Enhancement-Mode MISHFETs

Enhancement-mode Si₃N₄/AlGaN/GaN metal-insulator-semiconductor HFETs (MISHFETs) with a 1-mum gate footprint are demonstrated by combining CF₄ plasma treatment technique and a two-step Si₃N₄ deposition process. The threshold voltage has been shifted from -4 [for depletion-mode HFET] to 2 V using the techniques. A 15-nm Si₃N₄ layer is inserted under the metal gate to provide additional isolation between the gate Schottky contact and AlGaN surface, which can lead to reduced gate leakage current and higher gate turn-on voltage. The two-step Si ₃N₄ deposition process is developed to reduce the gate coupling capacitances in the source and drain access region, while assuring the plasma-treated gate region being fully covered by the gate electrode. The forward turn-on gate bias of the MISHFETs is as large as 7 V, at which a maximum current density of 420 mA/mm is obtained. The small-signal RF measurements show that the current gain cutoff frequency (f_T) and power gain cutoff frequency (f_max) are 13.3 and 23.3 GHz, respectively