Reliability and high field related issues in GaN-HEMT devices; Part I

In recent years, the number of published research studies on GaN HEMT reliability has increased exponentially, extrapolated mean time to failure values in excess of 106 hours have been claimed; however, failure mechanisms remain partially unknown, making extrapolation (largely based on Arrhenius-like models) questionable. Identified failure mechanisms include time-dependent gate leakage increase during reverse bias tests, hot-electron-induced drain current degradation, gate and ohmic contact degradation, delamination of passivation, electron trapping. This tutorial will present an overview of the main reliability issues in Gallium Nitride HEMTs, including a survey of DC and pulsed electrical characterization techniques, current collapse and other dispersion effects, accelerated testing methods and failure analysis. The effects of surface, interface and bulk traps will be discussed, on the basis of experimental data obtained by means of pulsed measurements, Deep Level Transient Spectroscopy, photocurrent spectroscopy and of 2D device simulation results. Failure modes and mechanisms of GaN HEMTs will be critically reviewed, on the basis of the experience gained within various European projects.