Ultrahigh-Speed GaN High-Electron-Mobility Transistors With $f_{T}/f_{\mathrm {max}}$ of 454/444 GHz

This letter reports record RF performance of deeply scaled depletion-mode GaN-high-electron-mobility transistors (GaN-HEMTs). Based on double heterojunction AlN/GaN/AlGaN epitaxial structure, fully passivated devices were fabricated by self-aligned-gate technology featuring recessed n+-GaN ohmic contact regrown by molecular beam epitaxy. Record-high fT of 454 GHz and simultaneous fmax of 444 GHz were achieved on a 20-nm gate HEMT with 50-nm-wide gate- source and gate-drain separation. With an OFF-state breakdown voltage of 10 V, the Johnson figure of merit of this device reaches 4.5 THz-V, representing the state-of-the-art performance of GaN transistor technology to-date. Compared with previous E-mode GaN-HEMTs of similar device structure, significantly reduced extrinsic gate capacitance and enhanced average electron velocity are the key reasons for improved frequency characteristic.