This paper outlines the systematic isolation of an electrostatic discharge defect on a depletion-mode FET. Topics covered are fault isolation, FIB-STEM cross-section and EDS analysis, and defect simulation. Multiple GaAs PA devices were submitted for analysis after failing different reliability stresses. Fault isolation revealed ESD damage on a DFET connected to the VMODE0 pin. Simulation of the failure showed that, most likely, the defect was caused by CDM stress. A design change of inserting a resistor between the VMODE0 pin and the DFET made the device more robust against CDM stress.