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IEEE ELECTRON DEVICE LETTERS, VOL. 24, NO. 5, MAY 2003

289

10-W/mm AlGaN–GaN HFET With a Field Modulating Plate
Y. Ando, Y. Okamoto, H. Miyamoto, T. Nakayama, T. Inoue, and M. Kuzuhara
Abstract—AlGaN–GaN heterojunction field-effect transistors (HFETs) with a field modulating plate (FP) were fabricated on an SiC substrate. The gate–drain breakdown voltage ( ) was significantly improved by employing an FP electrode, and the highest of 160 V was obtained with an FP length ( FP ) of 1 m. The maximum drain current achieved was 750 mA/mm, together with negligibly small current collapse. A 1-mm-wide FP-FET ( FP = 1 m) biased at a drain voltage of 65 V demonstrated a continuous wave saturated output power of 10.3 W with a linear gain of 18.0 dB and a power-added efficiency of 47.3% at 2 GHz. To our knowledge, the power density of 10.3 W/mm is the highest ever achieved for any FET of the same gate size. Index Terms—FET, field modulating plate (FP), GaN.
Fig. 1. Schematic AlGaN–GaN FP-FET structure.

I. INTRODUCTION ECENTLY, wide bandgap semiconductor GaN has received increased attention because of its great potential for microwave power devices [1]–[13]. Small periphery GaN of several hundred microns FETs with a gate width 10 W/mm have demonstrated very high-power densities (e.g., 11.7 W/mm for m [1] and 10.3 W/mm for m [2]). The power density, however, decreases as increases to 1 mm, where a power density of 6.3 W/mm was reported [3]. For larger periphery devices, the obtainable power density further decreased to a level of several Watts per millimeter (e.g., 4.3 W/mm (102 W) continuous wave (CW) power from a 24-mm-wide FET hybrid amplifier [4] and 3.5 W/mm (113 W) pulsed power from a 32-mm-wide single FET chip [5]). In previous work, SiN film was mostly used as a surface passivation, since it reduces response of the surface traps and thus suppresses the effect of current collapse. Collapse-free devices using SiN passivation, however, exhibited significant decrease [5]. This tradeoff in the gate–drain breakdown voltage relation between current collapse and breakdown characteristics has been known to be a main concern for realizing high-voltage high-power operation using AlGaN–GaN FET technology. Conventionally, a field modulating plate (FP) has been ap, but also to suppress the surplied not only to improve face trap effect for Si- or GaAs-based power devices [14]. For AlGaN–GaN FETs, however, only a few studies have been re[15], [16], and no work ported on FP structure to improve has been reported on RF power characteristics. In this letter, we report an AlGaN–GaN heterojunction field-effect transisManuscript received January 21, 2003; revised February 21, 2003. The review of this letter was arranged by Editor D. Ritter. The authors are with the Photonic and Wireless Devices Research Laboratories, NEC Corporation, Otsu 520-0833, Japan (e-mail: y-ando@bp.jp.nec.com). Digital Object Identifier 10.1109/LED.2003.812532

R

tors (HFETs) with an FP electrode (FP-FET) fabricated on a SiC substrate. The dependency of dc and RF properties on the is described. Finally, RF power measurement FP length results are reported for the developed AlGaN–GaN FP-FET. II. DEVICE STRUCTURE AND PROCESSING Fig. 1 illustrates a schematic of the fabricated FET structure. The drain side edge of the gate overlaps the SiN film by a length . Devices with various values m of were fabricated. The gate length and the gate–drain were fixed at 1.0 and 2.5 m, respectively. An spacing undoped AlGaN–GaN heterostructure was grown by metal organic chemical vapor deposition on a 330- m-thick SiC substrate. Thickness of the AlGaN layer is 30 nm, and the Al mole-fraction is approximately 25%. For comparison, conventional FETs without an FP electrode were also fabricated on the same wafer. The fabrication process began with ohmic contact formation. Ti–Al contact metals were evaporated and alloyed using rapid thermal annealing at 650 C. Device isolation was accomplished by nitrogen ion implantation. A 120-nm-thick SiN film was then deposited using plasma-enhanced chemical vapor deposition. After the gate footprint was opened through the SiN film using buffered HF solution, Ni–Au gate metals were evaporated and lifted off. Finally, a standard Au-plated air-bridge process was used to complete multifingered FETs. III. DC AND SMALL-SIGNAL CHARACTERISTICS Fig. 2 shows the maximum drain current measured of 1 V and measured at a gate at a gate voltage of 1 mA/mm, as a function of . Since current values of these devices range from 600 to 800 mA/mm, a deon is not clearly observed. was pendency of just 50 V for the conventional FET but was improved from 50 from 0 to 1 m. This improvement to 160 V by increasing

0741-3106/03$17.00 ? 2003 IEEE

290

IEEE ELECTRON DEVICE LETTERS, VOL. 24, NO. 5, MAY 2003

Fig. 2.

I

and BV

versus FP length

(W = 50 m; L = 2:5 m).

in is due to the depletion layer formed under the FP electrode, since it reduces the electric field strength at the drain edge gradually decreased of the gate [14]. On the other hand, m. This is because the electric field strength at for the FP edge increases due to the reduced spacing between FP of 1 m was found to be the and drain electrodes. Thus, most suitable for high-voltage power operation. To investigate the quality of the SiN film, the leakage current through the SiN m on the was measured using an isolated device same wafer. Conductivity of the SiN is sufficiently small, since V. the leakage current was less than 1 A/mm at To investigate the effect of FP on current collapse, we have compared current–voltage ( – ) characteristics measured by a curve tracer under two different sweeping conditions. Fig. 3(a) shows low bias (10 V sweeping range) and high bias (80 V sweeping range) – characteristics for an FP-FET, and Fig. 3(b) shows those for a conventional FET. We have as the ratio of the reduced defined a collapse factor with 80-V sweeping range to the with amount of 10-V sweeping range. The FP-FET exhibited negligibly , while the conventional FET showed a small . The current collapse is comparatively larger generally explained by the carrier freeze-out effect due to the negatively charged surface traps between gate and drain. In an FP-FET, however, a voltage signal applied to the FP electrode successfully modulates the carrier density between gate and drain, even when the surface traps remain negatively charged [14]. We believe this carrier modulation effect is responsible for the absence of current collapse in the FP-FET. Small-signal characteristics for 100- m-wide FETs were characterized by on-wafer S-parameter measurements from 0.5 to 40 GHz. The conventional FET showed a unity current gain of 9.8 GHz and a maximum oscillation cutoff frequency of 46 GHz. These values were obtained frequency of 10 V and a drain current at a drain voltage of 100 mA/mm. At the same biasing condition, the FP-FET and (7.8 and 29 GHz, respectively showed inferior m) due to the additional feedback capacitance for resulting from FP. However, the influence of FP on the feedback increases. This capacitance becomes less significant as is due to the drain depletion layer extended toward the drain

Fig. 3. Low bias (10-V sweeping range) and high bias (80-V sweeping range) I –V characteristics for (a) an FP-FET and (b) a conventional FET. Both devices have W of 50 m and L of 2.5 m. Vertical and horizontal divisions are 5 mA and 10 V, respectively. Gate sweep begins at V V with 1 V steps. No hysteresis is observed for 10-V sweep, while a counterclockwise hysteresis is caused due to the current collapse for 80-V sweep. Along the point at V V hysteretic (80-V sweep) curve, an instantaneous I increases from a “low” level (denoted by a circle L) to a “high” level (denoted by a circle H). “High” I point almost coincides with an I point for 10-V sweep. I values of the FP-FET are 40 and 39 mA (defined by the “low” level) for 10- and 80-V sweeping conditions, respectively, while these values of the conventional FET are 40 and 34 mA. Collapse factor I is <3 and 15% for the FP-FET and the conventional FET, respectively.

= +1

0

= 10

%

1

electrode with a high drain voltage. Thus, the FP-FET exhibited (29 to 45 GHz for m) drastic improvement in from 10 to 30 V. For the conventional FET, by increasing (46 to 52 GHz) was relatively smaller in improvement in . the same range of IV. LARGE-SIGNAL CHARACTERISTICS On-wafer load-pull measurements were performed at 2 GHz m and a conventional FET. for an FP-FET of 1 mm with a gate finger length of Both devices have

ANDO et al.: 10-W/mm AlGaN–GaN HFET WITH FIELD MODULATING PLATE

291

was obtained with m. of 750 mA/mm was achieved together with negligibly small current collapse. At V, a 1-mm-wide FP-FET m packaged , 47.3% PAE, into a ceramic carrier demonstrated 10.3 W at 2 GHz. To our knowledge, the power density and 18.0 dB of 10.3 W/mm is the highest ever achieved for any FET of the same gate size. These results indicate that an AlGaN–GaN FP-FET is promising as a high-voltage operation device for high-power solid-state amplifier applications. ACKNOWLEDGMENT The authors would like to thank M. Mizuta, T. Uji, and M. Ogawa with NEC Corporation for their continuing support. They would also like to express appreciation to K. Kasahara with NEC Corporation for discussions and help. REFERENCES
[1] L. F. Eastman, “Experimental power-frequency limits of AlGaN/GaN HEMT’s,” in IEEE MTT-S Dig., 2002, p. 2273. [2] W. -. Wu, P. M. Chavarkar, M. Moore, P. Parikh, and U. K. Mishra, “Bias-dependent performance of high power AlGaN/GaN HEMT’s,” in IEDM Tech. Dig., 2001, p. 378. [3] N. X. Nguyen, M. Micovic, W.-S. Wong, P. Hashimoto, L.-M. McCray, P. Janke, and C. Nguyen, “High performance microwave power GaN/AlGaN MODFET’s grown by RF-assisted MBE,” Electron. Lett., vol. 36, p. 468, 2000. [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 MESFET’s and GaN HEMT’s in power amplifier design,” in IEEE MTT-S Dig., 2002, p. 1819. [5] Y. Ando, Y. Okamoto, H. Miyamoto, N. Hayama, T. Nakayama, K. Kasahara, and M. Kuzuhara, “A 110-W AlGaN/GaN heterojunction FET on thinned sapphire substrate,” in IEDM Tech. Dig., 2001, p. 381. [6] C. Nguyen, M. Micovic, D. Wong, A. Kurdoghlian, P. Hashimoto, P. Janke, L. McCray, and J. Moon, “GaN HFET technology for RF applications,” in 2000 IEEE GaAs Dig., 2000, p. 11. [7] Y.-F. Wu, P. M. Chavarkar, M. Moore, P. Parikh, B. P. Keller, and U. K. Mishra, “A 50 W AlgaN/GaN HEMT amplifier,” in IEDM Tech. Dig., 2000, p. 375. [8] J. W. Palmour, S. T. Sheppard, R. P. Smith, S. T. Allen, W. L. Pribble, T. J. Smith, Z. Ring, J. J. Sumakeris, A. W. Saxler, and J. W. Milligan, “Wide bandgap semiconductor devices and MMIC’s for RF power applications,” in IEDM Tech. Dig., 2001, p. 385. [9] R. Sandhu, M. Wojtowicz, M. Barsky, R. Tsai, I. Smorchkova, C. Namba, P. H. Liu, R. Dia, M. Truong, D. Ko, J. W. Yang, H. Wang, and M. A. Khan, “1.6 W/mm, 26% PAE AlGaN/GaN HEMT operation at 29 GHz,” in IEDM Tech. Dig., 2001, p. 940. [10] “IEICE Tech. Rep. ,”, ED2002-94, 2002. [11] T. Kikkawa, M. Nagahara, T. Kimura, S. Yokokawa, S. Kato, M. Yokoyama, Y. Tateno, K. Horino, K. Domen, Y. Yamaguchi, N. Hara, and K. Joshin, “A 36 W CW AlGaN/GaN-power HEMT using surface-charge-controlled structure,” in IEEE MTT-S Dig., 2002, p. 1815. [12] 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, p. 677. [13] Y. Okamoto, Y. Ando, H. Miyamoto, T. Nakayama, K. Kasahara, T. Inoue, and M. Kuzuhara, “An 80 W AlGaN/GaN heterojunction FET with a field-modulating plate,” in 2003 IEEE MTT-Symp. [14] K. Asano, Y. Miyoshi, K. Ishikura, Y. Nashimoto, M. Kuzuhara, and M. Mizuta, “Novel high power AlGaAs/GaAs HFET with a field-modulating plate operated at 35 V drain voltage,” in IEDM Tech. Dig., 1998, p. 59. [15] N.-Q. Zhang, S. Keller, G. Parish, S. Heikman, S. P. DenBaars, and U. K. Mishra, “High breakdown GaN HEMT with overlapping gate structure,” IEEE Electron Device Lett., vol. 21, p. 421, 2000. [16] J. Li, S. J. Cai, G. Z. Pan, Y. L. Chen, C. P. Wen, and K. L. Wang, “High breakdown voltage GaN HFET with field plate,” Electron. Lett., vol. 37, p. 196, 2001.

Fig. 4. 2-GHz power sweep for a packaged 1-mm-wide FP-FET biased at V = 65 V (L = 1 m).

100 m and a gate pitch of 30 m. At V, a saturated of 7.1 W, a power-added efficiency (PAE) output power of 15.3 dB were measured of 56%, and a linear gain for the FP-FET, while the conventional FET exhibited of 4.5 W, PAE of 49%, and of 18.9 dB. Measured for the FP-FET increases almost linearly as is increased up to 30 V. This result is in good agreement with the class-A A/mm. On the other hand, calculation assuming for the conventional FET deviates from the calculation V. Superior in the FP-FET is attributed to for V, of the the suppressed current collapse. At FP-FET was about 7 dB lower than that of the conventional for the FP-FET was found to increase FET. However, (10 to 30 V), while was (11 to 15 dB) by increasing almost unchanged (18 to 19 dB) for the conventional FET. Thus, the gain drop due to the increased feedback capacitance . in FP-FETs is partially compensated by increasing Since the applied voltage is limited due to the durability of the on-wafer measurement system, the 1-mm-wide FP-FET was packaged into a ceramic carrier, and its power performance was measured using a test fixture consisting of impedance matching capacitors and a transmission line on a dielectric substrate. The of 6.4 W at V. packaged FP-FET exhibited The difference between the on-wafer (7.1 W) and the packaged (6.4 W) results would be attributed to the loss of the test fixture 0.5 dB . Fig. 4 presents a power sweep at 2 GHz for the V. Without correction of packaged FP-FET biased at , 47.3% PAE, and 18.0 dB were the fixture loss, 10.3 W measured. To our knowledge, the power density of 10.3 W/mm is the highest ever achieved for 1-mm class GaN FETs. Also, this result is comparable to the record power density (11.7 W/mm) obtained from a 0.1-mm-wide GaN FET [1]. V. CONCLUSION To improve the tradeoff relation between current collapse and breakdown characteristics, FP structure was applied to was significantly improved by an AlGaN–GaN FET. of 160 V employing an FP electrode, and the highest


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