Normally Off-Mode AlGaN/GaN Heterostructure Field Effect Transistor Using P-Type Gate Contact
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Normally Off-Mode AlGaN/GaN Heterostructure Field Effect Transistor Using P-Type Gate Contact Norio TSUYUKUCHI, Kentaro NAGAMATSU, Yoshikazu HIROSE, Motoaki IWAYA, Satoshi KAMIYMA, Hiroshi AMANO and Isamu AKASAKI Faculty of Science and Technology, 21st Century COE Program “Nano-Factory”, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan ABSTRACT A normally off-mode AlGaN/GaN heterostructure field effect transistor (HFET) using a p-type GaN gate was fabricated and their static properties were compared with those of HFET having a Schottky gate. HFET having a p-GaN gate contact shows a very low leakage current density of 18.2 µA/mm at VGS and VDS of 0 V and 20 V, respectively. INTRODUCTION GaN-based electron devices have received much attention for high-power electronics applications due to the high breakdown electric field and high electron-saturation velocity of the materials. The breakdown electric field and the electron saturation velocity of GaN are about 10 times and 3 times larger than those of Si, respectively [1]. In high-power applications such as an inverter system, normally off-mode devices are strongly desired because of their circuit simplicity and low power consumption. However, most GaN-based field effect transistors (FETs) have been fabricated as depletion-mode devices, or in other words, normally on-mode devices, due to the high-density 2D electron gas caused by the polarization field. Enhancement-mode or normally off-mode devices using the Schottky contact have been realized [2,3], but the sweeping range of gate voltage (VGS) in those devices is limited to below +2 V due to the serious leakage currents through the Schottky gate. The Al2O3 insulated-gate AlGaN/GaN heterostructure FET (HFET) can control the channel current up to VGS=+4 V [4]. However, this is a quasi normally off-mode device due to the difficulty in increasing built-in voltage. On the other hand, a pn junction induces high built-in voltage of over 3 V. Hu et al. fabricated an AlGaN/GaN HFET by selectively growing p-GaN as a gate [5], thus forming a pn junction gate, and reported the on voltage of +3 V. However, leakage current at zero gate-source bias was not negligibly small. In order to achieve low-power-consumption circuits, reduction of the leakage current at zero gate-source bias is necessary. Regrowth of p-GaN may induce a leakage pass at the interface between the AlGaN barrier and p-GaN gate, thus leading to a leakage current at zero gate bias [6]. In this study, we fabricated an AlGaN/GaN HFET with a p-GaN gate contact without using
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any regrowth or selective growth techniques. Compared to the HFET with a Schottky gate contact, this device shows the normally off-mode with an extremely low leakage drain current (ID) at VGS=0 V. The maximum ID and the maximum transconductance (gm) were the highest in the pn junction gate HFET. EXPERIMENTS We simulated the device structure with a p-GaN gate contact using SiLENSe 1.0. The key factors in operating the device in the normally of
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