3.6.8 Polarization Effects

The understanding of the transport, especially of the high field transport in AlGaN/GaN HEMTs is far from complete due to the variations of the material properties (MBE/MOCVD growth) and the surface/interface effects due to the spontaneous and piezoelectric polarization effects. The polarization induces two-dimensional electron gases with very high electron concentration $\geq$ 10$^{13}$ cm$^{-2}$. The very high carrier concentrations lead to confinement and current densities next to the AlGaN/GaN interface. The conduction band edges are strongly modified due to the polarization [238]. As the carrier concentration is focussed to lengths of 1-3 nm [12], the interface quality at the AlGaN/GaN is extremely important especially, for high Al contents in the barrier leading to tight channels.

To account for these polarization effects additional surface charges are applied in the simulation, as suggested in [12,275], which is a straight forward, but feasible approach. Positive surface charges are considered at the AlGaN/GaN channel interface in the simulations, while negative charges are applied at the substrate and the barrier towards air/passivation, see Fig. 3.1. Furthermore, the ohmic contact situation is found to be situation (II) in Fig. 3.25. A drift-diffusion model is applied using the high field mobility of (3.45) and the thermionic field emission model are used. This accounts for the overshoot at about 200 kV/cm using and demonstrates the applicability of the two-dimensional model in comparison with measurements. However, better understanding of the interfaces at the GaN-metal, the GaN-SiN, and GaN/AlGaN interfaces is necessary [12].