Schottky diodes integrated in vertical GaN-on-Si transistors

École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland claims the first monolithic integration of vertical gallium nitride (GaN) metal-oxide-semiconductor field-effect transistors (MOSFETs) with freewheeling Schottky barrier diodes (SBD) grown on 6-inch silicon substrates by metal-organic chemical vapor deposition [Chao Liu et al, IEEE Electron Device Letters, published online 1 June 2018]. This builds on previous work [www.semiconductor-today.com/news_items/2018/jan/epfl_160118.shtml].

The researchers hope to find a lower-cost route to vertical GaN power devices than the very expensive processes based on high-price bulk or free-standing GaN substrates that are presently used in research. GaN substrates are used to avoid defects that reduce the critical electric field for breakdown. Vertical power devices should be able to handle higher voltages and currents than lateral architectures. The use of large-diameter silicon substrates should reduce costs, but growth of GaN on silicon tends to introduce many performance-killing defects.

Freewheeling diodes are used in power conversion applications to allow the flow of reverse-bias current in the off state, releasing energy stored in inductive elements. While the in-built body p-i-n diode of the EPFL structure does allow some current flow, the turn-on voltage is high, increasing losses during switching. SBDs have a much lower turn-on voltage, along with faster switching and lower resistance.