From Semiconductor Today: 

Massachusetts Institute of Technology (MIT) in the USA and Enkris Semiconductor Inc in China claim record performance for vertical gallium nitride (GaN) power diodes on foreign substrate [Yuhao Zhang et al, IEEE Electron Device Letters, published online 26 March 2018]. The researchers have developed a new platform for vertical GaN on silicon (Si) that creates a back-side trench, allowing a back contact to be made to the n-GaN layers.

The trench etching removes resistive buffer layers that have up to now mainly restricted GaN/Si power electronics to lateral or quasi-vertical structures with all contacts on the top side of epitaxial structures. The buffer layers are needed to bridge the lattice and thermal expansion mismatch between the foreign substrate and the GaN crystal structure. Fully vertical GaN power devices have generally been demonstrated on very expensive bulk or free-standing GaN substrates.

Vertical power devices are attractive because they push peak electric fields away from the surfaces and associated electron states that can cause premature breakdown. Vertical structures hence allow higher currents and voltages in more compact footprints. Also, lateral/quasi-vertical devices suffer from current crowding, which increases Joule heating.

Read more: Platform for fully vertical gallium nitride on silicon power devices, reposted by Silicon Valley Microelectronics Inc.