A New Post Annealing Method for AlGaN/GaN Heterostructure Field-Effect Transistors Employing XeCl Excimer Laser Pulses
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E9.2.1
A New Post Annealing Method for AlGaN/GaN Heterostructure Field-Effect Transistors Employing XeCl Excimer Laser Pulses Min-Woo Ha, Seung-Chul Lee, Joong-Hyun Park, Kwang-Seok Seo, and Min-Koo Han School of Electrical Engineering #50, Seoul National University, Shinlim-dong, Gwanak-gu, Seoul 151-742, Korea Tel.:+82-2-880-7254, Fax:+82-2-875-7254, E-Mail: [email protected] ABSTRACT A new post annealing method employing excimer laser pulses is proposed to improve the transfer characteristics and the breakdown voltage of the unpassivated AlGaN/GaN heterostructure field-effect transistor (HFET) and the passivated one. The XeCl excimer laser pulses with wavelength of 308 nm anneal the AlGaN/GaN HFET after the Schottky gate metallization. The interface defects between the Schottky gate metal and a GaN layer is decreased by the lateral heat diffusion of the laser pulses. Our experimental results show that the drain current and the maximum transconductance of the unpassivated AlGaN/GaN HFET after laser pulses annealing are 496 mA/mm and 134 mS/mm while a virgin device shows 434 mA/mm and 113 mS/mm, respectively. The proposed method anneals effectively the SiO2 passivated AlGaN/GaN HFET and the leakage current of the passivated device is decreased from 483 nA to 29 nA. INTRODUCTION Wide band gap semiconductor, GaN may be promising for the microwave applications and the high voltage switching devices due to a high breakdown field (> 3 MV/cm) and a low intrinsic carrier generation [1]. Various AlGaN/GaN heterostructure field-effect transistors (HFETs) have been reported for high voltage switching devices due to a high sheet carrier density (1013 /cm2) and a high electron mobility (1500 cm2/Vs) [2-3]. The decreasing power loss of the AlGaN/GaN HFET is a critical factor for practical applications and that is determined by the on-state voltage drops and the leakage current. A conventional post annealing method, such as the thermal annealing may be useful to decrease the power loss of the AlGaN/GaN HFET [47]. When the AlGaN/GaN HFET is stressed more than optimized thermal annealing condition, the vertical diffusion of the Schottky gate metal into the GaN layer affects the strain and surface states of the AlGaN. That can decrease the polarization induced-charges and degrade the forward characteristics of the device [7].
E9.2.2
The purpose of this study is to propose a new post annealing method employing XeCl excimer laser pulses to suppress the leakage current and increase the drain current along with the breakdown voltage of the AlGaN/GaN HFET without degrading the other electric characteristics. The proposed method is rather simple to implement and the thermal stress of the device is reduced due to the instant duration time of the excimer laser pulses. FABRICATION AlGaN/GaN hetero structure was grown on a C-plane sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The epitaxial layer structure consisted of a 3 µm unintentionally doped (UID) GaN layer grown on an AlN nucleation layer, a 22 nm dope
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