Transient characteristics of Al_xGa_1-xN/GaN heterojunction field-effect transistors

Transient characteristics of drain-source current in response to picosecond pulsed gate-source voltages in Al_xGa_1-xN/GaN heterojunction field-effect transistors (HFETs) have been measured. It was found that the switching time constants were of the order of tens of picoseconds and depended strongly on the gate-source bias V_GS as well as drain-source bias V_DS. Slow transients caused by charge trapping effects such as those observed in AlGaAs/GaAs HFETs were absent in AlGaN/GaN HFETs. Our results suggested that the dependence of the effective electron mobility on the sheet density dictates the overall drain current transient characteristics as well as the device switching speed of AlGaN/GaN HFETs.