Multiple Defects Cause Degradation After High Field Stress in AlGaN/GaN HEMTs

Gate and drain bias dependences of hot carrier degradation are evaluated for AlGaN/GaN HEMTs fabricated via two different process methods. Both positive and negative threshold voltage Vth shifts are observed for each device type, depending on the mode and duration of the stress, indicating the presence of significant densities of donor-like and acceptor-like traps. Worst-case stress bias for transconductance degradation is the ON state for both device types. We find that transconductance degradation provides a more effective parameter to monitor defect buildup than Vth shifts, and that a single worst-case stressing bias condition cannot be defined for all varieties of AlGaN/GaN HEMTs. Low-frequency noise measurements versus temperature assist the identification of defects responsible for the observed degradation. Defect dehydrogenation and oxygen impurity centers are found to be particularly significant to the response of these devices.