Polarization doping modulated heterojunction electron gas in AlGaN/GaN CAVETs

Abstract

A common AlGaN/GaN current-aperture vertical effect transistor (CAVET) with a SiO2 current blocking layer on the GaN substrate is compared to two similar structures with the stepped and linearly graded AlGaN barrier layer, respectively. The approach resulted in high threshold voltages (Vth) of −2.6 V and −3.6 V, compared to Vth = −4.4 V for the common device. And the breakdown voltage of two modified CAVETs was increased from 541 V to 711 V and 613 V, respectively. This reveals the great potential of polarization doping for fabricating enhancement-mode and high-voltage power transistors. A mechanism accounting for the improvement in the device performance by modulating the heterojunction electron gas (HEG) is presented. Meanwhile the stepped graded AlGaN is discovered to be better than the linearly graded AlGaN in modulating the HEG. Furthermore, a trench structure is involved in the AlGaN/GaN CAVET with the stepped graded AlGaN in order to obtain an enhancement-mode device. A positive threshold voltage of 0.6 V and a breakdown voltage exceeding 800 V are demonstrated.