2D simulation of static surface states in AlGaN/GaN HEMT and GaN MESFET devices

Two-dimensional simulations of surface charge effects in AlGaN/GaN HEMT and GaN MESFET devices are performed. The influence of charges of different magnitude, sign and spatial distribution at the ungated surface on drain current characteristics is studied. We have found that positive and negative surface charges of the same magnitude produce very different modifications of drain current. The relative influence of polarization fields, surface charges and free hole accumulation at the top surface is considered and gives rise to the opposite behaviour in HEMT and MESFET devices. The implications of our study in current collapse and related dispersion effects in GaN FETs are discussed.