First Principles Modeling of High Field Transport in Ultra-Wide Bandgap Materials*

Ultra-wide band gap (UWBG) materials offer the potential for greatly improved power electronic device performance due to their predicted higher breakdown fields limited by avalanche breakdown, as well as their favorable transport characteristics such as high mobility and drift velocity, which reduce on-resistance and allow for high frequency operation in power conversion applications. Experimental data on the high field transport properties of UWBG materials such as the impact ionization coefficients are relatively limited. Hence, to understand the limits of performance of these materials, we report on first principles theoretical calculations of the high field transport properties of UWBG materials using a combination of ab initio calculations of the electronic and phononic structure coupled with particle based full-band Cellular Monte Carlo (CMC) high field transport simulation.

The electronic structure is computed using the GW method using the BerkeleyGW code suite. The phonon dispersion is calculated from DFPT (density functional perturbation theory) using Quantum Espresso. The full wave-vector dependent deformation potentials are computed using the GW wavefunctions and energies as input to the EPW (Electron-Phonon using Wannier) code to calculate the electron-phonon interactions from first principles. Based on these scattering mechanisms as input, transport quantities such as the velocity-field characteristics and impact ionization coefficients as a function of electric field are calculated from full band CMC simulation.

We have applied this framework initially to diamond in comparison to available high field transport data. One important observation is that while the critical field depends strongly on the material bandgap, the relative magnitude of the deformation potential plays an important role as well. We compare different approximations of the deformation potential in relation to the simulated impact ionization coefficients and their impact on breakdown. We then use the high field impact ionization rates in diamond to calculate its doping- and drift thickness-dependent critical fields and compare the results to available experimental measurements.