IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 11, NOVEMBER 2011 1519 High-Performance AlGaN/GaN Schottky Diodes With an AlGaN/AlN Buffer Layer Geng-Yen Lee, Hsueh-Hsing Liu, and Jen-Inn Chyi, Fellow, IEEE Abstract—High-performance AlGaN/GaN Schottky barrier diodes are fabricated on a composite AlGaN/AlN buffer layer with low screw-type and high edge-type dislocation densities. With- out edge termination, the devices with 30-µm anode-to-cathode spacing exhibit a high breakdown voltage (V B ) of 3489 V, a low leakage current (I R ) of less than 0.2 µA at −2000 V, and a low specific on-resistance (R on ) of 7.9 mΩ · cm 2 , resulting in a figure of merit (V 2 B /R on ) as high as 1.54 GW/cm 2 . Their switching characteristics as revealed by the reverse-recovery transient wave- form exhibit a short reverse-recovery time of 17 ns. Index Terms—AlGaN/AlN buffer, AlGaN/GaN, Schottky bar- rier diodes (SBDs). I. I NTRODUCTION R ECENTLY, wide-bandgap GaN-based Schottky barrier diodes (SBDs) have been intensively pursued for high- efficiency switching-mode power supplies. For power SBDs, having both low on-resistance (R on ) and high breakdown voltage (V B ) simultaneously is essential. The figure of merit (FOM ) defined as V 2 B /R on is a measure often used to evaluate the performance of SBDs. For low R on , the AlGaN/GaN heterostructure has been con- sidered a promising choice due to its high carrier mobility and density in the form of 2-D electron gas (2DEG) at the AlGaN/GaN interface [1]–[3]. For high V B , several approaches, such as guard rings and field plates, have been proposed to en- hance V B by lowering the peak electric field near the Schottky gate edge to reduce hot-carrier-induced impact ionization [4]–[7]. In addition, a high-resistivity buffer layer with low screw-type threading dislocation density is crucial for achieving low I R [8] and high V B . Using a wide-bandgap AlN buffer layer (6.2 eV) in an AlGaN/GaN high-electron-mobility tran- sistor (HEMT) structure not only provides a highly resistive buffer to sustain a high blocking voltage but also is capable of Manuscript received July 14, 2011; revised August 1, 2011; accepted August 6, 2011. Date of publication September 18, 2011; date of current version October 26, 2011. This work was supported in part by the National Science Council of Taiwan under Contract NSC 96-2628-E-008-072-MY3 and in part by Tekcore Company, Ltd. The review of this letter was arranged by Editor J. A. del Alamo. G.-Y. Lee and H.-H. Liu are with the Department of Electrical Engineering, National Central University, Jhongli 32001, Taiwan (e-mail: 965201041@ cc.ncu.edu.tw; 965401023@cc.ncu.edu.tw). J.-I. Chyi is with the Department of Electrical Engineering, National Central University, Jhongli 32001, Taiwan. He is also with the Department of Optics and Photonics, National Central University, Jhongli 32001, Taiwan, and also with the Research Center for Applied Sciences, Academia Sinica, Taipei 115-29, Taiwan (e-mail: chyi@ee.ncu.edu.tw). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2011.2164610 Fig. 1. Schematic diagram of the AlGaN/GaN SBD structure on (a) a GaN buffer (sample A) and (b) an AlGaN/AlN buffer (sample B). (c) Schematic device layout of the SBDs studied in this work. achieving ultralow screw-type dislocation density [9]. In 2007, Miyoshi et al. utilized a highly resistive AlN buffer layer to obtain AlGaN/GaN SBDs with a high V B of 3000 V [10]. How- ever, the R on of their devices was not revealed, and few studies have been reported to investigate the detailed characteristics of AlGaN/GaN SBDs with an AlN buffer layer. In this letter, we demonstrate high-performance GaN SBDs on an AlGaN/AlN composite buffer. The electrical character- istics of the SBDs are systematically investigated and corre- lated with the types of dislocations in the buffer. Low R on , reduced I R , and enhanced V B , which lead to a high FOM , are achieved. Fast switching characteristics of the SBDs are also demonstrated. II. EXPERIMENTS Two samples, denoted as A and B, were grown on a c-face sapphire substrate by low-pressure metal–organic vapor-phase epitaxy. Sample A was a conventional GaN HEMT structure, including a GaN cap (5 nm), an Al 0.23 Ga 0.77 N (30-nm) elec- tron supply layer, and a GaN (6-µm) buffer layer. Sample B had the same HEMT structure except the buffer layer, which was replaced by a GaN (600 nm)/Al 0.23 Ga 0.77 N (30 nm)/AlN (0.8 µm) composite buffer layer as shown in Fig. 1(a) and (b). Hall measurements show that the sheet carrier concentration, mobility, and sheet resistance are 1.89 × 10 13 cm −2 , 885 cm 2 / V · s, and 373 Ω/ for sample A and 1.05 × 10 13 cm −2 , 720 cm 2 /V · s, and 828 Ω/ for sample B, respectively. The lower electron mobility and the higher sheet resistance observed on sample B are attributed to its higher edge dislocation density in the buffer as will be shown in the latter section. For device isolation, mesas of 540 µm × 145 µm were formed by inductively coupled plasma etch using Ar/Cl 2 gases. The ohmic contacts were formed by alloying the Ti/Al/Ni/Au (25/125/45/55 nm) metal stack at 875 ◦ C in nitrogen ambient for 50 s, while the Schottky contact was formed by as-deposited 0741-3106/$26.00 © 2011 IEEE