Correlation between sidewall surface states and off-state breakdown voltage of AlGaN/GaN HFETs

Abstract

Correlation between the sidewall surface states and off-state breakdown voltage of AlGaN/GaN heterojunction field effect transistors (HFETs) is investigated for the first time. HFETs explored in this work were realized on a variety of isolation features including conventional mesa, non-slanted fin, and slanted fin. The output and transfer characteristics of the devices from all categories of the fabricated AlGaN/GaN HFETs were studied, and a link between the separation of isolation feature sidewalls in the drain access region and the breakdown voltage was observed. Simulation results showed that by shrinking the width of the isolation feature geometry, the peak of the electric field at the drain edge of the gate is reduced as a result of tailoring its profile when a more resistive path is imposed on the drain access region. While HFETs realized on fins of smaller width benefit more from the depleting effect of acceptor sidewall surface states and consequently a higher off-state breakdown voltage, they suffer from a lower current density in the on-state. The slanted fin isolation feature geometry that we proposed here, while maintaining high breakdown voltage in the off-state, reduces the resistance in the on-state, which is represented by its highest Baliga's figure of merit among the three categories of isolation feature geometries. The proposed solution for achieving an improvement to the off-state breakdown voltage of AlGaN/GaN HFETs relies on a technology that has already been explored as a successful alternative for the realization of enhancement-mode transistors (i.e., with positive threshold voltage).