2D Device Simulation of AlGaN/GaN HFET Current Collapse Caused by Surface Negative Charge Injection

Abstract

Drain collapse in AlGaN/GaN HFET is analyzed using a two-dimensional device simulator. Two-step saturation is obtained, assuming hole-trap type surface states on the AlGaN surface and a short negative-charge-injected region at the drain side of the gate. Due to the surface electric potential pinning by the surface traps, the negative charge injected region forms a constant potential like in a metal gate region and it acts as an FET with a virtual gate. The electron concentration profile reveals that the first saturation occurs by pinch-off in the virtual gate region and the second saturation occurs by the pinch-off in the metal gate region. Due to the short-channel effect of the virtual gate FET, the saturation current increases until it finally reaches the saturation current of the intrinsic metal gate FET. Current collapses with current degradation at the knee voltage in the I-V characteristics can be explained by the formation of the virtual gate.