Dear Ashutosh Srivastava ,

in order to get the breakdown voltage you have to apply a voltage on the capacitor such that the electric field in the material reaches the critical electric field sufficient for the breakdown.

So you can step the voltage and sense the current. The breakdown will be reached when the current increases appreciably.

You have to include the avalanche breakdown model to affect the mobile charge multiplication.

If this is not working please use n+in+ structure to inject primary electron currents in the i region.

Best wishes

from apply a voltage on the capacitor and field equal to critical eield sufficient for the breakdown.

DEAR aSHUTOSH,

Consider your capacitor as a simple semiconductor device (bulk AlGaN semiconductor), write an input file and go ahead to plot its I-V characteristics.

In the physical model section, choose a simple continuity model and accurately define the IPACT IONIZATION model, with accurate parameters of AlGaN.

You can start with uncoupled models and large voltage steps, as long as you're far from breakdown.

N.B. You can of course rapidly estimate the breakdown voltage from the only solution of Poisson's equation and checking the voltage at which the multiplication factor of charge carriers goes to infinity. The Multiplication factor is related to the impact ionization rate, which in turn related to the applied field E=V/d. More and more simpler relations can also work. For instance, for your parallel plate capacitor, C = Q/V = epsilon (AlGaN). A/d and the break-down voltage VBR = Breakdown field EBR x d. Both epsilon and EBR of AlGaN exist in the literature.

Best wishes