b)

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Figure 10.5 In (a), the CO/O2 and temperature-dependent binary switch of the response of Pt gate field effect sensors toward CO is exemplified, whereas (b) displays the disruption of the adsorbed CO layer on the Pt surface on hydrogen exposure. The spectral peaks at 1839, 2091, and 2064 cm1 (upper panel; no H2 exposure) correspond to CO adsorbed on Pt, whereas the peaks at wave numbers slightly below 2400 cm1 (lower panel; exposure to 500 ppm H2 in otherwise the same conditions as in the upper panel) represents gaseous CO2. In (c) and (d), the sensor response toward CO in the range of 125e1250 ppm in the absence/presence of hydrogen (500 ppm) is given for two different oxygen concentrations (lower panels), as well as the downstream H2 and CO2 partial pressures (upper panel). (a) is reprinted with €m T, Nilsson M, permission from the Andersson M, Everbrand L, Lloyd Spetz A, Nystro Gauffin C, Svensson H. A MISiCFET based gas sensor system for combustion control in small-scale wood fired boilers. Proceedings of the IEEE international conference on sensors, Proc IEEE Sensors 2007:962e5. Atlanta, USA Oct 28e31. © 2007 IEEE. (b) is reprinted with permission from the Becker E, Andersson M, Eriksson M, Lloyd Spetz A, Skoglundh M. Study of the sensing mechanism towards carbon monoxide of platinum-based field effect sensors, IEEE Sens J 2011;11(7):1527e34. © 2011 IEEE. (c) and (d) are reprinted with permission from the Darmastuti Z, Pearce R, Lloyd Spetz A, Andersson M. The influence of gate bias and structure on the CO sensing performance of SiC based field effect sensors. Proc IEEE Sensors 2011:133e6. Limerick, Ireland, October 28e31, 2011. © 2011 IEEE.79

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surface being practically covered with CO (unless the CO/O2 ratio is too small), almost excluding oxygen adsorption, also at CO concentrations well below the oxygen concentration. With no or very little oxygen on the surface, the CO oxidation rate is very low. At higher temperature or higher oxygen concentration, the poisoning of the sensor by adsorbed CO on the sensor surface recovers as the CO is removed and the Pt surface rapidly reverts to being dominated by adsorbed oxygen. A large response of Pt gate field effect devices to CO therefore correlates with a surface completely covered by CO, whereas a small CO response is encountered whenever the Pt surface is oxygen dominated. However, not only porous Pt gate contacts exhibit these characteristics. Dense films without any exposed oxide areas also show the same binary switch in sensor signal.81 Furthermore, on introduction of hydrogen at a constant concentration, the large CO response of Pt gate sensors can either increase or decrease, depending on the CO/O2 ratio and temperature (see Fig. 10.5(c and d)). It has also been concluded that the presence of hydrogen can break the self-poisoning of the CO oxidation (see Fig. 10.5(b)).78 This indicates that hydrogen may be able to penetrate/adsorb on a Pt surface covered by CO and, if the CO concentration in relation to the oxygen concentration is small, disrupt the CO coverage. If, instead, the CO/O2 ratio is too high in comparison with the hydrogen concentration, or the Pt surface temperature is too low, the surface remains covered by CO and, effectively, depleted of oxygen. Without any oxygen on the surface, there is no risk of hydrogen adsorbing on the Pt surface being oxidized. A much higher proportion of hydrogen atoms can therefore reach the interface. As a consequence, a CO-covered surface will exhibit a very much larger sensitivity detecting even small concentrations of hydrogen, suggesting the CO response partly being mediated through an increased sensitivity to the background concentration of hydrogen which is present in all gas mixtures. Further support for the influence of hydrogen on the CO response is given from UHV studies on Si-based field effect devices.82,83 As exemplified above, the application-specific performance of a sensor is thus influenced by adsorption, reactions between adsorbed species, diffusion of species on the surface, and desorption characteristics of the individual substances which are present in the gas mixture. These characteristics depend on the materials interacting with the substances, the structure of the materials, and the operating temperature; therefore, the selectivity and sensitivity to the gases of interest can be influenced by the choice of gate materials, their structure, and temperature. For the development of sensors for new

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applications, it is therefore important to gain knowledge about gasesolid interactions and sensor mechanisms to be able to tailor devices with good selectivity and sensitivity to the target substance(s).

10.3 Sensing layer development for improved selectivity of SiC gas sensors The ability of hydrogen atoms to diffuse through the commonly used gate materials renders most of the field effect sensors developed so far to exhibit sensitivity to hydrogen. In addition, nitride-based insulators have a tendency to oxidize over time, providing the necessary sites for hydrogen adsorption.59 In developing sensors for specific applications, the issue of cross-sensitivity to hydrogen and substances containing hydrogen therefore has to be considered. For most applications, this cross-sensitivity has been a limitation for the development of field effectebased devices for sensing of substances that do not contain hydrogen, such as oxygen, nitrogen oxides, and sulfur oxides. To widen the areas of application for field effect sensors by increasing selectivity toward other substances than hydrogen or hydrogen-containing gases, a line of development has been the introduction of new material combinations. However, also the nature of the transducer influences the gas response. SiC-FET devices were studied together with quartz crystal microbalance (QMB) sensors which employed the same porphyrin-based sensing layers. While the SiC-FET device responds to the charging of the gate introduced by the interaction of gases with the porphyrin layer, the QMB device responds (changes of the operating frequency) to the change in total mass of the device due to gas molecules absorbing in the sensing layer.84 Therefore, the combination of the SiC-FET device with the QMB device gives more information about a certain gas mixture. In Section 10.4, we will introduce temperature cycling operation mode and advanced data evaluation to improve selectivity and sensitivity of one sensor working as a virtual sensor array.

10.3.1 New material combinations From theoretical considerations and experimental results, there are indications suggesting that hydrogen terminationdand, thus, OH group formationdis energetically unfavorable on most magnesium oxide (MgO) surfaces.79,85 It has also been postulated that hydrogen adsorption at the insulator/metal interface of the MgO/Pt system would occur on the metal, rather than on the insulator side of the interface. Experimental results point

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in the same direction, showing that field effect sensors based on MgO/Pt structures exhibit no or very little response/sensitivity to hydrogen. The very small hydrogen-induced response is also in the opposite direction to the normal hydrogen response of SiO2/Pt structures, as briefly discussed earlier, indicating hydrogen adsorption to the metal side of the interface.56 Furthermore, sensitivity to CO of devices comprising dense Pt gate films on top of MgO is extremely low or nonexistent, providing further indications for the response to CO of SiO2/Pt-based sensors at least partly being mediated by an increase in sensitivity toward background hydrogen.

10.3.2 Tailor-made sensing layers for oxygen With the introduction of MgO as the top part of the insulating layer in field effect sensors, the cross-sensitivity to hydrogen or substances containing hydrogen can thus be markedly reduced. This has also been shown for field effect devices with other gate contacts than Pt. By using conducting oxides as gate materialdsuch as iridium oxide (IrO2) or ruthenium oxide (RuO2), for which the work function changes as a function of oxidation state86dit has been shown that the sensitivity toward oxygen, and thereby the gassensing abilities of field effect sensors, can also be retained when MgO is used as the insulator87 (see Fig. 10.6). This realizes oxygen sensors with

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no need for reference gas, unlike the lambda sensor (in the United States, universal heated exhaust gas oxygen, UHEGO).88,89 Partial oxidation or reduction on exposure to different oxygen concentrations at elevated temperatures changes the work function of the gate material at the gate material/insulator boundary and thereby, as discussed in Section 10.2, also the C/V or I/V characteristics of the device. Similar sensors employing ruthenium oxide nanoparticles deposited on SiO2 as gate material, on the other hand, exhibit more or less the same response characteristics to hydrogen and substances containing hydrogen as Pt/ SiO2 and Ru/SiO2 structures.90

10.3.3 Tailoring layers for CO2 and NOx Not only cross-sensitivity issues have been addressed in the development of new sensing materials and material combinations but also possible solutions for the detection of substances (e.g., CO2 and NO2), which have not been possible to detect with the field effect sensors developed so far, have been investigated. Ion-conducting materials sandwiched between a porous metal gate contact and the insulator have been studied since 2000.91 On exposure of such structures to the target gas (e.g., O2), the target gas adsorbs on the metal gate surface, picking up charges from the metal and thereby forming the corresponding ions (e.g., by formation of oxygen anions Oe). At the three phase boundaries between the metal, ion conductor, and gas phase, these ions spillover to, and can be incorporated in, the material at vacant positions. Most often, but not always, the material is partly composed by the same atoms/ions as the target gas for detection. In the case of oxygen, the ionic conductor is normally an oxide, such as zirconium oxide (ZrO2), commonly doped by another element, e.g., yttrium, to create more oxygen vacancies.89 At elevated temperatures, the ions start to become mobile, moving through the material from high to low concentration by diffusion through vacancies. As a result, charges are introduced into the electronically nonconducting material and to the interface between the ion-conducting and -insulating layers, thereby, as described in Section 10.2, changing the C/V or I/V characteristics of the device. This diffusion is counteracted by the drift due to the electrical field created between the interface and the gate electrode, the latter being held at a constant potential. At equilibrium, the net ion current is zero and the potential drop across the ion-conducting layer, DV, in simple terms is theoretically given by the Nernst relation (Eq. (10.5)):

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where Voffset is the potential difference inherent to the material or materials combination, R is the molar gas constant, T is the temperature, n is the number of electrons transferred per reaction, F is the Faraday constant, and [A]surface and [A]interface are the concentrations of species A at the surface and interface, respectively. For details, see, for instance.92,93 In conjunction with SiC-based field effect sensors, this concept was mainly used earlier for the detection of oxygen. Examples of ion-conducting materials introduced between the porous gate contact and the insulator include ZrO2,94,95 cerium dioxide (CeO2),96 and perovskite compounds such as barium tin oxide (BaSnO3).97,98 ZrO2 has been shown to work fairly well for oxygen assessments at high temperatures (600  C and above), whereas devices based on the ionic conductor lanthanum trifluoride (LaF3) exhibit a good oxygen sensitivity at lower temperatures, down to room temperature.99,100 Furthermore, the combination of MgO (as insulator) and LaF3 in field effect devices has also given indications on the possibility for oxygen sensing with markedly reduced cross-sensitivity to hydrogen or substances containing hydrogen. Possibilities for oxygen sensing at lower operating temperatures, as compared with the abovementioned MgO/conducting oxide combinations, have thus been demonstrated.87 Furthermore, the concept has also been utilized for the development of field effect devices sensitive to carbon dioxide (CO2) and nitrogen dioxide (NO2). A solid electrolyte was introduced, which facilitated incorporation   of carbonates (CO2 3 ) or nitrites/nitrates (NO2 /NO3 ) in the ion-conducting layer. In combination with a catalytic gate electrode, from which the carbonates/nitrates are generated on adsorption of CO2/NO2 (through the reaction with adsorbed oxygen anions), field effect sensor devices based on the same principles as oxygen sensors can be realized.101,102 In analogy with the oxygen sensors, the transfer of charge, in the form of  CO2 3 or NO3 , from the gate electrode into the electronically nonconducting solid electrolyte, gives rise to a change in the C/V or I/V characteristics of the device. Earlier investigations have also shown promising results regarding NO2 sensing with Si-based MIS capacitor structures employing sodium nitrite (NaNO2) as the ion-conducting material.101 For detection of CO2 with a carbonate-based electrolyte, device operating temperatures of at least 400  C are required. Therefore, SiC-based devices are chosen, see Section 10.1, for the development of a SiC field

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effect CO2 sensor based on the binary lithium carbonate (Li2CO3)/barium carbonate (BaCO3) solid electrolyte (see Fig. 10.7). The binary ion conductor exhibits, in addition to good sensitivity to CO2, an excellent stability also under humid conditions. Fig. 10.7 shows a device with electrolyte deposited on top of MgO and a highly porous Pt gate electrode with promising results regarding CO2 monitoring.103 In this case, MgO also acts as a passivation layer, preventing lithium ions (Liþ) from diffusing into the insulating layer during processing and operation of the device. Perovskites are used as NOx storage materials in catalytic converters for diesel engine exhaust after treatment.104 Strontium titanate (SrTiO3) has been employed as gate material in SiC-FET devices for NOx detection. Single digit ppm detection was demonstrated between 550 and 600  C, while at lower temperature, i.e., 530  C, the response to NOx was somewhat lower but compensated by improved selectivity to NH3.105 (a)

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10.4 Dynamic sensor operation and advanced data evaluation To improve selectivity toward certain gaseous substances for which detection, discrimination, and quantification otherwise might be difficult due to interference from other gases, the remedy has often been the introduction of more sensors, each with its own cross-sensitivity pattern. Normally, the combination of sensors and sensitivity patterns is very complicated, involving a large number of different kinds of sensors60 or similar sensors operated at different temperatures. The large number of sensor signals and their individual cross-sensitivities make necessary to reduce dimensionality by using multivariate statistical data analysis and pattern recognition methods to retrieve the desired information. The most common method to reduce dimensionality is principal component analysis (PCA).106,107 Multivariate methods such as PCA have, for example, been used in conjunction with SiC-based field effect devices to monitor the combustion process in biomass fueled power plants13 for the estimation of ammonia concentration in typical flue gases65 and for fast lambda control of a gasoline engine.108 Another example of a multivariate analysis method is linear discriminant analysis (LDA).109,110 In analogy with PCA, new variables (discriminant functions) are introduced as linear combinations of the original variables. Whereas PCA is an unsupervised method, in LDA the assignment of sensor observations into predefined groupsee.g., corresponding to concentrations of a certain target gaseis a prerequisite already when constructing the new variables. The linear combinations of sensor signals are calculated such that the distances between the centers of predefined groups are maximized in the new projected data set, while minimizing the scatter among observations within the different groups. This makes LDA a supervised method. As was discussed earlier, the interactions between a certain gate material and the substances of the surrounding gas matrix are temperaturedependent. Different substances show different temperature dependence, which is the reason why operation of a sensor at different temperatures can provide more information about the gas matrix composition, or the concentration of a specific gas in a background of other gases. Instead of an array of sensors, each of them operated at a different temperature, the operation of one sensor in a cycled temperature operation mode can provide just as much, or even more, information. In this way, not only the application of more temperatures is simplified but there is also the benefit of automatically obtaining

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information from nonequilibrium conditions, when changing from one temperature to another, aiding in the discrimination between gases and concentrations. The mean value of the sensor signal at different temperatures, as well as the derivatives of the signal corresponding to temperature changes, can then be extracted and treated by multivariate statistical methods (just as for the case of signals from many individual sensors). Another advantage related to the use of one sensor as a virtual sensor array includes a reduction of drift problems and, overall, a better control of the sensor signal and its stability over time. This approach has been developed using commercial resistive-type MOS sensorsdfor example, for early fire detection in coal mines.111,112 This concept is now also applied to field effect sensor devices based on SiC for detection and quantification of NO2, SO2, discrimination between different gases (such as H2, NH3, and CO), and different concentrations for both Pt and Ir gate field effect sensors.113,114,115 It was also possible to discriminate three different volatile organic compound (VOC) molecules, formaldehyde (50, 100, 150 ppb), benzene (1, 3, 5 ppb), and naphthalene (5, 20, 35 ppb) from each other in a mixture of them in humid air using an Ir-gated SiCFET and a 1-min temperature cycle.19 Gate bias ramping of SiC-FET devices introduced hysteresis in the sensor signal, the shape of which revealed more information about the gas mixture under testing. Therefore, gate bias cycling is another alternative for dynamic mode operation. The interaction between the various gaseous substances and the gate material is not only dependent on their identity and temperature but also on the gate potential. Temperature cycled operation combined with gate bias cycling improved the resolution when discriminating and quantifying NO2, CO, and NH3.116 Mixtures of four gases (NH3, CO, NO, and CH4) at two different concentrations (250 and 500 ppm) could be discriminated by employing LDA evaluation.117 Apart from the ambient condition, the shape of the hysteresis varied also with rate of the bias sweep and, of course, the temperature. This was assumed to show the existence of at least two competing chemical processes taking place on the sensor surface, which are also sensitive to the level of the applied gate bias. Fig. 10.8 shows an example of combined temperature and bias cycled operation (TCO-GBCO), feature extraction, and discrimination of NH3 and CO in a background of dry N2. Bastuck et al. investigated the complementary effects from using both MOS sensor devices and SiC-FET sensors in advanced operation modes. The MOS sensors were used in TCO mode, with and without a preconcentrator system, and the SiC-FET sensors were operated in a combined temperature cycled operationegate bias cycled operation (TCO-GBCO) mode.118

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The development of two different sensor operating modes may also openup possibilities regarding self-diagnostic sensor systems. Comparison of the data from two independent methodsde.g., temperature and bias cyclingdmay increase the chances for fault detection and self-diagnosis of the sensor. In the event of a sensor malfunction, it is not likely that the outcome of two separate evaluation schemes would be similar, the discrepancy between them therefore indicating problems. The concept has been demonstrated for a resistive-type metal oxide, MOX, semiconductor sensor utilizing simultaneous temperature cycling, and electrical impedance spectroscopy measurements.119

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10.5 Applications Except for long-term stable sensors, field measurements require suitable packaging of the sensors and functional electronics. In the following section, we will review important improvement in the packaging of SiC-FET gas sensors.

10.5.1 Sensor packaging The transistor outline (TO) header is an industrial standard that has been used for several decades to provide a mechanical basis for the installation of electronic and optical components such as semiconductors and laser diodes, while at the same time providing power to the components with the aid of pins. The SiC-FET gas sensor research improved considerably when TO headers were introduced for microelectronic packaging applications. Fig. 10.9 shows a SiC-FET sensor device mounted on a ceramic (Al2O3) substrate, with a thin resistive-type Pt heater wire on the backside, together with a Pt100 temperature sensor. The leads of the heater substrate and temperature sensor are spot welded to a gold-plated 16-pin TO8 header, whereas the sensors’ electrical contacts are connected to the pins of the TO8 header by gold wire bonding. Such sensor packaging enables operation temperatures even above 600  C, with good control of temperature and data acquisition. As an example of high temperature applications, TO headers have been used in engine exhaust systems and in flue gas channels in bioheaters. Over the last years, an innovative packaging technology based on LTCC has been developed, improving the performance of the SiC-FET sensors and widening the range of possible applications, see Fig. 10.9 (top right). This technology is characterized by hermetically sealed modules processed from sheets of unsintered LTCC, which

Figure 10.9 Four-inch diameter SiC wafer with about 2000 sensor chips commercially processed. Close-up of the wafer shows single transistor devices. After dicing the chips are mounted in a ceramic package (top right) or in a 16-pin TO8 header (bottom right).

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are provided by cavities and vias by laser cutting and electrical contact by screen printing. The sheets are then stacked and finally sintered in an oven at 850  C, which renders a ceramic component.120 Nowak et al. presented LTCC packaging of a SiC-based hydrogen sensor, which is glued on the screen-printed contacts.121 Sobocinski et al. demonstrated for the first time a SiC-FET sensor chip introduced in the LTCC stack and cofired in one single step to a packaged device, which do not need any glue or bond wires.122 This requires LTCC sheets, which do not shrink in the x-y direction (Hereaus Gmbh) during sintering.123 The electrical and sensing properties of the SiC-FET gas sensor are retained after the sintering process at 850  C.124

10.5.2 Applications and field tests The outstanding properties, e.g., in terms of long-term stability and high temperature performance of the SiC material in gas sensor devices are manifested in a range of successful applications and field tests. Loloee et al. demonstrated the robustness of SiC-based sensors using the same Pt-SiO2-SiC capacitive devices for continuous hydrogen monitoring in a coal gasification plant during 5 days and, after that, during 20 days in the laboratory.125 One SiC transistor device has also been operated in a small bioheater for more than 42 months. Control of the inlet air to the bioheater by two SiC-FET gas sensors and a temperature sensor increased the efficiency of the combustion of the wood fuel and considerably decreased the emissions of CO and hydrocarbons.7,14 Successful monitoring of ammonia in the exhausts of a diesel engine equipped with selective catalytic reduction system was demonstrated already 200516 and the sensors were successfully tested in two diesel trucks.4 Not only emissions from vehicles and industrial plants are a threat to our health, even indoor environments in private and public buildings need to be controlled. Indoor air pollution is one of the top five environmental risks to public health which significantly affect quality of life and economy. The list of top-10 gases in air pollution includes the so-called VOCs, a wide class of carbonehydrogen-containing chemicals which are normally found in many products of common use, e.g., tobacco smoke, paints, detergents, glues, construction materials, and pressed-wood products. In 2010, the World Health Organization (WHO) released guidelines for a range of hazardous VOCs, e.g., formaldehyde, benzene, and naphthalene, which are frequently found in indoor environments in concentrations of health concern. Formaldehyde, regarded as the most prevalent VOC, is classified as a probable human carcinogen with a recommended exposure limit of 81 ppb during 30 min of exposure. Benzene is classified as a known human carcinogen at any level of exposure. Naphthalene is reported as carcinogenic in animal experiments

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and a possible human carcinogen, the exposure limit for this substance is set to 1.9 ppb as an average annual level.126 Developing sensor systems specifically selective for such gases at the low ppb or even sub-ppb levels has become a market demand priority. Recently, it was demonstrated that the SiC-FET devices detect formaldehyde and naphthalene at concentrations below the recommended exposure limits, i.e., 10 ppb CH2O and below 0.5 ppb C10H8 under 60% RH. Moreover, the SiC-FET device has proven to detect benzene down to 0.2 ppb in 20% RH and 1e3 ppb in 60% RH, see also Fig. 10.3.8,18 In the last couple of years, different field test campaigns have been carried out in the framework of local collaborations or European projects (e.g., SENSIndoor, Key-VOCs). As an example, an experiment at an elementary school was carried out during a period of 3 months for specific detection of formaldehyde. A commercial formaldehyde monitor (FM-801, Graywolf) and a carbon dioxide concentration, temperature, and relative humidity transmitter (tSense Touch Screen CO2 þ RH/T Transmitter, SenseAir AB) were used as reference instruments. The FM-801 formaldehyde meter provides a measurement range from 20 ppb to 1 ppm and records one data point every 30 min. By using the SiC-FET-based sensor system in dynamic operation mode, continuous monitoring is significantly improved allowing data point recording approximately every minute (depending on the temperature cycle used). Fig. 10.10 displays a temperature cycle of 80 s (four temperatures) and extraction of virtual sensor signals in the school tests.20

Figure 10.10 Temperature cycle (blue, solid line), temperature (blue, dashed line), and the raw sensor signal (black line) during one cycle. The mean of four different areas is computed and the middle of each one, marked by a colored dot, can be regarded as a virtual sensor (left panel). In the right panel, the data of the virtual sensors is extracted from the raw sensor signal.

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CO2 conc r.h. Amb. temp. Form. conc. PLSR1 Ventilation Aug 20 Aug 21 Aug 22 Aug 23 Aug 24 Aug 25 Aug 26 Aug 27 Aug 28 Aug 29 Aug 30 2016

Figure 10.11 Sensors signals from reference instruments from a period of 11 days, standardized and shifted for visualization. The virtual sensors have been used to build the PLSR1 signal (red), smoothed with a window size of 22 (w30 min) for visualization. The bottom blue line represents the normal schedule of the ventilation of the school. The start of each day, i.e., midnight, is marked on the x-axis, and night from 6 p.m. to 6 a.m. is marked by darker areas. Note that Aug 20/21 and Aug 27/28 there is no school (weekends).

In Fig. 10.11, measurements from 11 days are shown. Using a multivariate regression model based on partial least squares regression (PLSR) on the sensor data, the experiment demonstrated a very good correlation between the SiCFET sensor and the FM-801 meter. The formaldehyde builds up at night and during weekends while the ventilation is switched off. The highest peak for the reference instrument was 34 ppb (August 21), while the computed PLSR1 signal from the SiC-FET sensor data had a peak of 24.5 ppb (August 28). In summary, the formaldehyde always stayed well below the threshold value of 80 ppb. The data evaluation also revealed some possible crosssensitivity of the SiC-FET sensor to other common VOCs that are emitted by breath (e.g., acetone and isoprene), which is an area to be further investigated.20

10.6 Summary The SiC-FET devices as high-temperature gas sensors are commercially available in sensor systems for combustion control, e.g., in smalland medium-scale power plants. Research and development has realized tailor-made sensing layers for, e.g., oxygen and carbon dioxide detection. Detection of toxic indoor gases, VOCs, below legally restricted levels has been demonstrated. Temperature and bias cycled operation modes together

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with advanced data evaluation based on multivariate statistics improved selectivity and sensitivity in complex gas mixtures. Field test campaigns have demonstrated the suitability of using the SiC-FET sensor as a selective formaldehyde sensor.

Acknowledgments Grants are acknowledged from the VINN Excellence Center in research and innovation on Functional Nanoscale Materials (FunMat), the Swedish Governmental Agency for Innovation Systems (VINNOVA #621-2012-4497), and the Swedish Research Council (VR #621-2012-4497). The authors also acknowledge funding from the European Union’s Seventh Programme for research, technological development, and demonstration under grant agreement No. 604311 (SENSIndoor), and from the COST Action TD1105 (EuNetAir). A.L.S. acknowledges the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Link€ oping University (Faculty Grant SFO-Mat-LiU No. 200900971). Dr Ruth Pearce is acknowledged for the contribution in the first edition of this book chapter.40 The epitaxial graphene sensor area has grown into an independent research area, exemplified in the introduction.

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