Positive temperature dependence of time-dependent breakdown of GaN-on-Si E-mode HEMTs under positive gate stress

We report an extensive analysis of the time-dependent breakdown of E-mode GaN-on-Si power HEMTs subjected to positive gate stress and demonstrate that TTF (time-to-failure) has a positive temperature dependence. The analyzed devices have a p-type gate; large (60A) power transistors were subjected to positive stress at different voltages and temperatures ranging from 25 degrees C to 250 degrees C. The original results collected in this paper demonstrate that (i) constant voltage stress induces a gradual decrease in gate leakage current, which is ascribed to hole-trapping in Mg-acceptors located at the p-GaN/AlGaN interface; (ii) this trapping process becomes less pronounced at high temperatures, due to a faster detrapping from the shallow Mg-related traps; (iii) the devices stressed at the same voltage and different temperatures show a similar initial gate leakage, due to the fact that in the analyzed regime, gate conduction is dominated by Fowler-Nordheim tunneling, a field-related, rather than thermally activated, process; (iv) the time-to-failure has a positive temperature dependence. The latter result is explained by considering that time-dependent breakdown occurs due to the accumulation of positive charges at the p-GaN/AlGaN interface, which results in an increased injection of 2DEG electrons into the p-GaN layer, where they are accelerated by the electric field. High temperatures favor the detrapping of holes, thus reducing this process and leading to a better stability.