Electrical continuity tests suggested that the gate was shorted to the drain and the source was intact. SEM analysis revealed a suspected damage site at the end of a gate finger (see below).
An elemental dot map at the breakdown site showed some slight disturbance of the aluminum metallization at the site.
Voltage contrast imaging confirmed that the suspect damage site was indeed the location of the short.
These results suggested that the device failed due to a voltage transient on the gate signal. The small dimensions of the damage site suggest that the transient was very fast (rough estimate 1E-9 to 1E-6 seconds). The damage could potentially be related to an ESD event. It could also potentially be caused by accumulated damage at the tips of the gate contacts due to excessive turn-on or turn-off dV/dt.
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