Power Semiconductor Devices and Technologies for High Power Density and Efficiency Converters

The emergence of wide bandgap power semiconductor device technologies has paved the path toward higher power densities and efficiency across a broad spectrum of applications. One of the leading device candidates is gallium nitride (GaN) based high electron mobility transistor (HEMT). The ability to grow high quality GaN epitaxial layers on inexpensive large diameter foreign wafers such as sapphire or silicon has made lateral GaN power devices an attractive choice for power devices.
This thesis presents for the first time the device physics, detailed characteristics, and circuit performance of high voltage GaN HEMTs based on a new technology referred to as polarisation superjunction (PSJ). The PSJ technology permits the design of high-voltage, high-performance diodes and switches with the prospective blocking voltage of up to 10 kV in a cost-effective manner.
According to the experimental results, PSJ devices offer an on-state resistance beyond the one dimensional material limit but without the limitations of conventional HEMTs such as dynamic on resistance. This is because of the uniformly distributed electric field due to the charge balance arising from the coexistence of positive and negative polarisation charges in a double heterojunction. PSJ GaN devices can operate effectively across a broad spectrum of conditions which renders them suitable for a variety of power electronic applications. One of the challenges in GaN HEMTs is their normally-on characteristic which is not desirable from a fail-safe requirements point of view. Thus, methods of normally-off configurations have been explored using cascode and direct drive configurations.
Additionally, detailed analysis of hybrid power switches (HPS) based on the latest silicon (Si), silicon carbide (SiC), and GaN technologies have been presented. The HPS is designed to provide high performance at a competitive cost, by combining the advantages of bipolar and unipolar device technologies. The results show that the HPS can effectively offer MOSFET-like switching performance while having the high current capability of the IGBT.