Vertical Gallium Nitride Technology

Abstract

The evolution of LEDs, lasers and HEMTs using gallium nitride as the semiconductor has taught us about its many unique capabilities including high breakdown electric field, polarization-induced high charge density in the channel and reliable operation at high temperature, to name a few. These superior performances due to GaN’s exceptional material properties make GaN ideal for power switching besides RF applications. Thus, power electronics got recently added to GaN’s portfolio showing great progress. Lateral HEMTs, already available as products for RF application, were the obvious first choice for designing power electronic switches. The last ten years have witnessed an incredibly fast advancement in the lateral HEMT technology building a market space for GaN in medium (up to 15 kW) power electronic applications. Although the maximum industrially feasible and economically viable limits of power conversion using the lateral GaN technology are yet to be determined, vertical devices start to look attractive for power conversion ranging above 15–20 kW. The availability of bulk GaN substrates has stimulated the development of vertical GaN technology. Vertical GaN devices, analogous to Si-DMOSFETs in some ways, can uniquely be designed with a high-mobility AlGaN/GaN channel combined to a thick-drift region in bulk GaN to offer very low on-resistance and high breakdown voltage—the two key parameters of benchmarking a power switch. While the channel of a vertical device can be designed either horizontally or vertically along the sidewalls, the peak electric fields in these devices are always buried in the bulk material, far from the surface. This allows the device to be reasonably dispersion-free without involving field plates, unlike used in lateral HEMTs. Attaining high electron mobility in bulk GaN that forms the drift region will be of key importance to outperform the competing technologies based on Si and SiC. This chapter will focus on the design space, challenges, current performance, cost and roadmap of vertical GaN devices for next-generation power conversion.