AlGaN/GaN HFETs and Insulated Gate HFETs DC and RF stability
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AlGaN/GaN HFETs and Insulated Gate HFETs DC and RF stability Alexei Koudymov, Salih Saygi, Naveen Tipirneni, Grigory Simin, Vinod Adivarahan, Jinwei Yang, and M. Asif Khan, Department of Electrical Engineering, University of South Carolina, Columbia, South Carolina, 29208, USA; e-mail: [email protected] ABSTRACT The comparative study of the DC parameters and RF power stability of nitride-based conventional HFETs and Metal-Oxide-Semiconductor HFETs (MOSHFETs) is presented. The average lifetime under DC stress is estimated to be as high as 2.5 years at room temperature. Under the large-signal RF stress, the gate leakage current of conventional HFETs increases significantly with time showing faster degradation as compared to DC stressing. MOSHFETs demonstrate superior RF performance stability, which perfectly correlates with the DC stability data. It is shown that in conventional HFETs, the combination of the self-heating and positive dynamic gate bias leads to the defect accumulation in the AlGaN barrier under the gate. In MOSHFETs, the absence of the gate leakage is a key to stable RF performance. INTRODUCTION Recent efforts in developing high power AlGaN/GaN Heterostructure field effect transistors (HFETs) [1,2,3] have led to a significant progress in improving material quality of these devices. High output RF powers in excess of 10 W/mm at relatively low (40-50 V) drain biases have been demonstrated by several groups using field plate [4] or gate recessing [5]. The gate leakage currents are known to be one of the major factors limiting the RF performance [6]. Several approaches for the gate isolation were reported utilizing SiO2, Si3N4 and other thin dielectric films [7, 8, 9]. In this paper, we present a comparative study of the performance stability of conventional Schottky gate AlGaN/GaN HFETs and SiO2/AlGaN/GaN insulated gate HFETs (MOSHFETs). Both AlGaN/GaN HFETs and MOSHFETs demonstrate decent DC stability for over 1000 hours at room temperature. The life time of the MOSHFETs under the DC bias was found to be around 2.5 years as extracted from the accelerated testing at elevated temperatures (200 – 400 °C) [10]. For the HFETs this extraction was not possible due to rapid increase of the gate leakage currents at high temperatures. It was also shown that the stability of the MOSHFETs under the RF stress correlates with its DC stability [10]. At the same time, the RF performance of the HFETs was found to degrade with time rapidly, allowing the record high output powers to be observed for only several hours or even several minutes. In this paper, we analyze the mechanisms responsible for the RF power degradation of AlGaN/GaN HFETs and report the improved RF stability for the insulated gate devices. EXPERIMENTAL DETAILS The HFET epilayers structures were grown over insulating SiC substrates using the lowpressure metal organic chemical vapor deposition (MOCVD) technique. The Ti-Al-Ti-Au ohmic contacts were annealed at 850 °C for 1 minute in nitrogen ambient. In order to fabricate the
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MOSHFETs,
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