IEEE ELECTRON DEVICE LETTERS, VOL. 25, NO. 3, MARCH 2004 117 30-W/mm GaN HEMTs by Field Plate Optimization Y.-F. Wu, Member, IEEE, A. Saxler, Member, IEEE, M. Moore, Member, IEEE, R. P. Smith, Member, IEEE, S. Sheppard, Member, IEEE, P. M. Chavarkar, Member, IEEE, T. Wisleder, U. K. Mishra, Fellow, IEEE, and P. Parikh, Member, IEEE Abstract—GaN high-electron-mobility-transistors (HEMTs) on SiC were fabricated with field plates of various dimensions for op- timum performance. Great enhancement in radio frequency (RF) current–voltage swings was achieved with acceptable compromise in gain, through both reduction in the trapping effect and increase in breakdown voltages. When biased at 120 V, a continuous wave output power density of 32.2 W/mm and power-added efficiency (PAE) of 54.8% at 4 GHz were obtained using devices with dimen- sions of 0.55 246 m and a field-plate length of 1.1 m. Devices with a shorter field plate of 0.9 m also generated 30.6 W/mm with 49.6% PAE at 8 GHz. Such ultrahigh power densities are a dra- matic improvement over the 10–12 W/mm values attained by con- ventional gate GaN-based HEMTs. Index Terms—Field plate, GaN, high-electron-mobility-transis- tors (HEMT), microwave power. I. INTRODUCTION W IDE BANDGAP GaN-based high-electron-mo- bility-transistors (HEMTs) have come a long way as microwave devices since their description in 1993 [1], and a demonstration of their power capability in 1996 [2]. Many research groups have presented devices with power densities exceeding 10 W/mm [3]–[5], a 10 improvement over con- ventional III-V devices. Most previous works covered material quality, choice of substrate, epi-layer structures and processing techniques. Less effort has been put on advanced device designs, leaving room for further improvement. An overlapping gate structure, or field plate, was used by Zhang et al. with GaN HEMTs for high-voltage switching applications [6]. Following this, Karmalkar et al. performed simulations for the field plate structure, predicting up to 5 enhancement in breakdown voltages [7]. However, fabricated devices at that time had low cutoff frequencies, not suitable for microwave operation. Ando et al.recently used a similar structure with smaller gate dimensions and demonstrated excellent performance of 10.3 W output power at 2 GHz using a 1-mm-wide device on SiC substarte [5]. Chini et al.implemented a new variation of the field-plate design with further reduced gate dimensions and obtained 12 W/mm at 4 GHz from a 150- m-wide device on sapphire substrate [8]. This letter presents a systematic study of the effect of field-plate dimensions on both cutoff frequencies and power performance. Optimized devices achieved dramatic improvement over previous state-of-the-art. Manuscript received October 21, 2003. This work supported in part by DARPA and monitored by Dr. H. Dietrich of ONR under Contract N00014-02-C-0306. The review of this letter was arranged by Editor D. Ritter. Y.-F. Wu, M. Moore, P. M. Chavarkar, T. Wisleder, U. K. Mishra, and P. Parikh are with the Cree Santa Barbara Technology Center, Goleta, CA USA 93117 A. Saxler, R. P. Smith, and S. Sheppard are with Cree, Inc., Durham, NC USA 27703 Digital Object Identifier 10.1109/LED.2003.822667 Fig. 1. Schematic of the GaN HEMT with a quasi-electric field plate. Important dimensions are: gate length , dielectric thickness , field plate length , separation between field plate and drain . II. DEVICE DESIGN AND FABRICATION The schematic of the devices in this letter is shown in Fig. 1 with a gate structure first used by Chini et al. [8] on nitride HEMTs. After a HEMT with conventional gate is fabricated, a layer of SiN is deposited on the wafer surface. Additional lithography is then performed to place a metal plate covering the gate and extending to the access region on the drain side. This metal plate is electrically connected to the gate on the gate pad outside of the active channel region. It tracks the potential of the gate electrode, acting as a quasielectric field plate. The function of the field plate is to reshape the distribution of the electric field on the drain side of the gate edge and to reduce its peak value. This not only increases device breakdown voltage but also reduces the high-field trapping effect, hence enhancing current-voltage swings at high frequencies. The trade-off of the field plate structure includes addition of the gate-drain capac- itance at low voltages and extension of the gate-drain deple- tion length at high voltages, which reduces gain. The goal of this study is to evaluate such a trade-off in order to achieve optimum performance. There are a few important dimensions in this structure. Gate length ( ) determines the transit time under the gate. The SiN thickness ( ) controls the onset voltage for additional channel depletion under the field plate while the field-plate length ( ) dictates the size of the field-reshaping region. To maintain good frequency performance, a basic de- sign guideline is to limit the addition of capacitance by the field plate to 10–15% of the original gate capacitance. With an of 0.5–0.6 m and an AlGaN thickness of about 250 Å, was chosen as 2000 Å and was varied from 0 to 1.1 m. The separation between the field plate and the drain ( ) was set to m to avoid premature breakdown. The epi-structure and processing steps were similar to pre- viously reported [4] with the following changes. In contrast to a conventional AlGaN–GaN HEMT, these devices included a 0741-3106/04$20.00 © 2004 IEEE