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-µm 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.
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