AlGaN/GaN HFETs have demonstrated excellent RF performance, but the devices still suffer from a reliability problem. The decrease of the dc current and RF output power over time is attributed to gate tunneling which is determined by the magnitude of electric field at the gate edge. In this work, in order to improve the reliability of AlGaN/GaN HFETs, a 2D drift-diffusion tool is used to explore the relationship between the magnitude of electric field and different device structures through modifications of the 2DEG sheet charge density, AlGaN barrier layer thickness, AlGaN doping concentration and gate to drain spacing. The effect of field plates is also investigated. It was found that decreasing 2DEG sheet charge density results in much improved reliability, although the current and output power are somewhat reduced.