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 @ 1 A/cm2) in AlGaN/GaN HEMTs.

Later, in the work, we tried to study the step-graded AlGaN transition layers (TL) in the HEMT stack to improve the breakdown voltage. The transition layers include three AlGaN epi-layers of 75%, 50%, and 25% Al-content, down-graded from bottom to top. The growth temperature and carbon doping are varied independently to assess the transition layer's role in leakage current. The introduction of C-doping in the top AlGaN transition layer with 25% Al-content improves lateral breakdown voltage in both mesa and 3-terminal configurations. The combination of HT AlGaN (75% Al-content) with C-doped AlGaN (25% Al-content) is found to be the optimal TL design.

After optimizing the stack for low off-state leakage current, we focused on evaluating the on-state performance of the AlGaN/GaN HEMT. In GaN HEMT devices, dynamic RON is considered the most crucial issue in high voltage switching applications. Dynamic RON is a phenomenon in which the on-resistance (RON) of the device increases under high voltage switching conditions. This chapter tried to analyze the effect of growth variations like carbon doping and high growth temperatures on dynamic RON.

A subtle balance between HEMT epitaxial stack and device design is crucial to achieving a high breakdown voltage in AlGaN/GaN HEMT. One of the main hurdles in the device design is surface passivation. In this work, we have investigated the material properties of PECVD (Plasma Enhanced Chemical Vapor Deposition) deposited amorphous SiN films and their influence on leakage current of AlGaN/GaN HEMT grown on Si. The device architecture is further improved by incorporating gate field plate design using the optimized SiN to enhance the breakdown voltage by distributing an electric field in the gate to the drain access region.

In conclusion, we propose epitaxial stack design guidelines to achieve high breakdown voltage with low dynamic RON. The optimized epitaxial stack of 1.65 µm has the potential to be converted as a cost-effective technology for high voltage applications (400 V), which is in demand in the EV industry.