Normally-off operation GaN HFET using a thin AlGaN layer for low loss switching devices
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Normally-off operation GaN HFET using a thin AlGaN layer for low loss switching devices Nariaki Ikeda, Kazuo Kato, Jiang Li, Kohji Hataya, Seikoh Yoshida Yokohama R&D Laboratories, The Furukawa Electric Co., Ltd. 2-4-3 Okano, Nishi-ku, Yokohama, Kanagawa 220-0073, Japan ABSTRACT We report on the novel normally-off GaN-based heterojunction field effect transistors (HFETs) on a Si substrate. The AlGaN/AlN/GaN heterostructure was grown using a metalorganic chemical vapor deposition (MOCVD). The HFET for a normally-off operation was fabricated using a precisely controlled thin-AlGaN layer as an electron supply layer. As a result, the HFET was operated at the condition of the positive gate bias. We also characterized the enlarged gate-width devices. The breakdown voltage of FET was over 300 V. A normally-off operation using GaN based HFETs with a thin-AlGaN/AlN/GaN heterostructure on the silicon substrate were thus confirmed for the first time. INTRODUCTION Nitride-based semiconductor devices are attractive due to excellent performances as well as SiC compared with conventional Si devices. Since GaN has an excellent figure of merits for the operation under high-power, high-frequency, and high-temperature conditions, GaN-based field effect transistors (FETs) have been developed [1-4]. Especially, the specific on-state resistance (Ron) of the FET is expected to be lower than that of Si based FET. That is, GaN-based electronic devices enable us to reduce the power loss of switching devices, such as inverters or converters, compared with that of using conventional Si devices, and consequently to reduce the cooling system. Since a GaN based FET is also expected to have a higher switching speed, and a high frequency operation, a high-efficiency power supply circuit can be realized by using GaN based devices. We have already demonstrated an inverter circuit using normally-on AlGaN/GaN HFETs on a sapphire substrate. This inverter was composed of a DC converter and an AC inverter using normally-on GaN based HFETs [5]. A normally-off FET is necessary for switching devices because of simplification of the conventional circuit of power electronics in order to maintain a fail safe system. However,
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normally-off operation using GaN based HFETs is difficult to be realized. One candidate for a normally-off operation, is the enhancement mode GaN based HFET using an isolated gate structure. However, in the case of nitride-based FETs, it was very difficult to fabricate the isolated gate structure in the same manner as the Si-MOSFETs, owing to a high quality gate insulator could not be formed. Another candidate is utilizing a thin AlGaN beneath the gate electrode in order to obtain a shallow depletion region. It was also reported enhancement-mode HFETs using a recess gate structure [6]. However this reports was incompletely normally-off operation because the drain current slightly flowed at Vgs = 0 V. We have demonstrated the normally-off HFET using a C-doped GaN as a channel layer [7]. The operation mechanism is that the c
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