http://www.semiconductor-today.com/news_items/2017/apr/asu_260417.shtml
Arizona State University in the USA has been studying the effects of gallium nitride (GaN) buffer layer thickness and drift layer doping on the performance of vertical p-n and Schottky barrier diodes [Houqiang Fu et al, IEEE Electron Device Letters, published online 4 April 2017]. Without passivation or field plates, some p-n devices reached breakdown voltages of more than 1000V and achieved specific on-resistances as low as 3mΩ-cm2.