Implications of Various Charge Sources in AlGaN/GaN Epi-Stack on the Drain & Gate Connected Field Plate Design in HEMTs

We have established that a design strategy for drain and gate connected field plates should be adopted while keeping in mind the interplay of various charge sources across AlGaN/GaN epi-stack, which governs the electric field distribution across HEMT. The investigations in this work are carried out for Schottky, MIS and p-GaN gate stacks while accounting for possible GaN buffer types (Fe-doped and C-doped). The role of gate and drain field plates was found to be different in the Fe-doped buffer compared to the C-doped buffer. More than suppression of avalanche generation, mitigation of gate injection by shifting the peak electric field position away from the gate edge was found to be the dominant cause of breakdown voltage improvement when field plates were adopted. In a few cases, however, the widening of the depletion region near the gate or dominance of the buffer field was the reason for breakdown voltage improvement with a gate field plate. On the other hand, the drain field plate was found to be effective only for lower polarization % and lower surface trap concentration. The role of buffer trap parameters, surface/passivation trap concentration, interface trap concentration at the gate, and passivation thickness in defining the optimum field plate strategy are discussed.