Statistics and localisation of vertical breakdown in AlGaN/GaN HEMTs on SiC and Si substrates for power applications.

•Probability distribution functions of GaN buffer breakdown voltage were determined.•A time dependent behaviour of vertical breakdown was observed.•Vertical breakdown paths were localised by backside infrared microscopy.•The strong localisation of the breakdown paths is due to current filamentation.•Roles of buffer defects and self-heating effects have been discussed.