Investigation of Buffer Design and Carbon doping in AlGaN GaN HEMTs for High Breakdown Voltages

Abstract

III-nitride HEMTs are strong contenders for next-generation power electronic applications. The superior material and electrical properties render GaN-based transistors suitable for high-power switching. The material characteristics such as high breakdown voltage, high electron mobility, and high operating temperature make GaN score over Si. Further, low ON resistance and high switching speed responsible for the subsequent reduction in both switching and ON/OFF state losses render GaN-based HEMT a foreboding device for power electronic applications. In this Doctoral Dissertation, we investigate the impact of epitaxial stack design and transistor architecture on the breakdown voltage characteristics and dynamic performances of AlGaN/GaN HEMT for power switching applications. The focus of the thesis starts with understanding the effect of threading dislocations in vertical leakage. This study discusses the mechanism of dislocation-mediated vertical leakage in MOCVD-grown Carbon (C)-doped AlGaN/GaN HEMTs on a 6-inch silicon wafer. Substrate bias polarity-dependent I-Vs, temperature-dependent fitting, and band diagram analysis pointed to the Poole-Frenkel type of conduction mechanism for vertical transport in the devices. We propose that higher dislocation density leads to shallower traps in the buffer and build an analytical model of dislocation-mediated vertical leakage around this. Based on the above evidence, vertical leakage can be reduced in an epitaxial HEMT stack with a higher dislocation density by introducing carbon. It reduces the unintentional doping in GaN, which acts as a carrier, conducted through the dislocation mediated leakage paths. Initially, we tried to incorporate carbon in GaN buffer by reducing the growth temperature, i.e., by auto doping and using CBr4 as an external source. The later part of the chapter reports on the experimental and analytical determination of the optimum carbon concentration in GaN to achieve enhanced breakdown voltage (voltage at 1 A/cm2) in AlGaN/GaN HEMTs...