Introduction to semiconductor gas sensors

CHAPTER FOUR

Introduction to semiconductor gas sensors: a block scheme description Arnaldo D’Amico, Corrado Di Natale Department of Electronic Engineering, University of Rome Tor Vergata, Roma, Italy

Contents 4.1 Introduction 4.2 The sensor blocks 4.2.1 The response curve 4.2.2 Sensitivity 4.2.3 Resolution 4.2.4 Example of the evaluation of resolution 4.3 Metal oxide semiconductor capacitor: the case of the hydrogen gas sensitivity of Pd-SiO2-Si 4.4 Light-addressable potentiometric sensor 4.5 Metal oxide semiconductor field-effect transistor 4.6 Metal oxide semiconductors 4.6.1 SnO2 bands 4.6.2 Band diagram modulation 4.7 Conclusions References

133 135 136 137 138 139 142 144 148 151 151 153 156 156

4.1 Introduction Several human activities require the measure of the concentration of gases and volatile compounds. Industrial processes and pollution control have been the traditional applications of gas sensors.1 In the last years, a number of novel and fascinating applications emerged, not least the detection of the volatile metabolites as biomarker for several diseases.2 In this context, analytical instruments are ready to provide the necessary accuracy for monitoring gases and vapors. However, although the great efforts of miniaturization of traditional instruments, such as gas chromatographs and Semiconductor Gas Sensors, Second Edition ISBN: 978-0-08-102559-8 https://doi.org/10.1016/B978-0-08-102559-8.00004-5

© 2020 Elsevier Ltd. All rights reserved.

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j

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mass spectrometers,3,4 these are still bulky, expensive, and require trained and skill technicians for proper operation and for their data analysis and interpretation. Solid-state sensors have been always considered the solution to overcome some of these drawbacks and to facilitate gas analysis avoiding the mandatory use of dedicated laboratories. Furthermore, solid-state sensors can take advantage of the micro/nanoelectronic technologies; thus, they can be cheap and reliable and furthermore readily available for the latest information technologies such as the so-called Internet of things.5 Semiconductor gas sensors are solid-state devices based on semiconductor materials and/or semiconductor devices. Then, when we talk of semiconductors, we may intend either those materials (inorganic or organic) that show some intrinsic sensitivity to gases or the devices that constitute the basis for transducers such as, for instance, those based on field effects where changes in the electronic properties of the gate can control either the output current or voltage. The parameters of devices and materials depend on the quality and quantity of the gases and vapors to which they are in contact. Semiconductor materials, particularly in transducer systems, offer the perspective of integration of the sensors in the microelectronic realm taking advantage of the integrated electronic interfaces for signal processing. In this paper we will consider a variety of sensors. Before describing the working principles of sensors, it is worthy to introduce some basic definitions of sensor properties.6,7 These definitions define a common language necessary for an objective comparison among sensors. Ideal gas sensors should be sensitive, selective, and stable. Additional properties are reversibility, accuracy, response time, and linearity. For many applications, additional properties such as low cost, small size, unsensitivity to radiation and temperature, and robustness are also required. Finally, sensors should always be compatible with standard preprocessing electronics. In practice, being almost impossible to satisfy at once all these constraints an acceptable compromise is always sought satisfying only a part of the above-mentioned features. In this paper, we will describe the sensors through a block scheme which helps to decompose the sensor principle in a number of elementary steps each endowed with its own sensitivity. The scheme might help to compare different sensors emphasizing where the different performance arises. Thus, instead of paying attention to the basic theories, widely available in literature

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and in many chapters of this book, this chapter stays focused on the basic intrinsic sensitivities. To facilitate the comprehension of the basic mechanisms of the sensors based on field effects, we will consider as an example the chemical sensitivity of palladium films with respect to the hydrogen gas.8 This example points out the basic properties of the devices and it can be easily generalized to semiconductor devices made of either inorganic or organic materials.

4.2 The sensor blocks Fig. 4.1 shows a complete block scheme of a general sensor system. Not only the whole system can be considered as a sensor but also elements or subsets of the scheme can be identified as sensors itself. The whole appraisal of such a scheme is mandatory to understand the role of devices. Sensors are the interface between the electronic circuits and the outer world. It is simple to understand that the properties of sensor signals, namely the electric signals depending on the outer world quantities, are determined both by the features of the sensor as a device and by the electric circuit at which the sensor is connected.

Outer world

measurand

sensor Y=f(M)

Y

electronic interface

v,i

amplifier

v,i

v,i

filter

A/D conversion

energy

N

μP

actuation

storage

communication

display

Figure 4.1 General scheme of a sensor system. The intrinsic sensor is the interface between the electronic system and the outer world.

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Fig. 4.1 shows the general scheme of a sensor-based electronic system. The intrinsic sensor element provides the interface with the outer world. The parameter Y is the electric parameter affected by the interaction with the gas. This may be, for instance, the resistivity, dielectric constant, or work function. The quantity Y itself is not observable until it is transformed by the interface circuit into a voltage or a current signal. The signal can then be amplified and filtered to facilitate its measurement. The measurement is usually performed through an analog to digital conversion, and the digital signal is processed to actuate some action on the outer world (e.g., a regulation system), stored, communicated, or displayed. Finally, it is important to keep in mind that each part of the system needs energy. All steps of the processing chain contribute to the overall sensitivity. Each block is characterized by a proper input/output relationship and the combination of all of them makes the total sensitivity.

4.2.1 The response curve The response is any quantity useful to represent the state of a sensor as a device or of the system at which the sensor is connected or of a sensor system that can be simple (formed by only one block) or rather complex made by more than one block as described in Fig. 4.1. In some cases, the response is defined in reference to a baseline value that is the response in absence of stimulus. In such a case, the response can be given as the difference, ratio, or relative change with respect to the baseline. Independently from the definition, the response is a function of the concentration of the gas. The functional relationship depends on the nature of the intrinsic sensing element and in case of a sensor system on the circuit parameters. The response curve (see Fig. 4.2) is usually represented in a Cartesian plane identified by two axes carrying the sensor signal and the gas concentration (generally called measurand), respectively. Because of the unavoidable noise, the origin of the plot cannot be reached, but rather the curve stops in correspondence of the noise level of the output signal. The response curve is the result of the sensor calibration. This operation corresponds in measuring the sensor signal when the device is exposed to known concentrations. The fitted experimental points give rise to the analytical response curve that is normally used to estimate the concentration once the output of the sensor is measured.

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Vout Vout, max

Vout, min=Vnoise Mmin

measurand

Mmax

Figure 4.2 Example of a typical response curve, when the sensor response is the output voltage signal (Vout).

4.2.2 Sensitivity It is defined as the derivative of the response curve with respect to the measurand: dVout (4.1) dM The sensitivity is the slope of the response curve (Fig. 4.2), and because the response is limited, the increase of sensitivity narrows the range of concentration. In case of gas sensors, a large sensitivity implies the capability to measure tiny changes of concentration. Where the response curve is flat, the sensitivity is null; this means that in this region any small change of the measurand does not change the output signal. Fig. 4.3 shows a subset of blocks of Fig. 4.1; it represents the steps leading from the concentration to the measured digital quantity. Taking into consideration these steps, it is convenient to express the partial sensitivities according to the nomenclature given in the same figure. It appears clearly that the overall sensitivity (the sensitivity of all the chain) is only partially related to the sensitivity of the first block, which is the most important. Rather the whole sensitivity includes also many steps of the electronic processing of the signal. The sensitivity of the intermediate blocks may be called the internal sensitivities. As shown along this chapter, the internal sensitivity can be further divided in many others, depending on the complexity of the sensor system. S¼

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S=

dN dN dv f dva dv0 dY df AD df F df A df C df S = × × × × = × × × × dM dv f dva dv0 dY dM dv f dva dv0 dY dM

Figure 4.3 The total sensitivity is given by the product of the sensitivities (derivatives of the transfer functions) of all the blocks of the chain leading from the measurand to the measured quantity.

Fig. 4.3 shows elements of a typical sensor system. The intrinsic sensor is characterized by a parameter that is affected by the measured M, the sensor is connected to an interface circuit and then any variation of Y produces a variation of a signal v0, the signal can be amplified giving rise to a larger signal va, noisy components could be removed by a filter and vf is the filtered signal, and finally the analog signal can be converted in a number (N) by an analog to digital converter. The definition of sensitivity can be applied to any block of the chain. Note that in case of the amplifier, the sensitivity is an alternate definition of the gain and the two quantities are coincident when the output of the amplifier is linearly proportional to the input. The total sensitivity (dN/dM) is then obtained with as the product of the sensitivity of each block.

4.2.3 Resolution The resolution is the necessary consequence of the peculiar nature of the quantity represented in the vertical axis of the response curve. Indeed, the sensor signal is the result of a measurement and it is subject to measurement errors. Errors of measurement are contributed by both the finite accuracy of the measurement instrument (in modern electronic systems this is ruled by the analog to digital conversion) and by the electronic noise. The electronic noise fixes the lower limit for the measurement error. The resolution (Mres) is the smallest measurable change of the measurand. Given a sensor signal Vout, the corresponding resolution is Mres ¼

Vout Vnoise ¼ Vout /Vnoise S S lim

(4.2)

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The sensitivity is calculated in the neighbor of the measurand at which Vout is measured. Of course, in case of a nonlinear response curve, both the resolution and the sensitivity are functions of the measurand. Vout can be considered at the output of each block in Fig. 4.3. As each electronic block adds its own noise, shorter the block chain, better the resolution. It is worth pointing out that noise and sensitivity measurements are of paramount importance for the sensor characterization to evaluate the resolution of a given sensor. Different types of noise may be encountered when we are dealing with sensors, and not all sensors have the same noise, while some sensors may show more than one kind of noise. In this context, the relevant parameter for the noise characterization is the noise spectral density s( f ). The root mean square of the output signal (Vrms) is the measurable manifestation of the noise. The relationship between S( f ) and the root mean square of the output signal is 2 31=2 Z f2 6 7 Vrms ¼ 4 sðf Þdf 5 (4.3) f1

The integral is calculated in the frequency interval practically defined by the measurement time. Shorter the measurement time, wider the frequency interval and then the noise contribution to the sensor signal. The most important types of noise are listed in Table 4.1.9 Excess noises are manifested as an additional contribution to the current and then they occur only in biased materials. It is worth mentioning that in gas sensors, additional sources of noise come from the fluctuation of the concentration at the sensor surface of the compounds at which the sensor is exposed and from the fluctuation of adsorption/desorption processes. These terms contribute to the overall spectral density of sensor noise as an additional flicker-like noise.10

4.2.4 Example of the evaluation of resolution To fix the ideas about the relationship between sensitivity and resolution, let us consider the simple circuit in Fig. 4.4 where a resistance temperature detector (RTD) is connected to a simple readout circuit. The relationship between temperature and resistance can be easily generalized to any resistive gas sensor. Let us consider the sensor represented by a linearized response curve

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Table 4.1 Characteristics of the most typical noises found in semiconductors. c, c0 , and c00 are constants; g is a factor close to 2; a is a factor close to 1; d is a factor ranging from 2 3; b is a factor ranging from 0.8 to 3. q is the electron charge and k is the Boltzmann constant. s is the recombination time of generationerecombination (GR) processes. Finally, u ¼ 2pf, i is current, sv is spectral density of voltage, and si is spectral density of current. Denomination Definition Spectral density

Thermal noise

Shot noise

Flicker noise

Burst noise

GR noise

Manifestation of the thermal motion of electric charges (electrons in solids); its magnitude is proportional to the resistance. Excess noise typical for space charge regions and therefore present in all junction devices (e.g., diodes). Excess noise likely due to a continuous distribution of traps, typical for semiconductors. Excess noise emerging in semiconductors likely due to impurity atoms. Excess noise due to GR processes in semiconductors.

RS ðT Þ ¼ R0 $½1 þ a$ðT  T0 Þ

sv ¼ 4$k$T $R

si ¼ 2$q$i

g

sv ¼ c Vua

sn ¼ c 0Vub

d

sn ¼ c 00 1þus 2 s2

(4.4)

R0 is the resistance known at the reference temperature T0 and a is the temperature coefficient. The block scheme of the sensor in Fig. 4.4 is shown in Fig. 4.5. The RTD is made of a material whose resistivity is affected by changes of temperature. The transducer circuit that generates the sensitive signal is made by the current source I0, the signal is then amplified by the noninverting amplifier, and, in case, it can be converted into a digital quantity. The overall sensitivity of Vout with respect to the temperature can be written as

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R2

R1

Vout (T)

RS(T)

I0

Figure 4.4 RS is a resistive sensor, a resistance temperature detector (RTD) in the example, connected to a noninverting amplifier.

R(T) (R+ΔR)

Transducer circuit

Amplifier (R+ΔR)*I*A

(R+ΔR)*I

A/D conversion

N

ρ+Δρ ΔT

ρ(T)

Figure 4.5 Block diagram of the sensor systems based on the circuit of Fig. 4.4.

dVout dVout dVin dRS ¼ ¼ A$ST $SR ¼ dT dVin dRS dT



 R2 1þ $I0 $R0 a R1

(4.5)

where A is the amplification, ST is the sensitivity of the transducer circuit, and SR is the intrinsic sensitivity of the thermistor. The resolution can be directly calculated from the definition. Vnoise (4.6) A$ST $SR Note that the magnitude of noise depends on the single block characteristics; for instance, the amplifier amplifies both the signal and the noise and then it is irrelevant in improving the resolution, rather it worsens the resolution because it adds its own noise to the total signal. This last expression makes evident that the noise of the output signal and the sensitivity of each block are necessary to evaluate the sensors performance. On the other hand, the knowledge of the intrinsic sensor (the temperature sensor in this case), although fundamental for the final performance, does Tres ¼

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not provide any knowledge about the actual performance until the electronic blocks are defined and characterized.

4.3 Metal oxide semiconductor capacitor: the case of the hydrogen gas sensitivity of Pd-SiO2-Si The first solid-state gas sensor that we take into consideration is the metal oxide semiconductor (MOS) capacitor schematically shown in Fig. 4.6. Let us consider the case when the gate metal is palladium.11 The catalytic properties of palladium to favor the dissociative adsorption of hydrogen gas and the following diffusion of atomic hydrogen are known, as well the H2 sensing properties of this device. The basic structure is formed by a stack of four regions: an ohmic contact, p-type silicon, silicon dioxide, and the thin film of palladium. In the depletion mode, the total capacitance Ct is given by the series of the oxide and the depletion layer capacitances. In a reference atmosphere where hydrogen gas is not present, the difference between the work functions of Pd and Si (p-type) is such to generate a depletion layer.12 As a consequence of H2 adsorption, the depletion layer size changes. This results in a variation of the depletion capacitance and then in the total capacitance. The intrinsic mechanism of sensitivity is reassumed in Fig. 4.7. The global sensitivity is the derivative of the total capacitance with respect to the hydrogen concentration. It is worth pointing out that each block is characterized by a proper response. In fact the first block is supposed to be the slowest being due to chemical reactions at the palladium surface and to the diffusion process of

P

Figure 4.6 Schematic of Pd-OS capacitor. The capacitance is the series of the oxide capacitance (fixed) and the depletion layer capacitance (variable). The depletion layer size changes as a consequence of the exposure to H2.

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Δ(H2) Δδ

ΔΦ

Δ VFB Δ xd Δ Ctot

Δ Vout

Figure 4.7 The exposure to hydrogen gas concentration ([H2]) results in a dissociative adsorption of H2 at the palladium surface. It produces a concentration of atomic hydrogen that quickly diffuses toward the oxide surface. The layer of atomic hydrogen forms a dipole layer (d) at the oxide surface that can be interpreted as a change of the work function difference (DF) between palladium and silicon, the consequence of which is a change of the flat band voltage (VFB) that is related to the size of the depletion layer (xd) and then to the total capacitance (DCtot). Finally, a proper transducer circuit can convert the capacitance changes into a voltage change (DVout).

hydrogen atoms from the palladium surface to the Pd/SiO2 interface. The electronic time constant should not be neglected; however, the overall response time is mainly controlled by the diffusion processes, which are rather slow being the diffusion rate of atomic hydrogen through palladium of the order of 5 ms/Å. From Fig. 4.7, the total sensitivity St can be expressed as dVout dVout dCtot dxd dVFB dDF dd ¼ (4.7) d½H2  dCtot dxd dVFB dDF dd d½H2  From the above expression, all the physical terms contributing to the overall sensitivity are visible. The last term dd/d[H2] is the intrinsic contribution of palladium, while all the other terms, summarized in dCtot/dd, are related to the MOS structure, and then they depend on the oxide thickness, permittivity of oxide, semiconductor, and the doping concentration in silicon, and the area of the MOS structure. Eventually, the first term depends on the particular circuit used to convert the capacitance changes into a voltage change. Fig. 4.8 shows an example of a circuit that can be used to measure the value of the capacitor C as a function of the applied DC voltage V0. The MOS capacitor is biased by the sum of a DC voltage (V0) and AC voltage (vi); then the total applied voltage, if R1 ¼ R2, is VS¼ V0  Vm cos(ut). It is important that Vm  V0. In this way, the AC signal is a small perturbation necessary to extract a signal proportional to C, but C depends almost completely on V0. Of course vi must be as small as possible but of a S¼

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R2

R1 R3

V0

R1

Figure 4.8 Example of a circuit to measure the capacitance of metal oxide semiconductor structure as a function of the applied voltage V0.

sufficient level to give a measurable output with at least a signal to noise ratio not less than about 6. The output signal is given by dVs (4.8) ¼ R3 Ctot uVm sinðutÞ dt The above circuit can be used to measure the relationship between Ctot. and V0; this is an outmost characteristic of MOS devices, and for sensing purposes, all circuit parameters can be optimized and kept fixed to measure the changes of Ctot. The scheme shown in Fig. 4.7 describes the chain of sensitivity when the MOS capacitor is not biased in inversion. In case of inversion, the depletion layer size is fixed, rather in this case, the change of the flat band voltage affects the threshold voltage at which the inversion layer is formed. This condition is exploited in the metal oxide semiconductor field-effect transistor (MOSFET) device discussed below. Vout ¼ R3 Ctot

4.4 Light-addressable potentiometric sensor The intrinsic photoconductivity of semiconductors is exploited in MOS structures to give rise to an interesting device called lightaddressable potentiometric sensor (LAPS).13 As shown in Fig. 4.9, the output voltage across a load resistance RL is generated by an internal equivalent voltage generator which is developed

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145

P O

+qε

D

ε

−qε S

Figure 4.9 Principle of light-addressable potentiometric sensor. Photons absorbed in silicon give rise to electronehole couples. The couples generated in the space charge region are separated by the built-in electric field. The displacement charges the metal oxide semiconductor (MOS) capacitor and results in a signal observable across a load resistor in series to the MOS capacitor. Keeping constant the photon flux, the signal is proportional to the volume of the space charge region and then it is affected by work function changes.

once pulsed light of suitable time width and of specific wavelength is absorbed in the semiconductor. In a first approximation, the electrical equivalent circuit of LAPS is shown in Fig. 4.10. Vin is the internal voltage source generated across the depletion region whose magnitude is due, by the product of the depletion region impedance and the current of the electrons and holes produced by the adsorbed photons. Electrons and holes are separated by the built-in electric field located inside the space charge region; the capacitor is the series of the oxide and the depletion capacitances, while RL is the load resistance. When the device is shined by pulses of light, the output voltage corresponds to the derivative

Vi

RL

Figure 4.10 Light-addressable potentiometric sensor equivalent circuit.

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Light intensity

Time V0

Time

Figure 4.11 Typical output voltage of a light-addressable potentiometric sensor in case of pulsed light.

of the voltage pulses (and not current pulses); the time constant is equal to CRL (see Fig. 4.11). The gas sensing mechanism is illustrated by the block scheme in Fig. 4.12. The gas sensitivity is due to the same processes occurring in the MOS capacitor; the difference here is that the size of the depletion layer modulates the amount of electronehole pairs that are separated by the built-in electric field. In practice, instead of measuring the change of the MOS capacitor, here a voltage proportional to the flux of impinging photons and to the

Δ(H2)

Δδ

∆ VF B

ΔΦ Δxd

vout n−p Couples

in; ip

Figure 4.12 Block diagram of light-addressable potentiometric sensor.

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volume of the space charge region are generated. Actually the sensor is sensitive to both light intensity and gas concentration and the device becomes selective to gas only once the intensity of light has been kept constant. S¼

dVout dVout dq dxd dVFB dDF dd ¼ $ d½H2  dq dxd dVFB dDF dd d½H2 

(4.9)

The quantity dq namely the amount of photo-induced electronehole pairs is proportional to the intensity of the impinging light. Nevertheless, the sensitivity of the LAPS is proportional to the intensity of light. In other words, the sensitivity lies in the number of electronehole couples related to the adsorbing volume defined by the depletion layer width and the section area of the device. It is worth to consider that the light pulse is a probe of the extension of the space charge region. The dependence from the light introduces an additional noise because of the fluctuation of the light source; this is equivalent to a shot noise proportional to the root square of the light intensity. In terms of sensitivity improvement, the key quantity is dxd/dVFB. The doping concentration of the semiconductor can actually determine a larger variation of the depletion region with respect to changes of the flat band voltage. LAPS sensors can be easily integrated in arrays.14 Fig. 4.13 shows a pictorial example. Each element of the array could be made of a different gate material or could be probed by light of different wavelengths. The light can be addressed sequentially to each element of the matrix. The n-multiple repetition of the scan and the storage data may allow a significant pffiffiffi noise reduction evaluated by a factor of n where n is the number of light scans per each element.

Light Pt

Pd

W

Au

Figure 4.13 Pictorial view of an array of light-addressable potentiometric sensor.

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4.5 Metal oxide semiconductor field-effect transistor MOSFETs are among the most reliable and versatile transducers for gas sensor applications. In electronics the MOSFET structure has been continuously modified and improved along the years and it is at the basis of both analog and digital electronics.12 More than four decades ago, the MOSFET structure was transformed into chemical sensors replacing the gate with either an electrolyte or a catalytic metal15,16 because that MOSFET has been the basis for the development of several sensors and biosensors for a plethora of different applications. Being the MOSFET an extension of the MOS capacitor, also in this case the sensitivity is activated anytime, the interaction between the gas and gate produces an amount of charges or dipoles at the gate SiO2 interface. This layer of charges or dipoles generates an additive gate voltage and then a variation of the drain source current. Fig. 4.14 shows a schematic drawing of a MOSFET. In its first implementation as gas sensor, the gate was a film of hydrogen-sensitive palladium.12 The charge control equation of the MOSFET in the quasi-linear region is approximately12

VS

VD VG

Metal

Metal

Oxide n+

Metal Oxide Inversion Depletion

Oxide n+

p-type Si p+ Metal

VB

Figure 4.14 General scheme of a metal oxide semiconductor field-effect transistor device.

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 2  w VDS IDS ¼ mn $Cox $ ðVGS  VT Þ$VDS  (4.10) L 2 In the saturation region, the current becomes largely independent from VDS: w IDS ¼ mn $Cox $ ðVGS  VT Þ2 (4.11) 2L The above relationship holds beyond the pinch-off point and neglects the effective channel length change due to VDS. In the above formulas, VGS and VDs are the voltages applied between the gate and source and drain and source, respectively, and IDS is the current from drain to source. mn is the effective electrons mobility in the channel, Cox is the oxide capacitance, w and L are the width and the length of the channel, respectively, and VT is the threshold voltage. This is a key parameter of the device because, at first approximation, only when VGS > VT the channel is formed and the current can flow. The threshold voltage depends on all the relevant quantities of the MOS structure such as the flat band voltage and the oxide capacitance. The saturation condition is obtained from Eq. 4.10 under the condition that the current IDS does not depend on VDS, namely dIDS/dVDS ¼ 0. This condition is achieved when VDS ¼VGS  VT. Eqs. (4.10) and (4.11) are plotted in Fig. 4.15. This is the so-called output characteristics of the MOSFET. The MOSFET should be polarized to fix the quiescent point in the saturation region. The block scheme of the sensing mechanism of the Pd-FET is shown in Fig. 4.16. It corresponds to the block scheme of MOS capacitor and LAPS, except that changes in the flat band voltage in this case affect the threshold voltage. Then if the biasing parameters of the MOSFET are kept constant, the IDS changes. The original Pd-FET structure was modified to extend the sensitivity to other species than hydrogen and to use more sensitive materials. A first interesting development consisted in the use of ultrathin metal films as gate. In this condition, the metal film is not homogeneous but rather it is characterized by a number of cracks that leave the oxide exposed to the ambient air.17 In this way, the analyte does not necessitate traveling through the metal but it can reach directly the oxide surface. This offered the possibility to use more catalytic metal and to expand the sensitivity to other analytes such as ammonia.18 Additionally, a cracked gate layer can also accommodate

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0.05 Vgate= 1V Vgate=1.5V Vgate= 2V Vgate=2.5V Vgate= 3V Vgate=3.5V Vgate= 4V Vgate=4.5V Vgate= 5V Vgate=5.5V

0.045 0.04

Drain current [mA]

0.035 0.03 0.025 0.02 0.015 0.01 0.005 0 0

1

2 3 Drain-source voltage [V]

4

5

Figure 4.15 Currentevoltage curves of the metal oxide semiconductor field-effect transistor where the gate voltage is kept as a parameter.

Δ(H2)

Δδ

∆ ςΦ Β

ΔΦ ∆ ςΤ

∆ςουτ Δ ΙΔΣ

Figure 4.16 Block diagram of Pd field-effect transistor (FET). The adsorption of hydrogen gas changes the flat band voltage of the metal oxide semiconductor (MOS) structure, and the flat band voltage affects the threshold voltage and then the currentevoltage characteristics of the device.

nonconductive organic layers as chemically sensitive material. This opportunity has been exploited to develop porphyrins functionalized MOSFETs.19 The MOSFET structure was also fabricated with organic semiconductors.20 These devices are usually made as thin film transistor architecture, and the small thickness of the organic semiconductor enables the accumulation mode and the sensitive materials as the organic semiconductor.21,22

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4.6 Metal oxide semiconductors MOS are probably the more diffused gas sensors. A number of different metal oxides can be actually used; the first of these sensors was made of ZnO23 but the most popular of these materials them became the tin oxide (SnO2).24 In the following section, the working principles of SnO2 are illustrated.25 Imperfect stoichiometric ratio between tin and oxygen occurs in real materials; the vacancy of oxygen atoms leaves a quota of electrons of tin atoms not engaged in covalent bonds and weakly bond to their atoms. At room temperature, this results in a light doping of the material that shows an n-type character. The surface chemistry of tin oxide is complex, and here the simplest reactions involving oxygen are described. The principle of operation of SnO2-based semiconductor gas sensor is mainly related to the change in conductivity occurring when reducing gases (such as CO, CH4) interact with chemisorbed oxygen ions. The adsorption of O2 at the surface of tin oxide results in charged species that subtract electrons from the semiconductor conductance band. As a consequence, a surface depletion layer appears and the surface conductivity is strongly reduced. The reduction of conductivity is manifested in polycrystalline materials, where the current moves from grain to grain and then it is subject to the built-in potentials associated to the surface depletion layers. The interaction with reducing gases removes the surface oxygen species, electrons are released in the conduction band and the intergrain potential barriers are also lowered, and as a consequence, the conductivity increases. The above-described process is temperature-activated and normal operation temperatures are between 150 and 600 C, depending on metal oxide and gas. Surface addition of nanoparticles deposited can greatly improve the sensitivity and the selectivity.26 More recently, organic surface functionalization was also introduced.27,28

4.6.1 SnO2 bands At thermal equilibrium (no voltage applied), many situations are possible at the SnO2 surface. As an example we consider the case of a SnO2-based CO sensor for which conduction and valence band bandings are mainly because of the following charge modifiers: traps (due to surface defects), additives,

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D

I

–

Δφ

Δσ

T

Figure 4.17 Block diagrams of metal oxide semiconductor gas sensors in case of CO, or any other reducing gas, detection. Oxygen ions adsorbed at the metal oxide surface interact with airborne CO molecules, and as a consequence, the depletion layer size and the surface barrier are decreased. In a polycrystalline material, it results in a change of the total conductivity. A transducer circuit, such as that shown in Fig. 4.4, transforms the change of conductivity into a voltage signal.

adsorbed oxygen and oxygen ions formation, and formation of CO2 in presence of CO. Fig. 4.17 shows the block schemes of the sensing principle. Here, we consider two main cases, taking into account that mixed situations involving not all the modifiers may be possible. Fig. 4.18 shows a typical cross section of a SnO2 sensor with two ohmic contacts, a layer of polycrystalline SnO2 of a given thickness and an electric insulator but thermal conductor substrate and a heater on the other face. Fig. 4.19 shows the band diagrams of a monodimensional sequence of adjacent grains, in thermal equilibrium. The equilibrium condition is reached through a rearrangement of electrons across the interfaces between grains. In Fig. 4.19, the charge, in the deep depletion approximation, and the electric field are shown. When a voltage is applied across the electrodes, we can suppose that the potential is distributed only across the depletion layer. This is a typical assumption when electronic devices are modeled.12 Then the voltage drop reduces the barrier height from one side of the contact between grain, and it results in a net current flowing in the material (see Fig. 4.20). Clearly, the bulk region of the grains does not participate to the gas sensitivity or to the conduction. In nanocrystalline materials, the bulk is actually eliminated and the whole material is exploited for sensing and conduction purposes. Electric contact

Polycrystalline material

Insulating substrate Heater

Figure 4.18 Cross section of a typical polycrystalline sensor.

Electric contact

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(a)

(b)

(c)

Figure 4.19 (a) Band diagram, (b) charge distribution, and (c) electric field in a monodimensional arrangement of metal oxide grains. Note that the chain is terminated at both sides by a junction with the metal electrode. Fm and FSnO2 are the work functions of the metal electrode and the sensor material, respectively. fi is the Schottky barrier between SnO2 and the metal electrode, fi is the intergrain potential barrier. ND is the electrons density in the conduction band corresponding to the donor concentration and xd is the depletion layer size at the intergrain interface.

4.6.2 Band diagram modulation Each step of the sensing principles illustrated in Fig. 4.17 in the block scheme changes the band diagram of the material at the metal oxideeair interface. Fig. 4.21 shows four steps leading from the equilibrium configuration of inert material to the consequences of the interaction with a reducing gas. Here again the carbon monoxide is explicitly mentioned. Fig. 4.21(a) shows the band diagram of inert material. This condition is met either in vacuum or at a low temperature. Low temperature means less than about 150 C, namely when the adsorption of airborne oxygen is not favored. At high temperature and in air, the adsorption of oxygen can take place. Oxygen can be adsorbed both as atomic and molecular species.

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E V=0 Fermi level

φS – VA

E

φS

V>0

E V>0

φS – VA

φS

Figure 4.20 Changes in the conduction band diagram of the junction between two adjacent grains at the equilibrium (V ¼ 0) and under bias positive and negative. VA is the portion of the total applied voltage across a single junction. Owing to the numerosity of grains, VA is small enough to ensure the quasi-equilibrium condition. Both thermionic and tunnel currents can be simultaneously present. Both the contributions give a current inversely proportional to the exponential of the barrier height.

In Fig. 4.21(b), the case of atomic oxygen adsorption is shown. The molecular oxygen undergoes a dissociative adsorption onto the metal oxide surface and two atomic oxygens are adsorbed. The bond is provided by two electrons which are displaced from the conduction band to the oxygen atoms. As a consequence, the surface region of the semiconductor is depleted of electrons, the bands bend upward, and a surface potential (qfS) and a work function change appear. Surface oxygen can further react, at the optimal temperature, with a reducing gas molecule (such as CO). The consequence is the formation of a volatile CO2 molecule and the release of an electron in the conduction band. This elicits a reduction of the surface barrier (qf0 S) and the work function. The full picture is much more complex because of the multiple oxygen species, each adsorbed at different energy. Furthermore, the presence of additional species in air, e.g., water vapor, makes the involved chemistry more complex. However, the above description provides a sufficient introduction to the main phenomena involved in the gas sensitivity of MOS.

(b) In air at high tempearture

(c) In air and reducing gas at high temperature: step 1

(d) In air and reducing gas at high temperature: step 2

Introduction to semiconductor gas sensors: a block scheme description

(a) In air at low tempearture

Figure 4.21 Band diagram modulation in the different steps of reducing gas detection. (a) In air at low temperature, (b) in air at high temperature, (c) in air and reducing gas at high temperature (step 1), and (d) in air and reducing gas at high temperature (step 2).

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4.7 Conclusions In this chapter, a general introduction to the topic of semiconductor gas sensors has been provided. General phenomena related to the sensing properties of semiconductor materials and semiconductor devices have been introduced, discussing the properties of MOS (resistors) and MOS device (capacitors) and the related FET. The discussion has been maintained at a general level focusing the attention on the basic processes responsible of the gas sensitivity. For this reason, block schemes have been introduced to help the reader to localize the sensitivity sources. The main purpose of these diagrams is to introduce a decomposition of the global sensitivity into elementary phenomena to make evident where the sensitivity emerges, which are the steps to improve the sensor, and finally how to modify the sensor to extend its property. We would like to propose this approach as a general method to present sensor properties. This in particular is necessary for novel sensors where a block scheme with the partial sensitivity values enables the comparison with other similar or, sometimes, identical sensors. A last point is concerned with the overall response time which is made up by the contribution of each block and then the knowledge of the response time of each elementary element is necessary for the development of more performant sensors.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

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