AlGaN/GaN-based power semiconductor switches

Abstract

AlGaN/GaN-based high-electron-mobility transistors (HEMTs) have great potential for their use as high efficiency and high speed power semiconductor switches, thanks to their high breakdown electric field, mobility and charge density. The ability to grow these devices on large-diameter Si wafers also reduces device cost and makes them easier for wide market adoption. However, the development of AlGaN/GaN-based power switches has encountered three major obstacles: the limited breakdown voltage of AlGaN/GaN transistors grown on Si substrates; the low performance of normally-off AlGaN/GaN transistors; and the degradation of device performance under high voltage pulsed conditions. This thesis studies these issues and presents new approaches to address these obstacles. The first part of the thesis studies the breakdown mechanism in AlGaN/GaN-on-Si transistors. A new quantitative model-trap-limited space-charge impact-ionization model- is developed. Based on this model, a set of design rules is proposed to improve the breakdown voltage of AlGaN/GaN-on-Si transistors. New technologies have also been demonstrated to increase the breakdown voltage of AlGaN/GaN-on-Si transistors beyond 1500 V. The second part of the thesis presents three technologies to improve the performance of normally-off AlGaN/GaN transistors. First, a dual-gate normally-off MISFET achieved high threshold voltage, high current and high breakdown voltage simultaneously by using an integrated cascode structure. Second, a tri-gate AlGaN/GaN MISFET demonstrated the highest current on/off ratio in normally-off GaN transistors with the enhanced electrostatic control from a tri-gate structure. Finally, a new etch-stop barrier structure is designed to address low channel mobility, high interface density and non-uniformity issues associated with the conventional gate recess technology. Using this new structure, normally-off MISFETs demonstrated high uniformity, steep sub-threshold slope and a record channel effective mobility. The thesis concludes with a new dynamic on-resistance measurement technique. With this method, the hard- and soft-switching characteristics of GaN transistors were measured for the first time.