Polarization Engineering on Buffer Layer in GaN-Based Heterojunction FETs

To improve the pinched-off characteristics of an AlGaN/GaN heterojunction field effect transistor (HJFET), the conduction band potential of an incorporated AL
_x
Ga
_1-x
N buffer is designed to be upwardly convex in a band diagram. This approach utilizes the polarization effects specific to GaN-based materials by lowering the Al content x from 30% to 5% almost linearly toward the front side. Fabricated field effect transistors (FETs) adopting the designed buffer have demonstrated the following advanced characteristics in comparison to those of a FET adopting a conventional GaN buffer: less than one-tenth of the buffer leakage current, a gate-to-drain breakdown voltage BV
_gd
twice or more as high, and remarkably improved carrier confinement and pinched-off behavior. The FETs are operated in an enhancement mode with a gate-to-channel distance thick enough to prevent tunneling current through the gate.