Off-state breakdown in power pHEMTs: the impact of the source

Conventional wisdom suggests that in pseudomorphic high electron mobility transistors (pHEMTs), 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 two 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 pHEMTs, 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 L_G is the gate length and x_D 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