Temperature dependences of an In0.46Ga0.54As/In0.42Al0.58As based metamorphic high
electron mobility transistor (MHEMT)
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Temperature dependences of an
In
0.46
Ga
0.54
As/In
0.42
Al
0.58
As based
metamorphic high electron mobility
transistor (MHEMT)
Chun-Wei Chen
1, Po-Hsien Lai
1, Wen-Shiung Lour
2,
Der-Feng Guo
3, Jung-Hui Tsai
4and Wen-Chau Liu
11Institute of Microelectronics, Department of Electrical Engineering, National Cheng-Kung University, 1 University Road, Tainan, Taiwan 70101, People’s Republic of China
2Department of Electrical Engineering, National Taiwan-Ocean University, 2 Peining Road, Keelung, Taiwan 20224, People’s Republic of China
3Department of Electronic Engineering, Chinese Air Force Academy, PO Box 90277-4, Kangsun, Kaoshiung County, Taiwan, People’s Republic of China
4Department of Electronic Engineering, National Kaohsiung Normal University, 62 Shenjhong Road, Yanchao Township, Kaohsiung County, Taiwan 82444, People’s Republic of China
E-mail:wcliu@mail.ncku.edu.tw
Received 4 May 2006, in final form 14 June 2006 Published 10 August 2006
Online atstacks.iop.org/SST/21/1358
Abstract
In this paper, an interesting thermally stable In0.42Al0.58As/In0.46Ga0.54As
metamorphic high electron mobility transistor (MHEMT) is fabricated and investigated. Good dc and RF characteristics are obtained by precisely depositing gold (Au) upon the In0.42Al0.58As barrier layer as the Schottky
contact metal. For a MHEMT with gate dimensions of 1× 100 µm2, high gate–drain breakdown voltage, high turn-on voltage, low gate leakage current density, high maximum transconductance with broad operating regime and low output conductance are obtained even at ambient
temperatures up to 510 K (240◦C). The studied device also shows a very good microwave performance at room temperature. Moreover, the relatively low variations of the device performance are achieved over a wide
temperature range (from 300 to 510 K). Therefore, the studied device has a good thermally stable performance that is suitable for high-speed and high-power electronic applications.
1. Introduction
Due to the high low-field electron mobility, high peak electron velocity, large conduction-band discontinuity (EC)
between Schottky and channel layers, and high sheet-carrier density, InAlAs/InGaAs high electron mobility transistors (HEMTs) lattice matched to InP substrates have shown better microwave and low-noise characteristics than AlGaAs/InGaAs pseudomorphic high electron mobility transistors (PHEMTs) on GaAs substrates [1–4]. Since InP substrates generally suffer from mechanical fragility, wafer size and high cost, the GaAs substrate is more suitable
for manufacturing large-scale millimetre-wave integrated circuit (MMIC) [5]. Growing the InAlAs/InGaAs structure metamorphically on GaAs substrates can eliminate this substrate issue. In this kind of structure, the metamorphic buffer used to accommodate the large lattice mismatch between the active layer and the GaAs substrate plays an important role on the device performance [6,7]. In addition, the active layer also presents several advantages including that (1) the wide-gap InAlAs has a good Schottky barrier quality; (2) the large conduction-band discontinuity leads to high sheet-carrier density and good electron confinement; (3) the unstrained material is good for high electron velocity
Temperature dependences of an In0.46Ga0.54As/In0.42Al0.58As based MHEMT
S.I. GaAs Sub. Drain Metamorphic Buffer 3000 Å-In0.42Ga0.58As 200 Å-In0.46Ga0.54As 50 Å-In0.42Al0.58As 300 Å-In0.42Al0.58As 50 Å-In0.43Ga0.57As undoped undoped n+ =5¥1018cm-3 Gate Source 50 Å-In0.42Al0.58As 1.5 µm-undoped undoped undoped δ1(n+)=4¥1012 cm-2 δ2(n+)=2¥1012 cm-2
Figure 1. Schematic cross section of the studied InGaAs/InAlAs/GaAs MHEMT.
and (4) the wide-gap InAlAs also allows high drain voltage operation and reduces impact ionization [7].
In addition, the metamorphic growth gives a free choice of lattice constant and indium composition in the InGaAs channel. Usually, indium contents of 30–50% are preferred for high-power and low-noise applications. The use of the wide-gap InAlAs layer is expected to simultaneously obtain good Schottky characteristics and to suppress the substrate leakage current. However, the narrow-gap InGaAs channel is observed to easily initiate the impact ionizations that will further cause the kink effects [8–10]. Holes generated from the impact ionization in the channel will be injected across the Schottky layer and collected at the gate terminal. Note that the gate alloy recipes will dominate the characteristics including the Schottky barrier height and the interfacial valence-band discontinuity between Schottky barrier and channel layers. Thus, the kink effect heavily depends on the specific gate metal chosen. Although MHEMTs have widely been studied and reported in microwave applications, there are a few literatures regarding the related temperature-dependent characteristics. In this work, a double δ-doped InGaAs/InAlAs/GaAs (MHEMT) with a 46% indium mole fraction of an InGaAs channel and an Au gate is fabricated. The temperature-dependent dc and microwave characteristics are studied.
2. Material growth and device fabrication
The studied InAlAs/InGaAs MHEMT is grown on a (1 0 0)-oriented semi-insulating (SI) GaAs substrate by a molecular-beam epitaxy (MBE) system. The schematic cross section of the studied device is depicted in figure1. The device structure consists of a 1.5 µm thick InAlAs metamorphic buffer layer, a 3000 ˚A In0.42Al0.58As barrier buffer layer, a delta-doped
sheet of δ2(n+) = 2 × 1012 cm−2, a 50 ˚A In0.42Al0.58As
space layer, a 200 ˚A In0.46Ga0.54As channel layer, a 50 ˚A
In0.42Al0.58As space layer, a delta-doped sheet of δ1(n+) =
4× 1012cm−2, a 300 ˚A In
0.42Al0.58As Schottky barrier layer,
and a 50 ˚A n+-In
0.43Ga0.57As (n+= 5 × 1018cm−3) cap layer.
Hall measurements are employed to measure the sheet-carrier density and electron mobility under a magnetic field of 5000 G.
The sheet-carrier density (ns) and electron mobility (µn) are
4.1× 1012(3.9× 1012) cm−2and 8000 (30 300) cm2V−1s−1 at 300 (77) K, respectively. The metamorphic buffer layer is used to accommodate the lattice mismatch between the active layer and the GaAs substrate. The wide-gap InAlAs layers for the Schottky barrier and buffer layer are used to obtain a good Schottky characteristic and to suppress the substrate leakage current. In addition, the double δ-doped sheets are designed to form the uniform carrier distribution in the InGaAs channel layer.
After the epitaxial growth, the device is processed by standard photolithography, conventional vacuum evaporation and a lift-off technique. First, the mesa isolation is performed by wet chemical etching. Drain/source ohmic contacts are then formed on the n+-In
0.43Ga0.57As cap layer
by alloying evaporated AuGe/Ni/Au metals at 330 ◦C for 30 s. Gate recess etching of the n+-In0.43Ga0.57As cap layer
is done by a highly selective PH-adjusted solution (succinic acid:H2O2:NH4OH). Gate Schottky contact is achieved by
evaporating Au metals on the 300 ˚A In0.42Al0.58As Schottky
barrier layer. Finally, the active layers underneath the gate feeder are completely removed by wet chemical etching so as to build the air-bridge gate structure that includes multiple piers under the gate-feeder metal. The gate dimensions are 1 × 100 µm2 with the drain-to-source spacing of 5 µm. The experimental dc current–voltage (I–V) characteristics are measured by an HP4156A semiconductor parameter analyser at different temperatures. The microwave performance of the studied device is measured by an HP8510C vector network analyser in conjunction with Cascade probes over the frequency ranging from 0.5 GHz to 20 GHz at room temperature.
3. Experimental results and discussion
Two-terminal gate–drain current–voltage (I–V) characteristics of the studied device at different temperatures are shown in figure 2(a). The corresponding temperature-dependent characteristics of the turn-on voltage Von, breakdown voltage
BVGD and gate current IG at VGD = −10 V are shown in
figure2(b). The source terminal is floating. The Vonvalues
-12 -8 -4 0 -0.5 0.0 0.5 1x100 µm2 330K 360K 390K 420K 450K 480K 510K Gate-Drain Voltage VGD (V) Gate Current I G (mA/mm) Source : floating 10-1 100 101 102 103 300 330 360 390 420 450 480 510 0.6 0.9 1.2 9 12 15 18 21 slope = -2.62 mV/K slope = 2.101 mA/K slope = -44.51 mV/K I G BV GD Von Temperature (K) IG (µ A/mm) at VGD =-10 V Turn-on Voltage V on (V) and Breakdown Voltage BV GD (V) (b)
Figure 2. (a) Gate–drain I–V characteristics of the fabricated device
measured at different temperatures. (b) The turn-on voltage Von, breakdown voltage BVGDand gate current IGat VGD= −10 V as a function of temperature.
defined as the gate voltage at which IG= 1 mA mm−1are
1.03 and 0.49 V at 300 and 510 K, respectively. The BVGD
values defined as the VGD voltage at IG = −0.5 mA mm−1
are 19.3 and 10.3 V at 300 and 510 K, respectively. The corresponding IG at VGD = −10 V are 0.43 and 434 µA
mm−1, respectively. The variations of Von (Von), BVGD
(BVGD) and IG (IG) from 300 to 510 K are 0.54 V,
6.03 V and 434 µA. The calculated degradation rate in
Von (∂Von/∂T), BVGD (∂BVGD/∂T) and IG(∂IG/∂T) are
−2.62 mV K−1, −27 mV K−1 and 2.1 µA mm−1 K−1,
respectively. Obviously, when the temperature is increased, the reduced Vonand increased IGare attributed to the reduction
of energy gap and the tunnelling mechanism [11]. However, in contrast to the conventional InP- and GaAs-based HEMTs [12–14], the studied device shows relatively temperature-independent characteristics in terms of IG. This good Schottky
and breakdown characteristics may be attributed to (1) the use of an undoped wide-gap In0.42Al0.58As Schottky barrier layer,
and (2) the enhanced carrier confinement resulting from the narrow-gap In0.43Ga0.57As channel sandwiched between two
In0.42Al0.58As layers. This significantly reduces the impact
ionization effect and gate leakage. Therefore, the degradation rate values of the studied MHEMT device show relatively temperature-independent characteristics in the temperature range of 300–510 K. 0.0 0.5 1.0 1.5 2.0 2.5 0 100 200 300 400 500 -2.0 V -1.5 V -1.0 V -0.5 V 0 V VGS= + 0.5 V VGS=-0.5 V/step 360K 390K 420K 450K 480K 510K Drain-Source Voltage VDS (V) Drain Current I D (mA/mm)
Figure 3. Common-source I–V characteristics of the device at
different temperatures. 300 330 360 390 420 450 480 510 -2.07 -2.03 -1.99 -1.95 -1.91 ∆V th Vth -150 -100 -50 0 mV/K -0.393 = ∂ ∂ T Vth 1x100µm2 VDS=2.0 V Temperature (K) D Vth (mV) Threshold Voltage V th (V)
Figure 4. Threshold voltage (Vth) and variations of Vth(Vth) as a function of temperature. The biased voltage is fixed at VDS= 2.0 V.
Figure 3shows the common-source I–V characteristics of the studied device at different temperatures. The applied gate–source voltage VGS is −0.5 V/step and the
maximum VGS is +0.5 V. After gaining enough energy
at high temperature, the background carrier concentration increases and the electrons within the channel tend to surpass over conduction-band discontinuities. This will cause the excessive leakage current. However, the studied device shows good pinch-off and insignificant kink effect. These may be because carriers are well confined in the channel by the large conduction-band discontinuity at the In0.42Al0.58As/In0.46Ga0.54As heterointerface. The measured
threshold voltage (Vth) and variations of Vth (Vth) as a
function of temperature of the studied device are illustrated in figure 4. The Vthvalues, at VDS= 2.0 V, are −1.96 and
−2.04 V at 300 and 510 K, respectively. According to the relationship Vth = Vth0 − K(T − T0) [15], the K value is
only of−0.393 mV K−1for the device within the temperature regime of 300 to 510 K. Clearly, the relatively temperature-insensitive performance of Vthis observed. This is attributed
to the good carrier confinement presented in the ‘narrow-gap’ In0.46Ga0.54As channel layer, which prevents the carriers from
Temperature dependences of an In0.46Ga0.54As/In0.42Al0.58As based MHEMT -2 -1 0 0 100 200 300 400 500 300K 420K 330K 450K 360K 480K 390K 510K 1x100 µm2 VDS=2.0 V 0 100 200 300 Gate-Source Voltage VGS (V) Transconductance g m (mS/mm)
Drain Saturation Current I
DS
(mA/mm)
Figure 5. Drain saturation current IDS,maxand transconductance gm versus gate–source voltage VGSfor the studied device at different temperatures. 300 330 360 390 420 450 480 510 180 240 300 360 420 480 I DS-OP IDS gm,max 1x100µm2 VDS=2.0 V 200 220 240 260 280 300
Maximum Drain Saturation Current I
DS,max (mA/mm) and I DS Operating Regime (>0.9 g m,max ) (mA/mm) Temperature (K) Maximum Transconductance g m,max (mS/mm)
Figure 6. Maximum transconductance gm,max, maximum drain saturation current IDS,maxand drain current operation regimes IDS-OP as a function of temperature for the studied device. The biased voltage is fixed at VDS= 2.0 V.
temperature. This also indicates that the increase of leakage current associated with high K value, caused by the increase of temperature, is indeed suppressed.
The drain saturation current IDS and transconductance
gmas a function of the gate–source voltage VGSat different
temperatures are shown in figure 5. Obviously, due to the increase of phonon scattering and hence the reduction of electron mobility with increasing the temperature, the maximum transconductance gm,maxand drain saturation current
IDS,maxare decreased. The corresponding gm,max, IDS,maxand
IDSoperation regimes (IDS-op) as a function of temperature
are shown in figure 6. At VDS = 2.0 V, the gm,max values
of the studied device are 274 and 232 mS mm−1at 300 and 510 K, respectively. The available IDS,max values, at VDS=
2.0 V and VGS = +0.5 V, are 434 and 403 mA mm−1,
respectively. As compared with the InP- and GaAs-based HEMTs [16, 17], the deviations of gm,max and IDS,max are
relatively insignificant as the temperature is increased. For instance, the gm,maxand IDS,maxvalues at 510 K still maintain
0.0 0.5 1.0 1.5 2.0 2.5 0 100 200 300 400 gds AV gm 0 100 200 300 1x100 µm2 T=300 K VGS=-0.8 V Transconductance g m and Output Conductance g ds (mS/mm) Voltage Gain A V Drain-Source Voltage VDS (V)
Figure 7. Transconductance gm, output conductance gdsand voltage gain AVversus drain–source voltage VDSat 300 K. The biased voltage is fixed at VGS= −0.8 V. 300 330 360 390 420 450 480 510 0 3 6 100 150 200 250 300 gds gm AV 0 100 200 1x100 µm2 VDS= 2.0 V VGS=-0.8 V Transconductance g m and Output Conductance g ds (mS/mm) Voltage Gain A V Temperature (K)
Figure 8. Transconductance gm, output conductance gdsand voltage gain AVas a function of temperature. The biased voltage is fixed at
VDS= 2.0 V.
85% and 93% of their values at 300 K. This is caused by the good Schottky and carrier confinement characteristics of the studied device over a wide temperature range. Moreover, the IDSoperation regime IDS-opis defined as the drain current
values within 10% drop from gm,max. The IDS-opvalues are
245 and 205 mA mm−1 at 300 and 510 K, respectively. Even at the temperature of 510 K, IDS-opis still higher than
200 mA mm−1. Based on the low degradations of gm,max,
IDS,max and IDS-op with increasing temperature, the studied
device is relatively temperature independent and suitable for high-power applications.
The transconductance gm, output conductance gds and
voltage gain AVversus drain–source voltage VDSfor the studied
device at 300 K are shown in figure 7. Clearly, the studied device exhibits a good FET performance. For instance, at
VGS= −0.8 V and VDS= 2.0 V, gds, gmand AVare 1.92 mS
mm−1, 268 mS mm−1and 140, respectively. The temperature dependences of gm, gdsand AVare illustrated in figure8. The
biased voltages are fixed at VDS= 2.0 V and VGS= −0.8 V
for the studied device. As the temperature is increased from 300 to 510 K, gds increases from 1.92 to 6.14 mS mm−1.
1 10 0 10 20 |H21| MAG MSG µ T=300 K V DS= 2.0 V V GS=-0.5 V fT = 17.5 GHz fmax = 42.2 GHz Frequency (GHz) Gain (dB )
Figure 9. Microwave characteristics of the fabricated device at
room temperature.
gm is decreased from 268 to 225 mS mm−1. Thus, the
corresponding AV (gm/gds) is decreased from 140 to 36.6.
Generally, it is known that the relatively large gds value is
caused by the kink effect that is associated with the occurrence of impact ionization within the channel [18]. However, due to the Al-rich Schottky barrier layer and the optimum gate recess width, the gdsvalues are reduced [19–21]. In addition,
owing to the improved carrier transport characteristics and gate modulation capability by using the unstrained In-rich channel and metamorphic buffer structure [5,18,19], the higher gm
values are obtained over a wide temperature range. Hence, a good amplification performance of the studied device can be achieved even at higher temperature (510 K) environment.
Figure 9 shows the microwave characteristics of the studied device at room temperature. The unit current gain frequency fTand maximum oscillation frequency fmax, under
the biased conditions of VDS= 2.0 V and VGS= −0.5 V, are
17.5 and 42.2 GHz, respectively.
The intrinsic fTand fmaxcan be expressed as [22]
fT≈ gm 2π(Cgs+ Cgd+ gm(Rs+ Rd)Cdg) ≈ gm 2π Cgs ≈ vsat 2π Lg (1) and fmax≈ fT 2√Rigds , (2)
where Cgs is the capacitance between the gate and source,
Cgd is the capacitance between the gate and drain, vsat is
the electron saturation velocity, Lg is the gate length, Ri is
the series resistance between the gate and source, gdsis the
output conductance and gmis the extrinsic transconductance.
fT is directly related to the device geometry according to
equation (1). In other words, fT is nearly proportional to
1/Cgs. The calculated Cgs of the studied device is 248f F.
Due to the relatively large Cgs, the microwave performance
of the studied MHEMT device is degraded. The measured fT
and fmaxversus IDSfor the studied device at room temperature
and VDS= 2.0 V are shown in figure10. The studied device
maintains 80% of its fTand fmaxpeak values over a large IDS
operation regime of 191 mA mm−1at 300 K. Therefore, the studied device also provides the promise for high-frequency electronic applications. 0 100 200 300 400 0 10 20 30 40 fT fmax VDS=2.0 V T=300 K
Drain-Saturation Current IDS (mA/mm)
Frequency (GHz)
Figure 10. The unity current gain frequency fTand maximum oscillation frequency fmaxversus drain saturation current IDS. The biased voltage is fixed at VDS= 2.0 V.
4. Summary
A double δ-doped In0.46Ga0.54As/In0.42Al0.58As/GaAs
MHEMT has been successfully fabricated and demonstrated. The studied device shows good dc and RF characteristics including the low leakage current, high breakdown voltage, high transconductance, voltage gain and good microwave performance. The gate leakage current of 0.43 (434) µA mm−1, breakdown voltage of 19.3 (10.2) V, maximum transconductance of 274 (232) mS mm−1, output conductance of 1.92 (6.14) mS mm−1 and voltage gain of 140 (36.6) are obtained, respectively, at 300 (510) K. In addition, the low temperature variation coefficients on gate leakage current (2.1 µA mm−1 K−1), gate–drain breakdown voltage (−27 mV K−1), and low decrease of maximum transconductance (−15%), drain saturation current (−7.1%) are obtained as the temperature is increased from 300 to 510 K. The corresponding fT and fmax are 17.5 and
42.2 GHz at room temperature. Significantly, the studied device provides the promising performance for microwave electronic applications.
Acknowledgments
Part of this work was supported by the National Science Council of the Republic of China under contract no 94-2215-E-006-060. The authors are also grateful to National Nano Device Laboratories (NDL) for RF measurements.
References
[1] Lai Y L, Chang E Y, Chang C Y, Chen T K, Liu T H, Wang S P, Chen T H and Lee C T 1996 IEEE Electron
Device Lett.17 229
[2] Lour W S, Tsai M K, Chen K C, Wu Y W, Tan S W and Yang Y J 2001 Semicond. Sci. Technol.16 826
[3] Chen Y J, Lee C S, Wang T B, Hsu W C, Chen Y W, Su K H and Wu C L 2005 Japan. J. Appl. Phys.44 5903
[4] Hsu W C, Chen Y J, Lee C S, Wang T B, Lin Y S and Wu C L 2005 IEEE Electron Device Lett.26 59
Temperature dependences of an In0.46Ga0.54As/In0.42Al0.58As based MHEMT [5] Chen Y J, Hsu W C, Lee C S, Wang T B, Tseng C H,
Huang J C, Huang D H and Wu C L 2004 Appl. Phys.
Lett.85 5087
[6] Win P, Druelle Y, Cappy A, Cordier Y, Adam D and Favre J 1993 Proc. IEEE/Cornell Conf. on Advanced Concepts in High Speed Semiconductor Devices and Circuits p 511
[7] Cordier Y, Bollaert S, Zaknoune M, di Persio J and Ferre D 1998 Int. Conf. on Indium Phosphide and Related Materials
(11–15 May) p 211
[8] Brown A S, Mishra U K, Chou C S, Hooper C E,
Melenders M A, Thompson M, Larson L E, Rosenbaum S E and Delaney M J 1989 IEEE Electron Device Lett.10 565
[9] Hori Y and Kuzubara M 1994 IEEE Trans. Electron
Devices41 2262
[10] Suemitsu T, Fushimi H, Kodama S, Tsunashima S and Kimura S 2001 13th Indium Phosphide and Related
Materials Conf. Proc. p 456
[11] Chan Y J and Yang M T 1995 IEEE Trans. Electron
Devices42 1745
[12] Su J S, Hsu W C, Lin D T, Lin W, Shaio H P, Lin Y S, Huang J Z and Chou P J 1996 IEE Electron Lett.
32 2095
[13] Yu K H, Chuang H M, Lin K W, Cheng S Y, Cheng C C, Chen J Y and Liu W C 2002 IEEE Trans. Electron
Devices49 1687
[14] Chang W L, Pan H J, Wang W C, Thei K B, Cheng S Y, Lour W S and Liu W C 1999 Japan. J. Appl. Phys.38 1385
[15] Ytterdal T, Moon B J, Fjeldly T A and Shur M S 1995 IEEE
Trans. Electron Devices42 1724
[16] Liu W C, Chang W L, Lour W S, Yu K H, Lin K W, Cheng C C and Cheng S Y 2001 IEEE Trans. Electron Device48 1290
[17] Chen Y J, Hsu W C, Chen Y W, Lin Y S, Hsu R T and Wu Y H 2005 Solid-State Electron.49 163
[18] Hsu W C, Chen Y J, Lee C S, Wang T B, J Huang C, Huang D H, Su K H, Lin Y S and Wu C L 2005 IEEE
Trans. Electron Devices52 1079
[19] Higuchi K, Matsumoto H, Mishima T and Nakamura T 1999
IEEE Trans. Electron Devices46 1312
[20] Boudrissa M, Delos E, Cordier Y, Theron D and Jaeger J C De 2000 IEEE Electron Device Lett.21 512
[21] Yuan K and Radhakrishnan K 2002 IPRM: Indium Phosphide
and Related Materials Conf. p 59
[22] Sze S M 1985 Semiconductor Devices: Physics and
Technology 2nd edn (New York: Wiley) p 250
