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AlGaAsSb/InGaAs/AlGaAsSb Metamorphic HEMTs Richard T. Webster
Air Force Research Laboratory AFRL/SNHA, 80 Scott DryHanscom AFB MA 0173 1 A. F.M.Anwar University of Connecticut Storrs CT 06269
John L. Heaton, Kirby Nichols, and Scott Duncan* BAE Systems
65 Spit Brook Rd, PO Box 868, Nashua NH 03061
* Present address: Opel, Inc 54 Ahern Ln, U5219 Merrit, Mansfield CT 06269
Abstract
Deep quantum well In08Gao.2As/AIGaAsSbMHEMTs on GaAs are described. The step-graded AlGaAsSb strain-relief buffer layer provided a high-quality surface for growth of the MHEMT layers. AlGaAsSb barrier layers offer flexibility in choosing the channel composition and the barrier height. Typical Hall mobilities were 11,000 cm2N-sec at 300K for carrier concentrations of 2.4 x 10l2cme2. Extrinsic DC transconductance of 820 mS/mm was obtained for an MHEMT with a 0.15 pm x 64 pm gate. Typical extrinsic unity current gain cutoff, ft, was 173 GHz with maximum frequency of oscillation, fma,of 474 GHz. Aside fiom layer growth, the MHEMTs were fabricated using only small changes fiom conventional GaAs PHEMT processing. This technology promises affordable production costs for high performance millimeter-wave low noise amplifiers.
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I. Introduction InGaAs has been established as the preferred channel material for high performance microwave and millimeterwave High Electron Mobility Transistors (HEMTs). Pseudomorphic InGaAdAlGaAs HEMTs on GaAs substrates with up to 30% indium are widely used in the microwave and low millimeterwave range and 6” wafer fabrication technology is well established. Increasing the indium content in the ternary alloy produces higher low- field mobility and higher saturation velocity, leading to higher gain and lower noise figures in the HEMT [ 11. On InP substrates, lattice matched Ino.s~Gao.47As/Ino.s2Al0.4~~ HEMTs achieve excellent performance beyond 200 GHz.
However, InP technology is not as mature as GaAs technology, and the cost of producing InP based HEMTs is higher. Metamorphic growth technology loosens the constraints of latticematched and pseudomorphic materials. With a strain-relief metamorphic layer, device quality InXGa(l,)As can be grown for the 1 1 1 range of Oixil on GaAs substrates, enabling high performance Metamorphic HEMTs (MHEMTs). Quaternary AlGaAsSb buffers provide considerable flexibility in growing high
performance MHEMT structures on GaAs. Gill, et al. [11, reported ft of 1SOGHz for 0.1 pm gate, Ino.s~Gao.47As/Ino.s2As0,48As MHEMTs made atop graded AlGaAsSb buffers on GaAs, performance comparable to InP based lattice matched HEMTs. W-band MMIC MHEMT low noise amplifiers [2] and V-Band MHEMT monolithic power amplifiers [3] demonstrate the promise of this graded AlGaAsSb buffer technology. A similar MHEMT, reported by Behet, et al. [43, was made on a 2 pm thick Alo.sGao.sAso.s5Sbo.45layer on GaAs. This strain~ ~ A s and the relaxed buffer was lattice-matched to an In0.53Gao.47As/I Q . ~ ~ A s o . structure 0.25pm gate MHEMT had an ft of 87GHz. Even more flexibility in choosing In concentration and band offsets can be obtained by using AlGaAsSb as the barrier material, MHEMT [SI. Miya, et al. [6,7] reported deepforming an AlGaAsSb/InGaAs/AlGaAsSb quantum-well, InAs channel MHEMTs using quaternary barriers of several different
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compositions all lattice matched to InAs. The 1.O pMHEMTs had ft’s of around 30 GHz, but the authors noted a strong kink effect, suggesting breakdown of the narrow band gap channel. In this paper, we report an In0,8Gao,2As/A1GaAsSbchannel HEMT with a graded AlGaAsSb buffer on GaAs. The indium concentration of 80% provides excellent transport properties with a bandgap of 0.47eV, providing channel breakdown voltage compared to InAs channels. In the following sections, we will present results of our initial effort to develop this material structure and the associated fabrication processes.
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11. Metamorphic Growth and Characterization
The layer structure, grown by MBE on a semi-insulating GaAs substrate, is shown schematically in Fig. 1 along with a band diagram. The antimony concentration of the 1.5 pm AlGaAsSb quaternary buffer layer was step-graded to shift the lattice constant from that
of the GaAs substrate to a value equivalent to In0.7Ga0.3As. This strain relief buffer layer confines dislocations to the AlGaAsSb and provides a suitable surface for growth of high quality channel material [l 1. A 30 8, InAlAs spacer was grown on the buffer to reduce buffer leakage current. The high mobility chinnel layer is 125 8, pseudomorphic In0@*.2As. A 358, AlGaAsSb spacer layer is followed by a 5 x 1OI2 cm-2 Te doping spike, and an additional 120 8, of undoped AlGaAsSb. A 10 8, undoped AlGaAs barrier was added to reduce gate leakage current. The final layer is a 125 8, InGaAs contact layer doped at 1 x 10l8cm”. The two dimensional electron gas (2DEG) is formed in the
[email protected] layer at the AlGaAsSbhGaAs heterointerface. The wide band gap of the quaternary provides a large conduction band offset, close to 0.8 eV in this structure, leading to good confinement of the channel electrons. Typical measured Hall mobilities were 11,000 cm2N-s at 300K for a 2DEG concentration of 2.4x 10l2 cm-2. Topside processing and metalization
was accomplished using the InP HEMT process as a starting point. Nonselective and
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selective etch chemistries were developed to form mesas and gate recesses in the Sb-based layer structure. Flash annealed ohmic contacts produced contact resistivity in the range of 0.13 to 0.19 ohms-mm. T-gates were formed by direct write E-beam with nominal gate
length of 0.15pm. Backside processing followed GaAs rules, including wafer thinning, dry etch vias, and backside metalization.
111. Device Measurements
Fig. 2 shows the typical measured current-voltage characteristics for a 0.15 pn x 64 pm AlGaAsSb/lno.gG*.2As HEMT. The source-to-drain spacing of this HEMT is 2.0 pm, with source-to-gate spacing of 0.5 pm. The device pinches off at V~=-0.5V. The output conductance remains below 150 mS/rnm for drain-to-gate voltages in the range of 0.65 V to 1.05 V. The maximum saturation current at vD=l.3 V and VG=O.OV is 345.0 d m m . In Fig.3, the DC transconductance and drain current are plotted as a function of gate voltage for V ~ = l . 3V. The transconductance peaks broadly around vG=-0.2 With a value of 820 mS/mm. The gate current plot shown in Fig. 4 reveals the characteristic hump associated with impact ionization similar to the one observed in InP-based HEMTs [8]. The hump
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becomes particularly prominent at higher drain-to-source voltages. For low noise applications, the MHEMT would be biased at lower voltages, avoiding this region of higher impact ionization. The leakage current is relatively high for this size device. Leakage tends to be higher for buffers with higher Sb concentrations. This dependence.needs further investigation. Higher aluminum concentration in the barriers would produce a higher hole barrier between the channel and the gate, but may risk instability of the layer due to oxidation.
FW performance of the fabricated devices was characterized by S-parameter measurements up to 60 GHz. The layout, shown in Fig. 5, embedded the transistor in
Figure 5: Layout of a 0.15pm x 64 pm MHEMT
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= 173GHz
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Figure 6. Maximum available gain, IS21r, and lhZ1fof an AlGaAsSbhnGaAs HEMT as a function of fiequency. vD=o.5v, ID=IO.OmA, and V ~ z 0 . 2 V .
microstrip lines and included two source vias and microstrip-to-coplanar transitions for wafer probing. On-wafer calibration standards were used to de-embed the measurements to the transistor gate and drain terminals. The maximum stable gain, IS21f, and lh21I2 are plotted in Fig. 6 as a h c t i o n of frequency with vD=o.5v,I ~ = l o m Aand vG=-o.Zv. Extrapolating to = 474 GHz and unity gain intercepts indicate that the maximum frequency of oscillation, fmax
the unity gain current cut-off frequency, ft = 173 GHz.
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IV. Conclusion
The AlGaAsSb/InGaAs/AlGaAAsSb MHEMT on GaAs is a strong candidate for millimeterwave low noise applications. The step-graded AlGaAsSb buffer layer on GaAs takes advantage of established wafer fabrication technology. Using AlGaAsSb as the MHEMT barrier allows flexibility in engineering the channel transport properties and the quantum well profile. The DC and RF results obtained from these devices indicate the high performance that can be obtained from this system with further optimization. Acknowledgement This work was supported in part by the Air Force Office of Scientific Research and
by AFRL. Contract F19628-95-C-0077.
References
[l] D.M. Gill, B. C. Kane, S. P. Svensson, D-W. Tu, P. N. Uppal, and N. E. Byer, “HighPerformance, 0.1 pm InAlAsAnGaAs High Electron Mobility Transistor on GaAs,”IEEE Electron Device Letters, vol. 17, no. 10, pp. 328-330, 1996. [2] K. C. Hwang, P. C. Chao, C. Creamer, K. B. Nichols, S.Wang, D. Tu, W. Kong, D. Dugas, and G. Patton, “Very High Gain Millimeter-Wave InAlAsAnGaAs/GaAs Metamorphic HEMT’s,”IEEE Electron Device Letters, vol. 20, no. 11, pp. 551-553, 1999. [3] 0. S.A. Tang, S . M. J. Liu, P. C. Chao, W. M. T. Kong, L. C. Hwang, K. Nichols, and J. Heaton, “Design and Fabrication of a Wideband 56- to 63-GHz Monolithic Power Amplifier
With Very High Power-Added Efficiency,” IEEE J Solid-state Circuits, vol. 35, no. 9, pp. 1298-1306,2000.
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[4] M. Behet, K. van der Zanden, G. Borghs, and A. Behes, “Metamorphic InGaAsfinAlAs
Quantum Well Structures Grown on GaAs Substrates for High Electron Mobility Transistor Applications,” Applied Physics Letters, vol. 73, no. 19, pp. 2760-2762, 1998. [5] A. F. M. Anwar and R. T. Webster, “Energy Bandgap of AlxGax~lAs~,Sby and Conduction Band Discontinuity of AlxGax-lAs1y Sby/InAs and AlxGax-lAs 1-,S b,/InGaAS Heterostructures,” Solid-state Electronics, vol. 42, no. 1 1, pp. 2 101-2 104, 1998. [6] S. Miya, S. Muramatsu, N. Kuze, K. Nagase, T. Iwabuchi, A. Ichii, M. Ozaki, and I.
Shibasaki, “AlGaAsSb BufferBarrier Layer on GaAs Substrate for InAs Channel with High Electron Properties,” Proc. I995 Indium Phosphide and Related Materials Conference, pp. 440-443. [7] S . Miya, S.Muramatsu, N.Kuze, K. Nagase, T. Iwabuchi, A. Ichii, M. Ozaki, and I.
Shibasaki, “AlGaAsSb Buffer/Barrier on GaAs Substrate for InAs Channel Devices with High Electron Mobility and Practical Reliability,” J Electronic Materials, vol. 25, no. 3, pp. 4 15-420, 1996.
[SI R. T. Webster and A. F. M. Anwar,“Impact Ionization in InAlAslInGaAsAnAlAs HEMTs,” IEEE Electron Device Letters, vol. 21, no. 5, pp 193-195,2000.
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