High-Speed AlN/GaN MOS-HFETs With Scaled ALD $\hbox {Al}_{2}\hbox {O}_{3}$ Gate Insulators

Highly scaled AlN/GaN metal-oxide-semiconductor heterojunction field-effect transistors (MOS-HFETs) with Al₂O₃ gate dielectrics of varying thicknesses deposited by atomic layer deposition (ALD) were fabricated, and their performance was compared with Schottky-barrier HFETs (SB-HFETs). MOS-HFETs with an ultrathin 2-nm-thick Al₂O₃ dielectric and a gate length of 40 nm had direct-current (dc) and radio-frequency (RF) performances similar to the SB-HFETs, with a high extrinsic transconductance of 415 mS/mm, f_T of 134 GHz, and f_max of 261 GHz. In contrast, the dc and RF performances of a MOS-HFET with a 4-nm-thick Al₂O₃ dielectric were degraded by short-channel effects. The 2-nm-thick Al₂O₃ gate insulator reduced the forward-bias gate current by more than two orders of magnitude. The data suggest the promise of ultrathin ALD Al₂O₃ gate dielectrics for next-generation high-speed GaN HFETs.