High Electron Velocity Submicrometer AlN/GaN MOS-HEMTs on Freestanding GaN Substrates

AlN/GaN heterostructures with 1700-cm<sup>2</sup>/V·s Hall mobility have been grown by molecular beam epitaxy on freestanding GaN substrates. Submicrometer gate-length (L<sub>G</sub>) metal-oxide-semiconductor (MOS) high-electron-mobility transistors (HEMTs) fabricated from this material show excellent dc and RF performance. L<sub>G</sub> = 100 nm devices exhibited a drain current density of 1.5 A/mm, current gain cutoff frequency f<sub>T</sub> of 165 GHz, a maximum frequency of oscillation f<sub>max</sub> of 171 GHz, and intrinsic average electron velocity v<sub>e</sub> of 1.5 ×10<sup>7</sup> cm/s. The 40-GHz load-pull measurements of L<sub>G</sub> = 140 nm devices showed 1-W/mm output power, with a 4.6-dB gain and 17% power-added efficiency. GaN substrates provide a way of achieving high mobility, high v<sub>e</sub>, and high RF performance in AlN/GaN transistors.

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