Electrical overstress in AlGaN/GaN HEMTs:: study of degradation processes

We study degradation mechanisms in 50 mum gate width/0.45 mum length AlGaN/GaN HEMTs after electrical overstresses. One hundred nanosecond long rectangular current pulses are applied on the drain contact keeping either both of the source and gate grounded or the source grounded and gate floating. Source-drain pulsed I-V characteristics show similar shape for both connections. After the HEMT undergoes the source-drain breakdown, a negative differential resistance region transits into a low voltage/high current region. Changes in the Schottky contact de I-V characteristics and in the source and drain ohmic contacts are investigated as a function of the current stress level and are related to the HEMT dc performance. Catastrophic HEMT degradation was observed after I-stress = 1.65 A in case of the 'gate floating' connection due to ohmic contacts burnout. In case of the 'gate grounded' connection, I-stress = 0.45 A was sufficient for the gate failure showing a high gate susceptibility to overstress. Backside transient interferometric mapping technique experiment reveals a current filament formation under both HEMT stress connections. Infrared camera observations lead to conclusion that the filament formation together with a consequent high-density electron flow is responsible for a dark spot formation and gradual ohmic contact degradation. (C) 2003 Elsevier Ltd. All rights reserved.