In this paper we present a general methodology, materials theory based modeling, for predicting device performance in technologically immature materials that can proceed relatively independently of experiment. The models incorporated within this general approach extend from a fundamental physics based, microscopic analysis to macroscopic, engineering based device models. Using this scheme, we have investigated the transport and breakdown properties of several emerging wide band gap semiconductor materials, i.e. GaN, InN, 3C-SiC, and 4H-SIC. The carrier drift velocities, mobilities, and impact ionization coefficients for these materials can be predicted using the materials theory based modeling method. Using these results, device level simulations can then be made. Here we report Monte Carlo and selfconsistent charge control modeling of GaN based devices. Comparison to experimental measurements is made when possible. Good agreement between the selfconsistent charge control model calculations and experiment is obtained. Some of the issues pertinent to heterostructure bipolar transistor modeling of GaN are discussed.