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

The consequences of surface states in field-effect devices based on GaN are analyzed in this work. DC and transient effects on drain current response are evaluated under several conditions of trap distributions at the ungated surface. Initially, a basic analysis with static surface charges is presented, revealing important consequences in the device characteristics and the type of traps. A detailed study focused on dynamic surface traps is also performed. Both, static and transient simulations provide a good understanding of the current collapse, gate-lag and related effects, showing a good agreement to experimental data. The study has been carried out in MESFET and HEMT devices using a numerical 2D simulation tool that includes polarization charges, trap ionization and hole accumulation at the surface in the electrostatic and transport equations. This model allows us to deduce the role of other device parameters such as contact resistance and leakage currents on the trap-related collapse and transient effects.