210-GHz InAlN/GaN HEMTs With Dielectric-Free Passivation

Lattice-matched depletion-mode InAlN/AlN/GaN high-electron mobility transistors (HEMTs) on a SiC substrate were fabricated, for the first time, with a dielectric-free passivation (DFP) process in which the device access region was treated by O
₂
/Ar plasma. Similar to dielectric passivation using SiN and Al
₂
O
₃
, the plasma treatment can effectively shorten the gate-length extension. As a result, the current gain cutoff frequency fT of a 60-nm rectangular-gate HEMT increased from 125 to 210 GHz after the plasma DFP; this RF performance is among the highest reported fT for GaN-based HEMTs. The device showed a dc drain current density of 2.1 A/mm and a peak extrinsic transconductance of 487 mS/mm after DFP. The Lg-fTproduct of 12.6 GHz ·μm is among the highest reported for a gate-physical-length-to-barrier-thickness aspect ratio of 5.6. Small gate lag and drain lag are observed in pulsed I-V measurements with a 300-ns pulsewidth.