(Invited) Ultra-Wide-Bandgap Semiconductors for Power Electronics

Dramatic improvements in Size, Weight, and Power (SWaP) for power conversion systems have recently been enabled by the adoption of Silicon Carbide (SiC) and Gallium Nitride (GaN) based power switching devices. Such improvements are of high interest for both civilian and defense applications. However, despite the significant progress in these Wide-Bandgap (WBG) materials and the dramatic advantages conferred by them relative to the incumbent Silicon (Si) based technology, various challenges related to performance and reliability remain to be solved. For example, SiC MOSFETs have suffered from gate oxide reliability problems, while non-MOS SiC devices may not be normally-off; similarly, GaN-channel HEMTs are intrinsically normally-on, and normally-off operation requires complex gate stack processing that may degrade long-term device reliability. Lateral GaN HEMTs are also limited in breakdown voltage, and do not avalanche and must thus be over-designed to ensure reliable operation. Improvement in WBG material and device technology has slowed as the technology has matured and become commercially available, and fundamental material properties now limit the performance. Thus, dramatic leaps in power electronics performance require a new generation of materials, the so-called “Ultra” Wide-Bandgap (UWBG) materials with bandgaps larger than 3.4 eV. Representative materials in this category include Aluminum Nitride (AlN), Diamond, and Gallium Oxide (Ga₂O₃).

This talk will focus on our efforts to develop the Aluminum Gallium Nitride (AlGaN) system with Al composition in the range of 30-100% into a viable material for next-generation power electronics. The basic advantages of AlGaN relative to Si, SiC, and GaN will be discussed, as evidenced by the superior unipolar Figure-of-Merit (FOM) for AlN as demonstrated in Figure 1. Here, AlN is shown to have a theoretical order-of-magnitude improvement relative to GaN; in the plot, high breakdown voltage (V_B, horizontal axis) and low specific on-resistance (R_on,sp, vertical axis) are desired. This FOM is based on a number of basic material parameters such as mobility and critical electric field, which will be covered in the talk. Fundamental challenges associated with substrates, epitaxial growth, doping, and point and extended defects will be discussed. Vertical and lateral (heterostructure-based) device architectures will likewise be covered, and the advantages and limitations imposed by the properties of the UWBG AlGaN materials will be examined. In particular, it will be demonstrated that an intrinsically normally-off device that exceeds the capability of GaN (as measured by the FOM) is viable. Experimental results at both the material and device levels will be presented.

This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.