Analysis of Leakage Currents in AlGaN/GaN Current Aperture Vertical Electron Transistors (CAVETs)
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Analysis of Leakage Currents in AlGaN/GaN Current Aperture Vertical Electron Transistors (CAVETs) Ilan Ben-Yaacov, Yee-Kwang Seck, Steven P. DenBaars, Evelyn L. Hu, and Umesh K. Mishra ECE Department, University of California Santa Barbara, CA 93106-9560, U.S.A. ABSTRACT A complete analysis of leakage currents in AlGaN/GaN Current Aperture Vertical Electron Transistors (CAVETs) with regrown aperture and source regions was carried out. The total observed leakage current was found to be a combination of both gate leakage and source leakage. Two paths for source leakage have been identified; electrons passing directly through the insulating layer to the drain region as well as electrons traveling through the aperture but underneath the 2DEG at the AlGaN/GaN interface. Source leakage through the insulating layer resulted from pits formed at the onset of regrowth, as the sample was heated to growth temperature, and was successfully eliminated by optimizing regrowth conditions. Source leakage underneath the 2DEG occurred when the unintentionally doped (UID) GaN layer above the insulating layer was not fully depleted and could be eliminated by reducing the thickness of the UID GaN layer. Gate leakage has been attributed to the enhanced incorporation of n-type impurities inside as well as above the aperture region during regrowth, resulting in a narrowing of the gate Schottky barrier.
INTRODUCTION During the past few years, enormous progress has been made in the development of IIInitride semiconductor materials for high voltage, high power, and high temperature electronics applications [1-3]. High breakdown field strength in GaN (2 MV/cm [4]) permits very high voltages to be sustained during operation of these devices. In HEMTs, breakdown results from an avalanche process that usually occurs near the gate edge on the drain side [5], where accumulation of charge at high gate-drain voltages results in large localized electric fields. Achieving a high breakdown voltage in a HEMT requires decreasing the electric field at the surface of the channel at the drain edge of the gate. In GaN HEMTs, this has been accomplished by Zhang et al. with the employment of an insulated gate structure, and source-drain breakdown voltages of over 1 kV have been achieved [6]. Another approach to achieving large breakdown voltages is to employ a current aperture vertical electron transistor (CAVET) structure. As illustrated in figure 3c, a CAVET consists of a source region separated from a drain region by an insulating layer containing a narrow aperture that is filled with conducting material. A device mesa is formed by reactive ion etching, and source contacts are deposited on either side of the aperture. The drain metal contacts the n-doped region below the aperture. Electrons flow from the source contacts through the aperture into the n-type GaN and are collected at the drain. A Schottky gate, located directly above the aperture, is used to modulate the current passing through the aperture. In a CAVET, because the virtual drain (or the
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