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IEEE ELECTRON DEVICE LETTERS, VOL. 25, NO. 9, SEPTEMBER 2004
9.4-W/mm Power Density AlGaN–GaN HEMTs on Free-Standing GaN Substrates K. K. Chu, P. C. Chao, M. T. Pizzella, R. Actis, D. E. Meharry, K. B. Nichols, R. P. Vaudo, X. Xu, J. S. Flynn, J. Dion, and G. R. Brandes
Abstract—High power microwave AlGaN–GaN high electron-mobility transistors (HEMTs) on free-standing GaN substrates are demonstrated for the first time. Measured gate leakage was 2 2 A/mm at 20 V and 10 A/mm at 45 V gate bias. When operated at a drain bias of 50 V, devices showed a record continuous-wave output power density of 9.4 W/mm at 10 GHz with an associated power-added efficiency of 40%. Long-term stability of device RF operation was also examined. Under room conditions, devices driven at 25 V and 3-dB gain compression remained stable in 200 h, degrading only by 0.18 dB in output power. Such results illustrate the potential of GaN substrate technology in supporting reliable, high performance AlGaN–GaN HEMTs for microwave power applications. Index Terms—Free-standing, GaN substrate, high electron-mobility transistor (HEMT), microwave power.
I. INTRODUCTION
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N recent years significant progress has been made in AlGaN–GaN HEMT technology for microwave power applications. The wide bandgap AlGaN–GaN material system inherently offers high voltage and high current capabilities, allowing excellent AlGaN–GaN HEMT power performance from S-band to Ka-band [1]–[3]. Successful demonstrations of hybrid amplifiers, as well as microwave monolithic integrated circuits (MMICs), have also been achieved [4], [5]. However, in spite of the excellent device and circuit demonstrations, there have been limited published reports regarding the reliability of AlGaN–GaN HEMT technology [6]–[8]. AlGaN–GaN layers grown on SiC or sapphire substrates typically have threading , increasing dislocation densities on the order of device leakage [9] and promoting metal diffusion along the dislocation sites [10]. These are both properties that can lead to reduced device lifetime, similar to that observed in GaN-based laser diodes [11]. The availability of a low-dislocation GaN substrate technology would help circumvent this problem. The first demonstration of AlGaN–GaN HEMT on freestanding GaN substrates was reported by Khan et al. in 2000 [12]. Device dc results comparable to that on SiC substrates were shown; yet no microwave data were reported. In this letter, we report on microwave AlGaN–GaN HEMTs fabricated
Manuscript received May 25, 2004; revised June 19, 2004. This work was supported in part by DARPA under Contract N00014-02-C-0321. The review of this letter was arranged by Editor T. Mizutani. K. K. Chu, P. C. Chao, M. T. Pizzella, R. Actis, D. E. Meharry, and K. B. Nichols are with BAE Systems, Information and Electronic Warfare Systems, Nashua, NH 03060 USA (e-mail:
[email protected]). R. P. Vaudo, X. Xu, J. S. Flynn, J. Dion, and G. R. Brandes are with Cree, Inc., Danbury, CT 06810 USA. Digital Object Identifier 10.1109/LED.2004.833847
on low-dislocation semi-insulating GaN substrates. Low gate leakage current, excellent microwave power performance as well as stable radio frequency (RF) operation are demonstrated. II. DEVICE FABRICATION The device structures in this study were grown by metalorganic chemical vapor deposition (MOCVD) on free-standing GaN substrates. The GaN substrates were produced by hydride vapor phase epitaxy (HVPE) and were Fe-doped to achieve a [13]. The room temperature resistivity in excess of epitaxial structure consisted of a 100-nm undoped GaN channel barrier. The layer followed by a 27-nm undoped homo-epitaxial surface was quite smooth, with RMS roughness less than 2 as measured by atomic force microscopy. Unlike SiC or sapphire, the use of GaN substrates allowed high quality epitaxial layers to be grown without utilizing a thick GaN buffer or a separate growth nucleation layer, simplifying the epitaxial growth process. Based on similar wafers, it is estimated that the dislocation density of the substrate and device layer was less . than Device processing included mesa isolation using a chlorinebased reactive-ion etch (RIE), followed by Ti–Al-based ohmic contact deposition and annealing. Using standard TLM techwith a niques, contact resistance was measured to be 1.5 sheet resistance of 340 per square. Isolation resistance above per square was measured between separate mesas after 1 ohmic contact formation, attesting to the excellent insulating qualities of the Fe-doped GaN substrate. Electron-beam lithography was then used to define Ni–Au T-gates with footprints of 0.15 m. Overlay metal was deposited for device connections, followed by silicon nitride passivation using plasma-enhanced chemical vapor deposition (PECVD). III. RESULTS AND DISCUSSION DC measurements were performed on the fabricated devices using a HP4145B unit. Maximum drain current was 1.1 A/mm V, while peak transconductance was at a gate bias of V. Pinch-off voltage 220 mS/mm at a gate voltage of was V. Two-terminal gate diode measurements were carried out to examine gate leakage on the devices. Fig. 1 shows the forward and reverse gate diode characteristics of a fully passivated device. Forward turn-on voltage (as measured at 1-mA/mm forward current) was 1.72 V. The reverse A/mm at a gate bias of V and gate current was A/mm at V. Two-terminal reverse breakdown voltage (defined at 1-mA/mm reverse gate current) was higher
0741-3106/04$20.00 © 2004 IEEE
CHU et al.: 9.4-W/mm POWER DENSITY AlGaN–GaN HEMTs ON FREE-STANDING GaN SUBSTRATES
Fig. 1. Forward and reverse gate-drain current densities for a passivated AlGaN–GaN HEMT on GaN substrate. Forward turn-on voltage was measured to be 1.72 V (inset). Reverse gate leakage was 10 A/mm at 45 V gate bias. Breakdown voltage was over 100 V.
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Fig. 2. Measured CW power performance of a 2 75 m AlGaN–GaN HEMT on GaN substrate at 10 GHz. Device was biased at 50 V and 150 mA/mm. A record output power density of 9.4 W/mm with an associated PAE of 40% was achieved.
TABLE I CW MICROWAVE POWER PERFORMANCE AT 10 GHZ FOR A 2 75 m ALGAN–GAN HEMT ON GAN SUBSTRATE UNDER VARIOUS QUIESCENT BIASES
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than 100 V, beyond the capability of our measurement system. Compared with AlGaN–GaN HEMTs on SiC with similar device structures, the current devices on GaN substrates have one to two orders-of-magnitude lower gate leakage values. The reduction in gate current is attributed to the reduced dislocation density achievable on free-standing GaN substrates. On-wafer microwave power measurements were performed using a Maury loadpull system. The devices were operated in Class AB mode with the input and output matches tuned for maximum output power. Table I lists the 10-GHz continuous-wave (CW) power output densities and associated power-added efficiencies (PAE) obtained at various bias conditions for a 2 75 m device. The linear scaling of power output density with drain bias indicates current dispersion was not significant in the devices. At 35 V drain bias, a CW output power density of 6.7 W/mm was measured at 10 GHz with an associated gain of 10.1 dB and a PAE of 51%. At 50-V drain bias, a CW power density of 9.4 W/mm was obtained with an associated gain of 11.6 dB and a PAE of 40%, as demonstrated in Fig. 2. To the authors’ knowledge, this is the highest microwave power density ever reported for AlGaN–GaN HEMTs based on GaN substrates. For AlGaN–GaN HEMT structures on SiC substrates with thicker GaN buffer layers, short gate devices designed for high gain (with gate length below 0.2 m) often suffer from high subthreshold leakage without the use of a compensation-doped buffer. Consequently device operation is limited to lower drain
Fig. 3. Room temperature RF stress test for an AlGaN–GaN HEMT on GaN substrate. Device was biased at 25 V and driven at 3-dB compression. Output power decreased by 0.18 dB in 208 h. Drain current decreased by 3.0%.
bias (30 V or below) and thus lower output power (5–6 W/mm). In this case, the devices on GaN substrates have similar performance at the same reduced drain biases. However, the thin GaN buffer together with the Fe-doped semi-insulating substrate improved electron confinement and allowed device operation up to 50 V, thereby significantly increasing the output power density achievable for 0.15- m gate devices. With lower dislocation density and reduced inherent strain in the epitaxial layers, AlGaN–GaN HEMTs on GaN substrates are expected to have improved device lifetime over their hetero-epitaxial counterparts on SiC or sapphire substrates. In this study, devices on GaN substrates were subjected to prolonged RF stress test under room conditions to examine their long-term stability. Quiescent drain bias was set at 25 V with a drain current of 200 mA/mm. The same loadpull test stand was used to optimize input and output matches for maximum output power. Input power level was kept constant at the initial 3-dB compression point of the devices. Fig. 3 shows the measured output power and drain current for a 2 75 m device over a period of 200 h. Initial device performance corresponds to a 3-dB compressed power of 4.6 W/mm with an associated PAE of 54%. After 200 h of continuous RF stress, device output power remained stable, degrading only by 0.18 dB. This translates to a post-stress output power of 4.4 W/mm and a PAE of 53%. Device drain current was found to track closely with
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IEEE ELECTRON DEVICE LETTERS, VOL. 25, NO. 9, SEPTEMBER 2004
output power, dropping 3.0% during the 200-hour test as shown in Fig. 3. This device degradation result compares favorably to other AlGaN–GaN HEMT degradation rates reported in literature [6], [7]. While more extensive multi-temperature testing is needed to extract actual device lifetime, this first demonstration clearly illustrates the excellent potential of GaN substrate technology in supporting reliable, high-power microwave devices. IV. CONCLUSION High-power microwave AlGaN–GaN HEMTs were demonstrated on free-standing GaN substrates for the first time. An excellent CW power density of 9.4 W/mm with an associated PAE of 40% was achieved at 10 GHz and 50 V drain bias. Two-terminal gate leakage of the devices was low, reaching A/mm at a gate bias of V. When operated at a drain bias of 25 V and 3-dB output compression, devices showed a 0.18-dB power degradation over 200 h. Such results demonstrate the excellent potential of AlGaN–GaN HEMTs on GaN substrates in achieving high microwave power with good reliability. ACKNOWLEDGMENT The authors would like to thank J. F. Bass for performing electron-beam lithography and L. M. Mt. Pleasant for help in RF measurements. Support from J. A. Windyka is also appreciated. REFERENCES [1] Y. F. Wu, A. Saxler, M. Moore, P. Smith, S. Sheppard, P. M. Chavarkar, T. Wisleder, U. K. Mishra, and P. Parikh, “30-W/mm GaN HEMTs by field plate optimization,” IEEE Electron Device Lett., vol. 25, pp. 117–119, Mar. 2004.
[2] V. Tilak, B. Green, V. Kaper, H. Kim, T. Prunty, J. Smart, J. Shealy, and L. Eastman, “Influence of barrier thickness on the high-power performance of AlGaN–GaN HEMTs,” IEEE Electron Device Lett., vol. 22, pp. 504–506, Nov. 2001. [3] K. Kasahara, H. Miyamoto, Y. Ando, Y. Okamoto, T. Nakayama, and M. Kuzuhara, “Ka-band 2.3 W power AlGaN/GaN heterojunction FET,” in IEDM Tech. Dig., 2002, pp. 677–680. [4] W. L. Pribble, J. W. Palmour, S. T. Sheppard, R. P. Smith, S. T. Allen, T. J. Smith, Z. Ring, J. J. Sumakeris, A. W. Saxler, and J. W. Milligan, “Applications of SiC MESFETs and GaN HEMTs in power amplifier design,” in IEEE MTT-S Dig., vol. 3, 2002, pp. 1819–1822. [5] B. M. Green, V. Tilak, S. Lee, H. Kim, J. A. Smart, K. J. Webb, J. R. Shealy, and L. F. Eastman, “High-power broad-band AlGaN/GaN HEMT MMICs on SiC substrates,” IEEE Trans. Microwave Theory Tech., vol. 49, pp. 2486–2493, Dec. 2001. [6] H. Kim, V. Tilak, B. M. Green, H. Cha, J. A. Smart, J. R. Shealy, and L. F. Eastman, “Degradation characteristics of AlGaN-GaN high electron mobility transistors,” in Proc. IEEE Int. Reliability Physics Symp., 2001, pp. 214–218. [7] C. Lee, L. Witkowski, M. Muir, H. Q. Tserng, P. Saunier, H. Wang, J. Yang, and M. A. Khan, “Reliability evaluation of AlGaN/GaN HEMTs grown on SiC substrate,” in Proc. IEEE Lester Eastman Conf. High Performance Devices, 2002, pp. 436–442. [8] J. A. Mittereder, S. C. Binari, P. B. Klein, J. A. Roussos, D. S. Katzer, D. F. Storm, D. D. Koleske, A. E. Wickenden, and R. L. Henry, “Current collapse induced in AlGaN/GaN HEMTs by short-term DC bias stress,” in Proc. IEEE Int. Reliability Physics Symp., 2003, pp. 320–323. [9] J. W. P. Hsu, M. J. Manfra, R. J. Molnar, B. Heying, and J. S. Speck, “Direct imaging of reverse-bias leakage through pure screw dislocations in GaN films grown by molecular beam epitaxy on GaN templates,” Appl. Phys. Lett., vol. 81, pp. 79–81, July 2002. [10] C. Y. Hsu, W. H. Lan, and Y. C. S. Wu, “Effect of thermal annealing of Ni/Au ohmic contact on the leakage current of GaN based light emitting diodes,” Appl. Phys. Lett., vol. 83, pp. 2447–2449, Sept. 2003. [11] S. Nakamura, “III–V nitride-based blue LDs with modulation-doped strained-layer superlattices,” in Proc. IEEE Int. Symp. Compound Semiconductors, 1997, pp. 1–4. [12] M. A. Khan, J. W. Yang, W. Knap, E. Frayssinet, X. Hu, G. Simin, P. Prystawko, M. Leszczynski, I. Grzegory, S. Porowski, R. Gaska, M. S. Shur, B. Beaumont, M. Teisseire, and G. Neu, “GaN-AlGaN heterostructure field-effect transistors over bulk GaN substrates,” Appl. Phys. Lett., vol. 76, pp. 3807–3809, June 2000. [13] R. P. Vaudo, X. Xu, A. Salant, J. Malcarne, and G. R. Brandes, “Characteristics of semi-insulating, Fe-doped GaN substrates,” Phys. Stat. Sol. (a), vol. 200, pp. 18–21, 2003.
