Polarization engineering in GaN power transistors

Abstract

This paper reviews novel design of epitaxial structure for GaN power transistors to reduce the on-state resistance and to increase the breakdown voltage. It takes advantages of the fixed charges at the interfaces caused by the material's inherent polarization, and thus it can be called as polarization engineering. Both AlGaN/GaN superlattices and polarization-matched InAlGaN quaternary alloy effectively reduce the on-state resistance by using them as capping layers over AlGaN/GaN combined with the recessed-gate structure. These capping layers successfully eliminate the electron depletion which is caused by the balancing of the fixed charges. The series resistance across the interface is fully reduced so that these capping layers enable low on-state resistance. Proposed natural super junction (NSJ) model well explains the fact that the breakdown voltages of AlGaN/GaN hetero-junction field effect transistors (HFETs) are increased by the extension of the gate and drain spacing. The model assumes that the hetero-junction can act an insulator since the polarization induced charges are always balanced at the top and the bottom surfaces. Taking advantages of the model, the fabricated multi-channel AlGaN/GaN diode with dual-recessed structure exhibits high breakdown voltage of 9400 V with low on-state resistance of 52 mΩ cm2, which reaches the predicted performance limit of GaN. The presented polarization engineering is very promising for future improvement of the device performances.