Effect of trapping on the critical voltage for degradation in GaN high electron mobility transistors

We have performed V_DS = 0 V and OFF-state step-stress experiments on GaN-on-Si and GaN-on-SiC high electron mobility transistors under UV illumination and in the dark. We have found that for both stress conditions, UV illumination decreases the critical voltage for the onset of degradation in gate current in GaN-on-Si HEMTs in a pronounced way, but no such decrease is observed on SiC. This difference is attributed to UV-induced electron detrapping, which results in an increase in the electric field and, through the inverse piezoelectric effect, in the mechanical stress in the AlGaN barrier of the device. Due to the large number of traps in GaN-on-Si, this effect is clearer and more prominent than in GaN-on-SiC, which contains fewer traps in the fresh state.