  For the development of reliable vertical GaN transistors, a detailed analysis
of the robustness of the gate stack is necessary, as a function of the process
parameters and material properties. To this aim, we report a detailed analysis
of breakdown performance of planar GaN-on-Si MOS capacitors. The analysis is
carried out on capacitors processed on different GaN bulk doping (6E18 Si/cc,
6E17 Si/cc and 2.5E18 Mg/cc, p-type), different structures (planar,
trench-like) and different geometries (area, perimeter and shape). We
demonstrate that (i) capacitors on p-GaN have better breakdown performance;
(ii) the presence of a trench structure significantly reduces breakdown
capabilities; (iii) breakdown voltage is dependent on area, with a decreasing
robustness for increasing dimensions; (iv) breakdown voltage is independent of
shape (rectangular, circular). TCAD simulations, in agreement with the
measurements, illustrate the electric field distribution near breakdown and
clarify the results obtained experimentally.
