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Threshold Voltage Instability in GaN HEMTs With p-Type Gate: Mg Doping Compensation

In this letter, we present an analysis of the threshold voltage shift induced by positive bias temperature instability stress in GaN-based power HEMTs with p-type gate, controlled by a Schottky metal/p-GaN junction. In particular, we show the positive effect of the magnesium compensation process in the p-GaN layer on the long-term threshold voltage instability. When a relatively high positive bias is applied to the gate (Schottky junction reverse-biased), holes are generated by impact ionization in the high-field depleted p-GaN region, then accelerated toward the AlGaN layer. The high-energy holes, combined with the high temperature effects, create defects in the AlGaN or at its interface with p-GaN, causing a long-term positive threshold voltage shift. A process variation in the p-GaN layer is introduced which promotes a wider depletion region near the Schottky interface with the metal, lowering the electric field and reducing the generation of holes due to impact ionization. As a result, the long-term threshold voltage instability is improved without altering the dc transistor parameters, such as threshold voltage, trans-conductance, and subthreshold slope.