AlGaN/GaN rake-gate HFET: A novel normally-off HFET based on stress and layout engineering

A novel normally-off AlGaN/GaN HFET (Rake-Gate HFET), based on stress and layout engineering without modifying typical fabrication processes is proposed. It is verified and optimized through TCAD simulation. Positive pinch off voltage is achieved by depositing compressively stressed passivation nitride surrounding gate tines, which induce significant negative piezoelectric charge under the tines. Punch through current is suppressed by including multiple tines connected as in a rake for excellent electrostatic control. It is showed that a properly designed Rake-Gate HFET has > 1000V punch-through and breakdown voltages. It also has smaller gate capacitance than regular HFETs with gates spanning similar areas. By applying the Rake-Gate HFET design to an existing normally-off HFET process (without modifying the fabrication process besides using 500nm -2GPa passivation layer), the pinch off voltage can be further increased by ~1.5V, resulting in the improvement of safe-operating margin and elimination of static energy loss in a 750V single power supply boost converter.