Microwave performance of 0.3-μm gate-length multi-finger AlGaN/GaN heterojunction FETs with minimized current collapse

We have fabricated multi-finger AlGaN/GaN heterojunction field effect transistors (HJFETs) with a sub-half-micron gate-length on a sapphire substrate that are suitable for near-millimeter-wavelength applications. Fabricated HJFETs with a 0.25-mu m gate-length had a small-signal power-gain high enough for K- and Ka-band applications, i.e., a cut-off frequency of 40 GHz, a maximum oscillation frequency of 97 GHz, and a maximum stable gain of 11.8 dB at 30 GHz. This high-frequency performance is attributed to the small parasitic capacitance enabled by air-bridge interconnection as well as to the sub-half-micron gate length formed by electron-beam lithography. The air-bridge interconnection also prevented surface-induced current collapse. Because the current-collapse was negligible, an HJFET with a 1.2-mm gate-width exhibited steady large-signal RF performance, i.e., a saturation output power of 1.47 W (1.22 W/mm) with a power-added efficiency of 51.5% at 1.95 GHz.