N-Polar Deep Recess GaN HEMT With a TiN Schottky Gate Contact Demonstrating 53.4% PAE and 3.7 W/mm Associated Pout at 94 GHz

A stack of TiN/Ru was employed as the Schottky gate metal for a nitrogen-polar deep-recess GaN high electron mobility transistor (HEMT) on a sapphire substrate. TiN/Ru Schottky diodes fabricated on the same HEMT epitaxy demonstrated a
$6\times $
reduction in reverse-bias leakage current compared to Ru-only counterparts. The addition of TiN also resulted in a clear improvement in breakdown voltage for scaled N-polar Schottky gate HEMTs, as measured by the drain-current injection method (DCI). TiN HEMTs exhibited a peak
${g}_{\text {m,dc}}$
of 843 mS/mm. A HEMT with a gate length (
$L_{\text {g}}$
) of 50 nm demonstrated a peak
${g}_{\text {m,RF}}$
of 1028 mS/mm, peak
$f_{t}$
of 193 GHz, and peak
$f_{\max }$
of 362 GHz. Load–pull was measured at 94 GHz with a
$V_{\text {DS}}$
of 12 V and
$I_{\text {DS}}$
of 250 mA/mm. The TiN HEMT demonstrated a power added efficiency (PAE) of 53.4%—a record for N-polar GaN at W-band. The associated output power density was 3.7 W/mm, and the linear power gain (Gp) was an exceptional 10.2 dB.