Off-state breakdown in power pHEMT's: The impact of the source

Conventional wisdom suggests that in pseudomorphic high electron mobility transistors (pHEMT's), the field between the drain and the gate determines off-state breakdown, and that the drain to gate voltage therefore sets the breakdown voltage of the device. Thus, the tno terminal breakdown voltage is a widely used figure of merit, and most models for breakdown focus on the depletion region in the gate-drain gap, while altogether ignoring the source. We present extensive new measurements and simulations that demonstrate that for power pHEMT's, the electrostatic interaction of the source seriously degrades the device's gate-drain breakdown. We identify the key aspect ratio that controls the effect, L(G) :x(D), where LG is the gate length and to is the depletion region length on the drain. This work establishes that the design of the source must be taken into consideration in the engineering of high-power pHEMT's.