Impact of P-GaN ohmic Contact Resistivity on Switching Time of GaN Super-Heterojunction FETs

Gallium nitride (GaN) Super-Heterojunction FET (SHJ-FET) utilizes a charge-balanced PN heterojunction, instead of a field-plate structure, to manage the E-field profile between the gate and the drain [1]. GaN SHJ-MOSFETs with a blocking voltage of 10 kV and an R_DS,ON C_O(tr) figure-of-merit of approximately 4.9 ps have been reported, indicating potential for efficient and fast switching medium voltage power electronics [2]. It relies on a source-connected p-GaN contact to enable hole injection and depletion during switching. One concern about the prospect of the SHJ FET is that the switching time could be limited by the low doping efficiency and low hole mobility in the P-GaN. Previous work [3] revealed that the turn-on time has a quadratic dependence on the SHJ length, reaching μs-range for 10-kV-class device designs. For lower-voltage applications, e.g. 48V-1V point-of-load DC-DC converters, the SHJ length will be scaled down substantially from ~80 μm to 3 μm or less. Contact resistance to the p-GaN is expected to play a more important role in the switching time [4]. Through physics-based and mixed-mode TCAD simulations, this paper reveals the impact of p-GaN ohmic contact resistivity on switching time.