Current Collapse and High-Electric-Field Reliability of Unpassivated GaN/AlGaN/GaN HEMTs

Long-term on-state and off-state high-electric-field stress results are presented for unpassivated GaN/AlGaN/GaN HEMTs on SiC substrates. Thanks to the thin GaN cap layer, devices show minimal current-collapse effects prior to high-electric-field stress, despite they are not passivated. This comes at the price of a relatively-high gate-leakage current. Under the assumption that donor-like electron traps are present within the GaN cap, two-dimensional numerical device simulations provide an explanation for the influence of the GaN cap layer on current collapse and for the correlation between the latter and the gate-leakage current. Both on-state and off-state stresses produce simultaneouscurrent-collapse increase and gate-leakage-current decrease, which can be interpreted to be the result of gate-drainsurfacedegradation and reduced gate electron injection. This study shows that, although the thin GaN cap layer is effective in suppressing surface-related dispersion effects in virgin devices, it does not, per se, protect the device from high-electric-field degradation and it should, to this aim, be adopted in conjunction with other technological solutions like surface passivation, pre-passivation surface treatments and/or field-plate gate.