IEEE TRANSACTIONS ON MICROWAVE THEORY ANDTECHNIQUES, VOL. 50, NO. 11, NOVEMBER 2002 2499 DC, RF, and Microwave Noise Performances of AlGaN/GaN HEMTs on Sapphire Substrates W. Lu, Senior Member, IEEE, V. Kumar, R. Schwindt, E. Piner, and I. Adesida, Fellow, IEEE Abstract—High-performance AlGaN/GaN high electron-mo- bility transistors with 0.18- m gate length have been fabricated on a sapphire substrate. The devices exhibited an extrinsic transcon- ductance of 212 mS/mm, a unity current gain cutoff frequency of 101 GHz, and a maximum oscillation frequency of 140 GHz. At V and mA/mm, the devices exhibited a minimum noise figure of 0.48 dB and an associated gain of 11.16 dB at 12 GHz. Also, at a fixed drain bias of 4 V with the drain current swept, the lowest of 0.48 dB at 12 GHz was obtained at mA/mm, and a peak of 11.71 dB at 12 GHz was obtained at mA/mm. With the drain current held at 40 mA/mm and drain bias swept, the increased almost linearly with the increase of drain bias. Meanwhile, the values decreased linearly with the increase of drain bias. At a fixed bias condition ( V and mA/mm), the values at 12 GHz increased from 0.32 dB at 55 C to 2.78 dB at 200 C. To our knowledge, these data represent the highest and , and the best microwave noise performance of any GaN-based FETs on sapphire substrates ever reported. Index Terms—GaN, AlGaN, HEMT, microwave noise. I. INTRODUCTION A lGaN/GaN high electron-mobility transistors (HEMTs) have demonstrated device characteristics, which make them excellent candidates for high-power, high-frequency, and high-temperature applications because of unique material prop- erties. State-of-the-art results of AlGaN/GaN HEMTs include a breakdown voltage of as high as 570 with a source–drain spacing of 13 m, a gate length of 0.5 m using an overlapping gate structure [1], a unity current gain cutoff frequency of 101 GHz, a maximum oscillation frequency of 155 GHz for a 0.12- m device [2], and an of 110 GHz for a 50-nm device [3], together with a power density of 9.1 W/mm at 8 GHz [4], as well as a total output of 40.7 W for a 12-mm-wide AlGaN/GaN transistor on SiC at 10 GHz [5]. Up to now, extensive investigations have been conducted on Manuscript received February 16, 2001. This work was supported by the Defense Advanced Research Projects Agency under Contract DAAD19-99-1- 0011, by the Office of Naval Research under Grant N00014-01-1-1000 and Grant N00014-01-1-1072 (monitor: Dr. J. Zolper). W. Lu was with Department of Electrical and Computer Engineering, Microelectronics Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA. He is now with the Department of Electrical Engineering, The Ohio State University, Columbus, OH 43210 USA. V. Kumar, R. Schwindt, and I. Adesida are with Department of Electrical and Computer Engineering, Microelectronics Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA. E. Piner was with ATMI/Epitronics, Phoenix, AZ 85027 USA. He is now with the Nitronex Corporation, Raleigh, NC 27606 USA. Digital Object Identifier 10.1109/TMTT.2002.804619 the potential of AlGaN/GaN HEMTs for power applications [6]–[9]. Investigations on microwave noise performances of GaN-based devices are of importance because the possibility of applications of these devices in low-noise front-end systems would eliminate the need for additional protection circuits with the advantages of high breakdown voltages. Though GaAs- and InP-based HEMTs have demonstrated excellent microwave noise performances, these devices generally suffer from low-breakdown voltages. At present, in front ends of microwave systems such as satellite communications, limiters or protection circuits are required to protect low-noise ampli- fiers (LNAs) because of low-breakdown voltages of GaAs- and InP-based low-noise HEMTs. Devices like GaN-based HEMTs with low noise figures and high breakdown voltages will remove the front-end protection circuits. Such robust low-noise devices will simplify system designs and the complexity of layer structures and device processing and possibly improve the integration of circuits. However, to date, a limited number of investigations have been reported on microwave noise per- formance of GaN-based heterojunction field-effect transistors (HFETs). These preliminary investigations have shown that AlGaN/GaN HEMTs on SiC exhibit excellent microwave noise properties that are comparable to those of AlGaAs/GaAs HEMTs. Specifically, 0.25- m AlGaN/GaN HEMTs with a minimum noise figure of 0.77 dB at 5 GHz and an of 1.06 dB at 10 GHz were reported [10]. An of 0.60 dB at 10 GHz was achieved in an AlGaN/GaN HEMT on SiC with a gate length of 0.15 m [11]. Recently, we reported AlGaN/GaN HFETs on an insulating SiC substrate with a gate length of 0.12 m, which exhibited less than 1 dB at 18 GHz, indicating a potential for broad-band applications of these devices [2]. All these previous studies concentrated on GaN-based HEMTs on SiC substrates because of less lattice-mismatch problems, hence, better material quality. Progress has been made in the growth of AlGaN/GaN HEMTs on sapphire with resulting excellent two-dimensional electron gas properties. Although sapphire has less desirable heat conduction properties than SiC, it is much cheaper. AlGaN/GaN HEMTs on sapphire are attractive especially for low-noise applications because low-noise operation imposes less severe self-heating problems than power operation does. Therefore, AlGaN/GaN HEMTs could provide cost-effective solutions for analog front-end systems. In this paper, for the first time, we report results on microwave noise characteristics of AlGaN/GaN HEMTs on sapphire substrates in comparison with our recently reported noise characteristics of GaN-based HEMTs on SiC substrates. 0018-9480/02$17.00 © 2002 IEEE