RF and DC characteristics in Al ₂O ₃/Si ₃N ₄ insulated-gate AlGaN/GaN heterostructure field-effect transistors with regrown ohmic structure

Al ₂O ₃/Si ₃N ₄ insulated-gate AlGaN/GaN heterostructure field-effect transistors (HFETs) have been fabricated, where the regrown ohmic structure was incorporated to reduce the contact resistance. Excellent DC and RF characteristics have been obtained together with the low gate leakage current as the result of employing the metal-insulator-semiconductor (MIS) structure. An HFET with a gate length (L _g) of 0.1 μm has exhibited a drain current density (I _d) and a transconductance (g _m) of 1.30 A/mm and 293 mS/mm, respectively, with a reduced contact resistance of 0.3 Ω mm. The gate leakage current (I _g) was as low as 1 × 10 ^-8 A/mm in the reverse vias region, and only 4 × 10 ^-5 A/mm even at a forward bias voltage of +3 V. In this device, the cutoff frequency (f _T) and maximum oscillation frequency (f _max) were estimated to be 70 and 90 GHz, respectively. In the HFETs with longer L _g of 0.7 and 1.0 μm,f _T and f _max were 20 and 48 GHz (L _g = 0.7 μm), respectively; and 14 and 35 GHz (L _g = 1.0 μm), respectively. Thus, the Al ₂O ₃/Si ₃N ₄ MIS HFETs have proved to also exhibit excellent RF characteristics.