- Email: [email protected]
GaN High Electron Mobility Transistor
Sensors and Actuators A 194 (2013) 247–251
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Effect of bias conditions on pressure sensors based on AlGaN/GaN High Electron Mobility Transistor E.D. Le Boulbar a,∗ , M.J. Edwards a,b , S. Vittoz c , G. Vanko d , K. Brinkfeldt e , L. Rufer c , P. Johander e , T. Lalinsky´ d , C.R. Bowen a , D.W.E. Allsopp a a
Department of Electronics and Electrical Engineering, University of Bath, UK Department of Mechanical Engineering, University of Bath, UK c TIMA Laboratory (UJF, CNRS, G-INP) Grenoble, France d Institute of Electrical Engineering, Slovak Academy of Sciences, Bratislava, Slovak Republic e Swerea IVF, Mölndal, Sweden b
a r t i c l e
i n f o
Article history: Received 5 September 2012 Received in revised form 11 February 2013 Accepted 12 February 2013 Available online 19 February 2013 Keywords: Stress sensor AlGaN/GaN HEMT Piezoelectric
a b s t r a c t This work reports the bias and pressure sensitivity of AlGaN/GaN High Electron Mobility Transistors (HEMTs) sensing elements strategically placed on a pressure sensitive diaphragm clamped at its edges. The sensitivity was over 150 times greater in the weak inversion regime than in the strong inversion regime of the HEMT, leading to a drain current change of >38% when a pressure of 50 bar was applied. The sensitivity of the HEMT to pressure followed an exponential dependence from atmospheric pressure up to 80 bar, behaviour explained by the response of the density of a two-dimensional electron gas to pressure induced changes in the HEMT threshold voltage in the weak inversion regime. Finally, it was found that the sensitivity of the HEMT was maximum when it was situated in the middle of the diaphragm, whereas a device mounted over the clamping point showed less than 0.02% change in drain current when pressure change of 50 bar was applied. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The potential for GaN based pressure sensors to operate at high-temperatures and in harsh environments has been described recently in a number of publications [1–3]. Researchers have employed a variety of devices and sensitive mechanisms to convert pressure into a readable signal in GaN based devices. Simple piezoresistors were initially used in early studies to demonstrate the proof of sensor concept [4]. The use of a High Electron Mobility Transistor (HEMT) as the sensing element resulted in a major increase in sensitivity [5,6]. The transduction mechanism in a GaN HEMT results from the variation in the conductivity of the two-dimensional electron gas (2DEG) in response to an externally applied force. The variations in the 2DEG conductivity derive from changes in the piezoelectric polarization at the interface between the gate barrier and the channel and from a possible piezoresistive contribution. Although a number of papers have revealed that HEMT electrical parameters such as gate bias (VGS ) and drain source voltage (VDS ) play a key role in optimization of pressure sensor sensitivity [5–7],
a systematic investigation of their importance of each parameter on the pressure sensitivity of GaN HEMT transducers. In this work we examine the sensitivity of AlGaN/GaN HEMT transducers for pressure sensing. The sensitivity was investigated under static pressure for three different HEMT working regimes: (1) strong, (2) moderate and (3) weak inversion regimes by applying gate bias voltages from 0 to −3.5 V in −0.1 V steps for a drain source voltage fixed at 1 V. The same experiment was repeated for different VDS to find the most sensitive regime. As a result, an optimum sensitivity was found in terms of VGS and VDS . The radial response of the chip was investigated by measuring sensitivity on six sensing elements placed in various distances from the diaphragm centre. Finally, the pressure dependence of the sensor in the weak inversion regime was investigated by introducing nitrogen gas at pressures up to 80 bar uniformly on one side of the device whilst the other was kept at atmospheric pressure. An exponential dependence of sensitivity with pressure is found to fit closely with a simple model of HEMT sheet carrier density in the weak inversion regime.
2. Experimental ∗ Corresponding author. Tel.: +44 01225 385315. E-mail address: [email protected] (E.D. Le Boulbar). 0924-4247/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2013.02.017
Fig. 1 shows an image of the drumskin sensor. The heterostructure consisted of a 3 nm GaN cap layer, a 20 nm thick
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Fig. 1. Microscopic view of drumskin sensor (8 mm × 8 mm).
undoped Al0.26 Ga0.74 N layer on a 3 m thick GaN layer grown by metal–organic chemical vapour epitaxy (MOCVD) on a 375 m thick sapphire (0 0 0 1) substrate. HEMTs were then fabricated using a conventional mesa isolation technology with Nb/Ti/Al ohmic contacts and Ni/Au Schottky gates [8]. The Schottky gate had a length of 10 m and was 600 m wide. The sources of all of the HEMT were connected to common contact pad and the gates likewise. The sources were all connected in parallel and the gates in series. This left six contact pads for each of the drains, meaning that every HEMT could act as an independent sensing element. 8 mm2 square chips were diced from the wafer. On each chip the six HEMTs were located −3.5, −1.8, −0.9, 0, 0.9 and 1.8 mm from the centre of the chip, where 0 indicates its geometric centre. The chips were then clamped in a stainless steel flange located over a 4 mm diameter hole which defined an active diaphragm located in the centre of the chip. The active radius of the drumskin was 2 mm, indicated by the dashed circular line in Fig. 1. Thus, the HEMT located −3.5 mm from the chip’s centre was placed over a region where the chip is clamped and acts as a reference since its I–V characteristics should show negligible change with respect to pressure. Fig. 2 shows a schematic of the measurement system. The chip was bonded over a hole in a flange using epoxy resin. The eight contact pads were then wire-bonded to gold TO headers, which were inserted in the flange around the chip and held in place using an epoxy. A copper gasket was added to the flange in order to seal the chamber and prevent gas leakage. Pressured pure nitrogen was used to apply a pressure up to 80 bar to the drumskin to displace the unclamped region of the chip (i.e. the dotted line region in Fig. 1). Possible heating resulting from adiabatically changes in the pressure is an important consideration and need to be carefully controlled. A K-type thermocouple and a separate pressure probe were used to monitor both temperature and pressure inside the
Fig. 3. IDS current characteristics for VGS applied for 0 to −3.5 V, for a fixed VDS = 1 V at atmospheric (full line) and 50 bar pressure (dashed line). The inset shows in detail the weak inversion regime with IDS plotted on a logarithmic scale.
chamber. Electrical measurements were carried out using a Keithley 236 source, allowing the drain–source current or voltage to be pulsed to reduce the degree of self-heating of the chip. All measurements were performed by applying VDS for 10 s and switching it off for 90 s. The response of each HEMT under the application of a pressure is then given by IDS =
IDSref − IDSp IDSref
(1)
where IDS is the sensitivity of the sensor response, IDSref is the reference drain current typically measured at atmospheric pressure and IDSp is the drain current at the applied pressure. In order to reduce noise and improve the accuracy of the measurements, an average of at least 100 readings is taken for each bias point. The uncertainty related to the response was calculated using the common total differential method for estimating errors or dispersion in the measured sensitivity. 3. Results and discussion The piezoresponse of the central HEMT was obtained at atmospheric and static pressure of 50 bar by measuring the IDS characteristic for applied gate bias (VGS ) from 0 to −3.5 V. Fig. 3 shows that an increase of pressure leads to a decrease in IDS current for all VGS in agreement with the literature [9]. As Fig. 3 shows, the HEMT characteristics can be divided into three regions: (1) a strong inversion regime where the gate voltage bias VGS does not significantly change IDS , (2) a moderate inversion regime where IDS varies almost linearly with VGS due to the reduced 2DEG channel length and (3) a weak inversion regime which corresponds to
Fig. 2. Schematic illustration of the pressure test set-up. Inset: the mounting of the drumskin sensor in the flange.
E.D. Le Boulbar et al. / Sensors and Actuators A 194 (2013) 247–251
Fig. 4. IDS current characteristics for VGS applied for 0 to −3.5 V, for a fixed VDS = 1 V with IDS measured at atmospheric pressure.
the 2DEG channel being pinched off. Each regime is governed by different transduction mechanisms [10]. Moreover, the substrate deformation caused by the application of 50 bar pressure leads to a reduction of the threshold voltage, VT , of 40 ± 5 mV. A similar change in VT was observed as a function of strain relaxation in an AlGaN/GaN HEMT by Yang et al. [11]. The response IDS of the drumskin sensor as a function of VGS was calculated based on Eq. (1) and is shown in Fig. 4. At higher gate bias, the response was seen to increase, as reported earlier [5,7]. A rapid increase of the sensitivity is observed as the HEMT moves from strong to weak inversion regime. Examination of the curve reveals that the three regimes highlighted in Fig. 3 can also be overlayed in Fig. 4. In region (1), the strong inversion regime, only a ∼1% change in the response was found as VGS was varied. In region (2), the moderate inversion regime, an increase in sensitivity of ∼6% could be achieved by applying a more negative VGS . Finally, in region (3), the weak inversion regime, an increase in IDS of ∼30% was obtained as VGS changed from −3.0 to −3.5 V. The behaviour of the sensitivity as a function of VGS closely follows an exponential curve with the sensitivity increasing by over 150 times from 0.005% bar−1 at VGS = 0 V to 0.76% bar−1 at VGS = −3.5 V. This behaviour is directly related to an exponential dependence of the 2DEG density on the net surface potential [10], including any effects of pressure. Since the results in Fig. 3 reveal that the pinch-off voltage is pressure dependant, this can be accounted for in determining the relationship beween sensitivity and the 2DEG density. This can be done by plotting log(IDS ) against IDS as shown in Fig. 5. Again IDSref is measured at atmospheric pressure. This plot has two distinguishable parts. The first correspond to the strong and moderate inversion regimes in Fig. 4. Under these bias conditions only a small increase in sensitivity response, from ∼0.2% to ∼9%, was observed. In the device tested this coincides with a descrease in IDS from 5 to 1 mA. For drain currents lower than 1 mA, where the HEMT operates in the weak inversion regime and a pure log-linear variation in the sensitivity was observed, improving from ∼9% up to ∼38% at VGS = −3.5 V. In the moderate-to-strong inversion regimes carrier screening causes the departure from the log-linear behaviour of the weak inversion region. The latter provides a readily calibrated operating regime in addition to optimum sensitivity. However, the improvement in sensitivity is offset by a greater susceptibility to dispersion or noise in the drain current measurement. This corresponds to an increase in the measurement uncertainty from ±0.05% at −3.0 V to ±0.29% at −3.5 V.
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Fig. 5. IDS current characteristics as a function of the logarithmic drain–source current obtained at atmospheric pressure for VGS applied for 0 to −3.5 V, for a fixed VDS = 1 V.
High sensitivities found in the weak inversion regime could be explained by the larger dependence of the current on the net AlGaN/GaN barrier interface height which is shifted by the straininduced piezoelectric charges [11], as suggested by Yilmazoglu [6]. In the weak inversion regime, the polarization-dependent sheet carrier density at atmospheric pressure and for an applied pressure can be written as [10]: nsref = 2No kT exp
V − V GS Tref
nsp = 2No kT exp
kT/q VGS − VTp kt/q
(2)
(3)
where No = 4m*/h2 is the conduction band density of states of a 2D system, k is the Boltzmann’s constant, T is absolute temperature, q is the value of an electron charge, VGS is the applied gate bias, VTref and VTp are the threshold voltage at the reference pressure atmospheric and applied pressure, respectively. In this regime, the sheet carrier density depends only on the threshold voltage, VT . In the case of a pressure sensing HEMT, and any shift in this value will lead to a decrease of IDS at constant VGS and VDS owing to the pressure dependence in VT of the type seen in Fig. 3. Based on Eqs. (2) and (3) the sheet carrier density will decrease exponentially with the pressure induced shift in VT . The influence of VDS on the sensitivity was investigated at VGS = −3.5 V, i.e. in the weak inversion regime. Fig. 6 shows that the sensitivity undergoes a small but linear increase from 35% to 39% as VDS increases from 0.1 to 0.5 V. On further increase in VDS to 7 V the sensitivity plateaus and then varies around the ∼40% level (inset of Fig. 6). For the device tested the optimal point for a reliable and sensitive measurement was found for VGS = −3.5 V and VDS = 0.7 V, where the response IDS is 39.0 ± 0.1%. The output characteristics of this HEMT were typical of others obtained from the same wafer, making the pressure response equally typical. However, for higher VDS the noise in IDS is larger: ±0.6% when VDS = 4.5 V which was about eight times larger than at VDS = 0.7 V. This noise seems to correlate with the point-to-point dispersion in IDS seen in the inset of Fig. 6 for VDS = 2–7 V. Possible explanations for the dispersion in the response at higher VDS , include hot carrier injection into traps in the gate barrier layer, and surface states, and self-heating. The effect of pressure change (1–80 bar) when the device was biased into the weak inversion, low IDS dispersion regime (VDS = 1 V, VGS = −3.5 V) is shown is Fig. 7. The data follow a {1 − exp(x)} response. This exponential behaviour departs from the linear dependence predicted by finite element models [12].
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Fig. 6. Variation in IDS as a function of the drain–source voltage obtained for a fixed VGS = −3.5 V.
Fig. 8. Radial sensitivity response at VDS = 1 V for a 50 bar applied as a function of same IDSref current.
This behaviour can be explained in the following way. At constant VGS and VDS , and constant temperature the change in drain current will depend only on the 2DEG density. Combining Eqs. (1), (2) and (3) yields a predicted response of
dependence) the pyroelectric reaction (temperature dependence) that occurs in GaN materials. On the other hand, all the HEMTs placed in the free-to-move drumskin region (circular region in Fig. 1) react to applied pressure but different amounts depending on location. As both sapphire and GaN moduli are mechanically isotropic in (0 0 0 1) plane [12,13], the radial dependence of IDS can be ascribed to the bending resulting from the applied pressure. The trend is for IDS to increase moving from edge to centre of the moveable diaphragm, with the most sensitive element being at the centre of the chip, in agreement with finite element modelling [14].
IDS = Io
1 − exp
qV T
kT
(4)
where Io is a pre-factor and VT = VTref − VTp . The validity or otherwise of Eq. (4) can be tested by noting that VT should be directly proportional to the change in displacement vector, i.e. polarization, at the gate barrier/channel interface and from Fig. 3 that VT = 40 ± 5 mV for a pressure increase of 50 bar from atmospheric pressure. The resulting fit is shown by the dashed line in Fig. 7. There is excellent agreement the theoretical behaviour predicted by Eq. (4) and the measured data. Finally, radial dependence of the piezo-response of the drumskin was measured using the HEMTs at different locations (Fig. 1). The measurements were performed for a static 50 bar applied pressure. As HEMT are not equivalent, IDS has been measured by setting same IDSref current at atmospheric pressure for every HEMT. Fig. 8 shows the results. A small (<0.02%) response is found on the HEMT located on the fixed periphery of the chip. As a result, this sensing element could be used as a reference for relative pressure measurements at high temperature to eliminate from the piezoresponse (pressure
4. Conclusions A detailed parametric investigation of a novel drumskin sensor based on GaN/AlGaN HEMTs has been presented. The sensitivity of the drain current of an AlGaN/GaN HEMT is enhanced by adopting a gate voltage that biases the device into the weak inversion regime. The drain source voltage was also shown to have an impact on the sensitivity and must be chosen cautiously to obtain reliable measurements. In this regime, the best response was found to be 39.0 ± 0.1% for the parameters VGS = −3.5 V; VDS = 0.7 V for the devices tested. The response was ∼170 times larger than the response found in the strong inversion regime. The sensitivity will also depend on the thickness and stiffness of the substrate (sapphire), but this was not varied in this work. An increase in response of nearly 20% each IDSatm decade was observed, indicating the potential to improve sensitivity if the signal-to-noise ratio can be made low enough to enable the measurement lower amplitude signals. The response to pressure variation was found to follow an {1 − exp(x)}, where x = qVT /kT. Such behaviour is characteristic of HEMT operation in the weak inversion regime.
Acknowledgements
Fig. 7. Response of the most sensitive sensing element (HEMT mounted in centre of drumskin) at VDS = 1 V, VGS = −3.5 V from atmospheric to 80 bar.
The authors wish to acknowledge support from the European Community’s Seventh Framework Programme FP7/2007-2011 under grant agreement n◦ 214610, project “MORGaN”. This publication reflects only the authors’ views and that the Community is not liable for any use that may be made of the information contained therein. The authors would like to acknowledge discussions with Ulrich Heinle and Peter Benkart of MicroGaN, Ulm, Germany and Sylvain Delage of III-V Labs, France.
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References [1] V. Tilak, J. Jiang, P. Batoni, A. Knobloch, GaN based high temperature strain gauges, Journal of Materials Science: Materials in Electronics 19 (2008) 195–198. [2] V. Cimalla, J. Pezoldt, O. Ambacher, Group III nitride and SiC based MEMS and NEMS: materials properties, technology and applications, Journal of Physics D: Applied Physics 40 (2007) 6386. [3] S.J. Pearton, B.S. Kang, S. Kim, F. Ren, B.P. Gila, C.R. Abernathy, J. Lin, S.N.G. Chu, GaN-based diodes and transistors for chemical, gas, biological and pressure sensing, Journal of Physics: Condensed Matter 16 (2004) R961. [4] M. Eickhoff, Piezoresistivity of Alx Ga1−x N layers and Alx Ga1−x N/GaN heterostructures, Journal of Applied Physics 90 (2001) 3383. [5] T. Zimmermann, M. Neuburger, P. Benkart, F.J. Hernandez-Guillen, C. Pietzka, M. Kunze, I. Daumiller, A. Dadgar, A. Krost, E. Kohn, Piezoelectric GaN sensor structures, Electron Device Letters IEEE 27 (2006) 309–312. [6] O. Yilmazoglu, K. Mutamba, D. Pavlidis, M.R. Mbarga, Strain sensitivity of AlGaN/GaN HEMT structures for sensing applications, IEICE Transactions (2006) 1037–1041. ´ V. Kutiˇs, S. Stanˇcík, I. Ryger, ´ A. [7] G. Vanko, M. Drˇzík, M. Vallo, T. Lalinsky, Benˇcurová, AlGaN/GaN C-HEMT structures for dynamic stress detection, Sensors and Actuators A: Physical 172 (2011) 98–102. ˇ ´ Zˇ . Mozolová, J. Liday, P. Vogrinˇciˇc, A. Vincze, F. Uherek, S. [8] G. Vanko, T. Lalinsky, Haˇscˇ ík, I. Kostiˇc, Nb–Ti/Al/Ni/Au based ohmic contacts to AlGaN/GaN, Vacuum 82 (2007) 193–196. [9] B. Kang, Pressure-induced changes in the conductivity of AlGaN/GaN highelectron mobility-transistor membranes, Applied Physics Letters 85 (2004) 2962. [10] Rashmi, A. Kranti, S. Haldar, R.S. Gupta, An accurate charge control model for spontaneous and piezoelectric polarization dependent two-dimensional electron gas sheet charge density of lattice-mismatched AlGaN/GaN HEMTs, Solid-State Electronics 46 (2002) 621–630. [11] Y. Yang, Y. Hao, J. Zhang, C. Wang, Q. Feng, Impact of strain relaxation of AlGaN barrier layer on the performance of high Al-content AlGaN/GaN HEMT, Science in China Series E: Technological Sciences 49 (2006) 393–399. [12] H.N.G. Wadley, Y. Lu, J.A. Goldman, Ultrasonic determination of single crystal sapphire fiber modulus, Journal of Nondestructive Evaluation 14 (1995) 31–38. [13] M.-L. Hicks, J. Tabeart, M.J. Edwards, E.D. Le Boulbar, D.W.E. Allsopp, C.R. Bowen, A.C.E. Dent, High temperature measurement of elastic moduli of (0 0 0 1), Gallium Nitride 133 (2012) 17–24. [14] M.J. Edwards, S. Vittoz, R. Amen, L. Rufer, P. Johander, C.R. Bowen, D.W.E. Allsopp, Modelling and optimisation of a sapphire/GaN-based diaphragm structure for pressure sensing in harsh environments, in: 8th International Conference on Advanced Semiconductor Devices & Microsystems (ASDAM), 2010, 2010, pp. 127–130.
Biographies E.D. Le Boulbar received his Master of Material science degree in 2007 from the University of Rennes I, France. He worked on the growth and characterization of oxide material grown by pulsed laser deposition and received his PhD in 2010, from the University of Orleans, France. He joined the III-Nitride Bath University research group as a research officer in 2010. His scope of interest currently lies on the growth of III-Nitrides nanostructured by MOVPE and devices characterization. M.J. Edwards was born in 1986 in North Walsham, Norfolk, UK. He attended the University of York from 2004 to 2008, where he graduated with a Master of Physics degree. In 2008, he started work on a PhD on GaN semiconductors sensors at the University of Bath. He successfully defended his thesis and graduated in 2012. S. Vittoz graduated from Institut National Polytechnique de Grenoble (Grenoble Institute of Technology) in France with a specialty in Semiconductor and Device
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Physics in 2008 and received, the same year, a Master degree in Micro and Nanoelectronics from Joseph Fourier University in Grenoble, France. From 2008 to 2011, he prepared his PhD preparation in TIMA Laboratory, focusing on the modelling and test of III-Nitrides based mechanical sensors for harsh environments. G. Vanko was born in Vel’ky´ Krtíˇs, Slovakia in 1981. He received the degree in engineering from Faculty of Electrical Engineering and Information Technology, Slovak Technical University, Bratislava in 2006. From 2006 to 2010 he was a PhD student in Department of Microelectronic Structures at Institute of Electrical Engineering of Slovak Academy of Sciences, Bratislava. Since that he is the member of the same department and his work is focused on the technology and characterization of the AlGaN/GaN based HEMT devices and their applications in the MEMS sensors for harsh environment. K. Brinkfeldt received his PhD degree from the Swedish Institute of Space Physics, Umeå University, Sweden in 2006. After his PhD work he spent time at the Department of Microtechnology and Nanoscience at Chalmers University of Technology, Göteborg, Sweden working on micromechanical actuator systems for particle measurement applications. Currently he holds a scientist position at Swerea IVF where his main research focus is packaging of electronics and sensors for harsh environments L. Rufer received both Engineer and PhD degrees from Czech Technical University, Prague, Czech Republic. Until 1993, he was with the Faculty of Electrical Engineering at the same university and since 1994, he has been Associate Professor and researcher both at Joseph Fourier University and INP Grenoble, France. In 1998, he joined TIMA Laboratory. His expertise is mainly in electro-acoustic and electromechanical transducers modelling, design, and fabrication applied to MEMS-based sensors and actuators, energy harvesting, and RF MEMS. P. Johander has published about 60 papers in international journals and at international conferences within the field of solid state physics, physical chemistry, solar energy, and electronics and micro system technology. At Swerea IVF he has been a project leader in national and international research projects since 1990, in addition to project leader in an UNDP/SIDA project, regarding ODS phase out in the electronics industry in Shanghai during the period 1997 to 2001. He has been manager and process leader for the Microwave Road association in Sweden, during 2002 and 2003, and since 2004 manager for the Ceramic cluster in the 4 M (MultiMaterial Micro Manufacturing) Network of Excellence in the 6th framework program. T. Lalinsky´ was born in Bratislava, Slovakia in 1951. He received the degree in engineering from Slovak Technical University, Bratislava in 1974, PhD degree on GaAs FETs technology from the Institute of Electrical Engineering, Slovak Academy of Sciences, Bratislava in 1981 and Doctor of Science degree on MEMS device technology from the same Institute in 2007. Since 1985, he is the head of Department of Microelectronic Structures at the above Institute. His research concerns the pseudomorphic HFET devices at millimetre wave frequency band. Since 1995, he is also the head of research team in the field of design and development of both III–V and III-N compound semiconductor based M(N)EMS devices. He has contributed more than 150 papers in referred journals and conference proceedings. C. Bowen is a Professor in Materials at the Materials Research Centre, Department of Mechanical Engineering, University of Bath. He has a DPhil from the University of Oxford on processing ceramic composites. Postdoctoral research was carried out at the Technical University of Hamburg-Harburg on reaction processing of ceramics, followed by a Research Fellowship at the University of Leeds on ionic conductivity of zirconia composites for sensors. He joined the University of Bath as a Lecturer in 1998 and is now taking up and ERC Advanced Fellowship in Novel Energy Materials, Engineering Science and Integrated Systems (NEMESIS). D.W.E. Allsopp received the B.Sc. degree in physics in 1971 and a Ph.D. degree in 1977, both from the University of Sheffield, Sheffield, U.K. Since 1999, he has been in the Optoelectronics Group, University of Bath, U.K. He is a Senior Research Fellow of the Royal Academy of Engineering/Leverhulme Trust and currently leads the IIInitride research group at Bath.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Effect of bias conditions on pressure sensors based on AlGaN/GaN High Electron Mobility Transistor E.D. Le Boulbar a,∗ , M.J. Edwards a,b , S. Vittoz c , G. Vanko d , K. Brinkfeldt e , L. Rufer c , P. Johander e , T. Lalinsky´ d , C.R. Bowen a , D.W.E. Allsopp a a
Department of Electronics and Electrical Engineering, University of Bath, UK Department of Mechanical Engineering, University of Bath, UK c TIMA Laboratory (UJF, CNRS, G-INP) Grenoble, France d Institute of Electrical Engineering, Slovak Academy of Sciences, Bratislava, Slovak Republic e Swerea IVF, Mölndal, Sweden b
a r t i c l e
i n f o
Article history: Received 5 September 2012 Received in revised form 11 February 2013 Accepted 12 February 2013 Available online 19 February 2013 Keywords: Stress sensor AlGaN/GaN HEMT Piezoelectric
a b s t r a c t This work reports the bias and pressure sensitivity of AlGaN/GaN High Electron Mobility Transistors (HEMTs) sensing elements strategically placed on a pressure sensitive diaphragm clamped at its edges. The sensitivity was over 150 times greater in the weak inversion regime than in the strong inversion regime of the HEMT, leading to a drain current change of >38% when a pressure of 50 bar was applied. The sensitivity of the HEMT to pressure followed an exponential dependence from atmospheric pressure up to 80 bar, behaviour explained by the response of the density of a two-dimensional electron gas to pressure induced changes in the HEMT threshold voltage in the weak inversion regime. Finally, it was found that the sensitivity of the HEMT was maximum when it was situated in the middle of the diaphragm, whereas a device mounted over the clamping point showed less than 0.02% change in drain current when pressure change of 50 bar was applied. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The potential for GaN based pressure sensors to operate at high-temperatures and in harsh environments has been described recently in a number of publications [1–3]. Researchers have employed a variety of devices and sensitive mechanisms to convert pressure into a readable signal in GaN based devices. Simple piezoresistors were initially used in early studies to demonstrate the proof of sensor concept [4]. The use of a High Electron Mobility Transistor (HEMT) as the sensing element resulted in a major increase in sensitivity [5,6]. The transduction mechanism in a GaN HEMT results from the variation in the conductivity of the two-dimensional electron gas (2DEG) in response to an externally applied force. The variations in the 2DEG conductivity derive from changes in the piezoelectric polarization at the interface between the gate barrier and the channel and from a possible piezoresistive contribution. Although a number of papers have revealed that HEMT electrical parameters such as gate bias (VGS ) and drain source voltage (VDS ) play a key role in optimization of pressure sensor sensitivity [5–7],
a systematic investigation of their importance of each parameter on the pressure sensitivity of GaN HEMT transducers. In this work we examine the sensitivity of AlGaN/GaN HEMT transducers for pressure sensing. The sensitivity was investigated under static pressure for three different HEMT working regimes: (1) strong, (2) moderate and (3) weak inversion regimes by applying gate bias voltages from 0 to −3.5 V in −0.1 V steps for a drain source voltage fixed at 1 V. The same experiment was repeated for different VDS to find the most sensitive regime. As a result, an optimum sensitivity was found in terms of VGS and VDS . The radial response of the chip was investigated by measuring sensitivity on six sensing elements placed in various distances from the diaphragm centre. Finally, the pressure dependence of the sensor in the weak inversion regime was investigated by introducing nitrogen gas at pressures up to 80 bar uniformly on one side of the device whilst the other was kept at atmospheric pressure. An exponential dependence of sensitivity with pressure is found to fit closely with a simple model of HEMT sheet carrier density in the weak inversion regime.
2. Experimental ∗ Corresponding author. Tel.: +44 01225 385315. E-mail address: [email protected] (E.D. Le Boulbar). 0924-4247/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2013.02.017
Fig. 1 shows an image of the drumskin sensor. The heterostructure consisted of a 3 nm GaN cap layer, a 20 nm thick
248
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Fig. 1. Microscopic view of drumskin sensor (8 mm × 8 mm).
undoped Al0.26 Ga0.74 N layer on a 3 m thick GaN layer grown by metal–organic chemical vapour epitaxy (MOCVD) on a 375 m thick sapphire (0 0 0 1) substrate. HEMTs were then fabricated using a conventional mesa isolation technology with Nb/Ti/Al ohmic contacts and Ni/Au Schottky gates [8]. The Schottky gate had a length of 10 m and was 600 m wide. The sources of all of the HEMT were connected to common contact pad and the gates likewise. The sources were all connected in parallel and the gates in series. This left six contact pads for each of the drains, meaning that every HEMT could act as an independent sensing element. 8 mm2 square chips were diced from the wafer. On each chip the six HEMTs were located −3.5, −1.8, −0.9, 0, 0.9 and 1.8 mm from the centre of the chip, where 0 indicates its geometric centre. The chips were then clamped in a stainless steel flange located over a 4 mm diameter hole which defined an active diaphragm located in the centre of the chip. The active radius of the drumskin was 2 mm, indicated by the dashed circular line in Fig. 1. Thus, the HEMT located −3.5 mm from the chip’s centre was placed over a region where the chip is clamped and acts as a reference since its I–V characteristics should show negligible change with respect to pressure. Fig. 2 shows a schematic of the measurement system. The chip was bonded over a hole in a flange using epoxy resin. The eight contact pads were then wire-bonded to gold TO headers, which were inserted in the flange around the chip and held in place using an epoxy. A copper gasket was added to the flange in order to seal the chamber and prevent gas leakage. Pressured pure nitrogen was used to apply a pressure up to 80 bar to the drumskin to displace the unclamped region of the chip (i.e. the dotted line region in Fig. 1). Possible heating resulting from adiabatically changes in the pressure is an important consideration and need to be carefully controlled. A K-type thermocouple and a separate pressure probe were used to monitor both temperature and pressure inside the
Fig. 3. IDS current characteristics for VGS applied for 0 to −3.5 V, for a fixed VDS = 1 V at atmospheric (full line) and 50 bar pressure (dashed line). The inset shows in detail the weak inversion regime with IDS plotted on a logarithmic scale.
chamber. Electrical measurements were carried out using a Keithley 236 source, allowing the drain–source current or voltage to be pulsed to reduce the degree of self-heating of the chip. All measurements were performed by applying VDS for 10 s and switching it off for 90 s. The response of each HEMT under the application of a pressure is then given by IDS =
IDSref − IDSp IDSref
(1)
where IDS is the sensitivity of the sensor response, IDSref is the reference drain current typically measured at atmospheric pressure and IDSp is the drain current at the applied pressure. In order to reduce noise and improve the accuracy of the measurements, an average of at least 100 readings is taken for each bias point. The uncertainty related to the response was calculated using the common total differential method for estimating errors or dispersion in the measured sensitivity. 3. Results and discussion The piezoresponse of the central HEMT was obtained at atmospheric and static pressure of 50 bar by measuring the IDS characteristic for applied gate bias (VGS ) from 0 to −3.5 V. Fig. 3 shows that an increase of pressure leads to a decrease in IDS current for all VGS in agreement with the literature [9]. As Fig. 3 shows, the HEMT characteristics can be divided into three regions: (1) a strong inversion regime where the gate voltage bias VGS does not significantly change IDS , (2) a moderate inversion regime where IDS varies almost linearly with VGS due to the reduced 2DEG channel length and (3) a weak inversion regime which corresponds to
Fig. 2. Schematic illustration of the pressure test set-up. Inset: the mounting of the drumskin sensor in the flange.
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Fig. 4. IDS current characteristics for VGS applied for 0 to −3.5 V, for a fixed VDS = 1 V with IDS measured at atmospheric pressure.
the 2DEG channel being pinched off. Each regime is governed by different transduction mechanisms [10]. Moreover, the substrate deformation caused by the application of 50 bar pressure leads to a reduction of the threshold voltage, VT , of 40 ± 5 mV. A similar change in VT was observed as a function of strain relaxation in an AlGaN/GaN HEMT by Yang et al. [11]. The response IDS of the drumskin sensor as a function of VGS was calculated based on Eq. (1) and is shown in Fig. 4. At higher gate bias, the response was seen to increase, as reported earlier [5,7]. A rapid increase of the sensitivity is observed as the HEMT moves from strong to weak inversion regime. Examination of the curve reveals that the three regimes highlighted in Fig. 3 can also be overlayed in Fig. 4. In region (1), the strong inversion regime, only a ∼1% change in the response was found as VGS was varied. In region (2), the moderate inversion regime, an increase in sensitivity of ∼6% could be achieved by applying a more negative VGS . Finally, in region (3), the weak inversion regime, an increase in IDS of ∼30% was obtained as VGS changed from −3.0 to −3.5 V. The behaviour of the sensitivity as a function of VGS closely follows an exponential curve with the sensitivity increasing by over 150 times from 0.005% bar−1 at VGS = 0 V to 0.76% bar−1 at VGS = −3.5 V. This behaviour is directly related to an exponential dependence of the 2DEG density on the net surface potential [10], including any effects of pressure. Since the results in Fig. 3 reveal that the pinch-off voltage is pressure dependant, this can be accounted for in determining the relationship beween sensitivity and the 2DEG density. This can be done by plotting log(IDS ) against IDS as shown in Fig. 5. Again IDSref is measured at atmospheric pressure. This plot has two distinguishable parts. The first correspond to the strong and moderate inversion regimes in Fig. 4. Under these bias conditions only a small increase in sensitivity response, from ∼0.2% to ∼9%, was observed. In the device tested this coincides with a descrease in IDS from 5 to 1 mA. For drain currents lower than 1 mA, where the HEMT operates in the weak inversion regime and a pure log-linear variation in the sensitivity was observed, improving from ∼9% up to ∼38% at VGS = −3.5 V. In the moderate-to-strong inversion regimes carrier screening causes the departure from the log-linear behaviour of the weak inversion region. The latter provides a readily calibrated operating regime in addition to optimum sensitivity. However, the improvement in sensitivity is offset by a greater susceptibility to dispersion or noise in the drain current measurement. This corresponds to an increase in the measurement uncertainty from ±0.05% at −3.0 V to ±0.29% at −3.5 V.
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Fig. 5. IDS current characteristics as a function of the logarithmic drain–source current obtained at atmospheric pressure for VGS applied for 0 to −3.5 V, for a fixed VDS = 1 V.
High sensitivities found in the weak inversion regime could be explained by the larger dependence of the current on the net AlGaN/GaN barrier interface height which is shifted by the straininduced piezoelectric charges [11], as suggested by Yilmazoglu [6]. In the weak inversion regime, the polarization-dependent sheet carrier density at atmospheric pressure and for an applied pressure can be written as [10]: nsref = 2No kT exp
V − V GS Tref
nsp = 2No kT exp
kT/q VGS − VTp kt/q
(2)
(3)
where No = 4m*/h2 is the conduction band density of states of a 2D system, k is the Boltzmann’s constant, T is absolute temperature, q is the value of an electron charge, VGS is the applied gate bias, VTref and VTp are the threshold voltage at the reference pressure atmospheric and applied pressure, respectively. In this regime, the sheet carrier density depends only on the threshold voltage, VT . In the case of a pressure sensing HEMT, and any shift in this value will lead to a decrease of IDS at constant VGS and VDS owing to the pressure dependence in VT of the type seen in Fig. 3. Based on Eqs. (2) and (3) the sheet carrier density will decrease exponentially with the pressure induced shift in VT . The influence of VDS on the sensitivity was investigated at VGS = −3.5 V, i.e. in the weak inversion regime. Fig. 6 shows that the sensitivity undergoes a small but linear increase from 35% to 39% as VDS increases from 0.1 to 0.5 V. On further increase in VDS to 7 V the sensitivity plateaus and then varies around the ∼40% level (inset of Fig. 6). For the device tested the optimal point for a reliable and sensitive measurement was found for VGS = −3.5 V and VDS = 0.7 V, where the response IDS is 39.0 ± 0.1%. The output characteristics of this HEMT were typical of others obtained from the same wafer, making the pressure response equally typical. However, for higher VDS the noise in IDS is larger: ±0.6% when VDS = 4.5 V which was about eight times larger than at VDS = 0.7 V. This noise seems to correlate with the point-to-point dispersion in IDS seen in the inset of Fig. 6 for VDS = 2–7 V. Possible explanations for the dispersion in the response at higher VDS , include hot carrier injection into traps in the gate barrier layer, and surface states, and self-heating. The effect of pressure change (1–80 bar) when the device was biased into the weak inversion, low IDS dispersion regime (VDS = 1 V, VGS = −3.5 V) is shown is Fig. 7. The data follow a {1 − exp(x)} response. This exponential behaviour departs from the linear dependence predicted by finite element models [12].
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Fig. 6. Variation in IDS as a function of the drain–source voltage obtained for a fixed VGS = −3.5 V.
Fig. 8. Radial sensitivity response at VDS = 1 V for a 50 bar applied as a function of same IDSref current.
This behaviour can be explained in the following way. At constant VGS and VDS , and constant temperature the change in drain current will depend only on the 2DEG density. Combining Eqs. (1), (2) and (3) yields a predicted response of
dependence) the pyroelectric reaction (temperature dependence) that occurs in GaN materials. On the other hand, all the HEMTs placed in the free-to-move drumskin region (circular region in Fig. 1) react to applied pressure but different amounts depending on location. As both sapphire and GaN moduli are mechanically isotropic in (0 0 0 1) plane [12,13], the radial dependence of IDS can be ascribed to the bending resulting from the applied pressure. The trend is for IDS to increase moving from edge to centre of the moveable diaphragm, with the most sensitive element being at the centre of the chip, in agreement with finite element modelling [14].
IDS = Io
1 − exp
qV T
kT
(4)
where Io is a pre-factor and VT = VTref − VTp . The validity or otherwise of Eq. (4) can be tested by noting that VT should be directly proportional to the change in displacement vector, i.e. polarization, at the gate barrier/channel interface and from Fig. 3 that VT = 40 ± 5 mV for a pressure increase of 50 bar from atmospheric pressure. The resulting fit is shown by the dashed line in Fig. 7. There is excellent agreement the theoretical behaviour predicted by Eq. (4) and the measured data. Finally, radial dependence of the piezo-response of the drumskin was measured using the HEMTs at different locations (Fig. 1). The measurements were performed for a static 50 bar applied pressure. As HEMT are not equivalent, IDS has been measured by setting same IDSref current at atmospheric pressure for every HEMT. Fig. 8 shows the results. A small (<0.02%) response is found on the HEMT located on the fixed periphery of the chip. As a result, this sensing element could be used as a reference for relative pressure measurements at high temperature to eliminate from the piezoresponse (pressure
4. Conclusions A detailed parametric investigation of a novel drumskin sensor based on GaN/AlGaN HEMTs has been presented. The sensitivity of the drain current of an AlGaN/GaN HEMT is enhanced by adopting a gate voltage that biases the device into the weak inversion regime. The drain source voltage was also shown to have an impact on the sensitivity and must be chosen cautiously to obtain reliable measurements. In this regime, the best response was found to be 39.0 ± 0.1% for the parameters VGS = −3.5 V; VDS = 0.7 V for the devices tested. The response was ∼170 times larger than the response found in the strong inversion regime. The sensitivity will also depend on the thickness and stiffness of the substrate (sapphire), but this was not varied in this work. An increase in response of nearly 20% each IDSatm decade was observed, indicating the potential to improve sensitivity if the signal-to-noise ratio can be made low enough to enable the measurement lower amplitude signals. The response to pressure variation was found to follow an {1 − exp(x)}, where x = qVT /kT. Such behaviour is characteristic of HEMT operation in the weak inversion regime.
Acknowledgements
Fig. 7. Response of the most sensitive sensing element (HEMT mounted in centre of drumskin) at VDS = 1 V, VGS = −3.5 V from atmospheric to 80 bar.
The authors wish to acknowledge support from the European Community’s Seventh Framework Programme FP7/2007-2011 under grant agreement n◦ 214610, project “MORGaN”. This publication reflects only the authors’ views and that the Community is not liable for any use that may be made of the information contained therein. The authors would like to acknowledge discussions with Ulrich Heinle and Peter Benkart of MicroGaN, Ulm, Germany and Sylvain Delage of III-V Labs, France.
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Biographies E.D. Le Boulbar received his Master of Material science degree in 2007 from the University of Rennes I, France. He worked on the growth and characterization of oxide material grown by pulsed laser deposition and received his PhD in 2010, from the University of Orleans, France. He joined the III-Nitride Bath University research group as a research officer in 2010. His scope of interest currently lies on the growth of III-Nitrides nanostructured by MOVPE and devices characterization. M.J. Edwards was born in 1986 in North Walsham, Norfolk, UK. He attended the University of York from 2004 to 2008, where he graduated with a Master of Physics degree. In 2008, he started work on a PhD on GaN semiconductors sensors at the University of Bath. He successfully defended his thesis and graduated in 2012. S. Vittoz graduated from Institut National Polytechnique de Grenoble (Grenoble Institute of Technology) in France with a specialty in Semiconductor and Device
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Physics in 2008 and received, the same year, a Master degree in Micro and Nanoelectronics from Joseph Fourier University in Grenoble, France. From 2008 to 2011, he prepared his PhD preparation in TIMA Laboratory, focusing on the modelling and test of III-Nitrides based mechanical sensors for harsh environments. G. Vanko was born in Vel’ky´ Krtíˇs, Slovakia in 1981. He received the degree in engineering from Faculty of Electrical Engineering and Information Technology, Slovak Technical University, Bratislava in 2006. From 2006 to 2010 he was a PhD student in Department of Microelectronic Structures at Institute of Electrical Engineering of Slovak Academy of Sciences, Bratislava. Since that he is the member of the same department and his work is focused on the technology and characterization of the AlGaN/GaN based HEMT devices and their applications in the MEMS sensors for harsh environment. K. Brinkfeldt received his PhD degree from the Swedish Institute of Space Physics, Umeå University, Sweden in 2006. After his PhD work he spent time at the Department of Microtechnology and Nanoscience at Chalmers University of Technology, Göteborg, Sweden working on micromechanical actuator systems for particle measurement applications. Currently he holds a scientist position at Swerea IVF where his main research focus is packaging of electronics and sensors for harsh environments L. Rufer received both Engineer and PhD degrees from Czech Technical University, Prague, Czech Republic. Until 1993, he was with the Faculty of Electrical Engineering at the same university and since 1994, he has been Associate Professor and researcher both at Joseph Fourier University and INP Grenoble, France. In 1998, he joined TIMA Laboratory. His expertise is mainly in electro-acoustic and electromechanical transducers modelling, design, and fabrication applied to MEMS-based sensors and actuators, energy harvesting, and RF MEMS. P. Johander has published about 60 papers in international journals and at international conferences within the field of solid state physics, physical chemistry, solar energy, and electronics and micro system technology. At Swerea IVF he has been a project leader in national and international research projects since 1990, in addition to project leader in an UNDP/SIDA project, regarding ODS phase out in the electronics industry in Shanghai during the period 1997 to 2001. He has been manager and process leader for the Microwave Road association in Sweden, during 2002 and 2003, and since 2004 manager for the Ceramic cluster in the 4 M (MultiMaterial Micro Manufacturing) Network of Excellence in the 6th framework program. T. Lalinsky´ was born in Bratislava, Slovakia in 1951. He received the degree in engineering from Slovak Technical University, Bratislava in 1974, PhD degree on GaAs FETs technology from the Institute of Electrical Engineering, Slovak Academy of Sciences, Bratislava in 1981 and Doctor of Science degree on MEMS device technology from the same Institute in 2007. Since 1985, he is the head of Department of Microelectronic Structures at the above Institute. His research concerns the pseudomorphic HFET devices at millimetre wave frequency band. Since 1995, he is also the head of research team in the field of design and development of both III–V and III-N compound semiconductor based M(N)EMS devices. He has contributed more than 150 papers in referred journals and conference proceedings. C. Bowen is a Professor in Materials at the Materials Research Centre, Department of Mechanical Engineering, University of Bath. He has a DPhil from the University of Oxford on processing ceramic composites. Postdoctoral research was carried out at the Technical University of Hamburg-Harburg on reaction processing of ceramics, followed by a Research Fellowship at the University of Leeds on ionic conductivity of zirconia composites for sensors. He joined the University of Bath as a Lecturer in 1998 and is now taking up and ERC Advanced Fellowship in Novel Energy Materials, Engineering Science and Integrated Systems (NEMESIS). D.W.E. Allsopp received the B.Sc. degree in physics in 1971 and a Ph.D. degree in 1977, both from the University of Sheffield, Sheffield, U.K. Since 1999, he has been in the Optoelectronics Group, University of Bath, U.K. He is a Senior Research Fellow of the Royal Academy of Engineering/Leverhulme Trust and currently leads the IIInitride research group at Bath.