Switching Transient Analysis for Normally-off GaN Transistor With p-GaN Gate in a Phase-Leg Circuit

Commercially available normally-off GaN power high-electron-mobility transistor (HEMT) devices have typically adopted a p-GaN gate structure. In the gate region, there exist a Schottky junction (between gate electrode and the p-GaN layer) and a p-GaN/AlGaN/GaN heterojunction. As the p-GaN layer is not directly shorted to the gate electrode and conducting channel, it can be considered as electrically 'floating.' With drain voltage changing during a switching process, the variation of net charge in the floating p-GaN layer would cause instability in threshold voltage. Besides, because of the distinctive features of the p-GaN gate HEMT structure under certain gate/drain bias, gate-related junction capacitances would exhibit behavior different from those of a Si mosfet. Consequently, the switching transient performance of a GaN transistor with a p-GaN gate could be significantly influenced by the aforementioned factors. In this work, the threshold voltage instability associated with the drain-to-gate voltage stress is first analyzed. Despite the difficulty in directly performing capacitance measurements inside the device structure, a hybrid physical-behavior modeling method is proposed. The model is capable of extracting the capacitance bias relationships with regard to the gate region from static terminal measurements. In previous works, the advanced analytical model would make modest change on a Si mosfet's model. As a result, without fully considering the specific features of a GaN transistor with the p-GaN gate, the simulated waveforms would exhibit 20-50% discrepancy from the experiment. In contrast, the proposed switching transient analytical approach would exhibit improved accuracy (<10% disparity). Consequently, the switching transient performance of the GaN transistor with the p-GaN gate could be more accurately evaluated.