Impacts of eliminating ungated access regions on DC and thermal performances of GaN-based MIS-HEMT

Abstract

Impacts of eliminating access regions on DC and thermal performances of GaN-based MIS-HEMT have been studied using a device of 4-μm gate length. Nominal absence of access regions was achieved by MOCVD-regrown heavily n-doped GaN contact region extended to the 2DEG channel (sheet resistance as low as 14 Ω/□ and ohmic contact resistance of 0.20 Ω·mm) and 22-nm-thick AlN/Al2O3/HfO2 insulator layers acting as both gate dielectrics and sidewall spacers. As a result, a low knee voltage (2.5 V @ VGS = + 2 V) comparable to deeply-scaled devices was attained, revealing the dominant role of access regions in knee voltages. High linearity at lower supply voltages (gate voltage swing of 7.3 V @ VDS = 5 V) and a faster gate voltage swing saturation trend with VDS increasing was observed benefiting from the improved utility of applied lateral voltage. Moreover, a much lower thermal resistance compared with that of the conventional MIS-HEMT structure (146 vs. 202 K/W) was extracted by a static-pulsed I-V measurement method. Simplified TCAD simulations were conducted to explain the underlying mechanisms, demonstrating that the enhanced surface heat flow covered by gate metal as well as the more uniform electric field along the 2DEG channel accounts for the better capability of heat management in the device free of access regions. Our results indicate how much enhancements in terms of DC and thermal performances can be obtained by eliminating access regions in a GaN-based MIS-HEMT structure.