W-band fully passivated AlN/GaN HEMT device with 56% power-added efficiency and 780 mW/mm output power density at 94 GHz

We report on deeply scaled AlN/GaN HEMTs demonstrating power added efficiency (PAE) of 56% and 780 mW/mm output power at 94 GHz. This high transistor efficiency at W-band is enabled by in-situ SiN passivation, a 40-nm T-gate, and by scaling the source-to-drain length to
$0.23\ \mu \mathrm{m}$
using electron-beam lithography. Scaled source-to-drain length and regrown n+ GaN ohmic contacts result in a HEMT on-resistance (
$R_{on}$
) of 0.35 ohm-mm, maximum drain current (
$I_{d, max}$
) of 2.3 A/mm, and a peak transconductance (
$g_{mp}$
) of 1.2 S/mm. Small-signal RF measurements show a 231 GHz cut-off frequency (
$f_{T}$
) and 503 GHz maximum oscillation frequency (
$f_{max}$
). Large-signal load-pull characterization shows that PAE is maintained above 46% with an associated saturated power of 3.1 W/mm at 94 GHz. This new T3.5 technology enables fabrication of a continuum of device geometries and is backwards compatible with HRL's current T3 foundry node.