Analysis of threshold voltage instability in AlGaN/GaN MISHEMTs by forward gate voltage stress pulses

We report on the investigation of the Vth drift behaviour of AlGaN/GaN MISHEMTs upon forward gate voltage stress in dependence of stress bias and stress time. The pulsed measurements allow for the evaluation of the operational regime for optimum device efficiency. We compared the effect of two different high-κ gate dielectric materials with similar equivalent oxide thickness ϵ0ϵr/thigh-κ on the Vth instability in order to separate the influence of the heterojunction design and the high-κ/GaN-cap interface from the bulk high-κ. The matched gate capacitance coupling of the studied Al2O3 and HfO2 gate dielectric results in an nearly identical critical forward gate voltage, where the AlGaN barrier potential is lowered and severe threshold voltage shift (ΔVth) into the positive voltage direction is induced. Beyond this critical forward voltage, detailed time-dependent stress pulse measurements from 1 µs to 1000 s revealed an immediate electron injection and trapping at the oxide/GaN interface for stress pulses with tstress ≥ 1 µs. The presented results of Vth drift analysis demonstrate the limits of the maximum tolerable forward gate voltage of the investigated Al2O3 and HfO2 MISHEMTs, although the excellent low-leakage currents of the insulated gate would imply a potentially higher gate-overdrive.