Reliability Physics of GaN HEMT Microwave Devices: The Age of Scaling
This paper reviews failure modes and mechanisms of 0.5 mm, 0.25 mm and 0.15 mm AlGaN/GaN HEMTs for microwave and millimeter-wave applications. Early devices adopting Ni/Pt/Au metallization were found to be affected by sidewall interdiffusion of Au and O, followed by electrochemical oxidation of AlGaN, a problem which was solved by adopting a new metallization and passivation scheme providing 0.25 mm devices capable of withstanding 24h at V DS = 60V, on-state, T ch = 375°C with no failure. 4000 h long-term thermal storage tests with no bias identified a non-monotonic behaviour of gate Schottky barrier height, causing a temporary increase of gate leakage current which presented no risk for device reliability. Beside contact-related degradation mechanisms, hot electron effects become increasingly more relevant during DC life tests (inducing a 10% increase of on-resistance), rf tests (creating or re-activating deep levels, which increase current-collapse and reduce rf output power and gain). RF tests are the harshest ones for hot-electron degradation, which represents the limiting factor for GaN HEMTs having L G < 0.15 mm.