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 <tex-math notation="LaTeX">×</tex-math> 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 <tex-math notation="LaTeX">g_m,dc</tex-math> of 843 mS/mm. A HEMT with a gate length ( <tex-math notation="LaTeX">L_g</tex-math> ) of 50 nm demonstrated a peak <tex-math notation="LaTeX">g_m,RF</tex-math> of 1028 mS/mm, peak <tex-math notation="LaTeX">fₜ</tex-math> of 193 GHz, and peak <tex-math notation="LaTeX">fₘₐₓ</tex-math> of 362 GHz. Load–pull was measured at 94 GHz with a <tex-math notation="LaTeX">VDS</tex-math> of 12 V and <tex-math notation="LaTeX">IDS</tex-math> of 250 mA/mm. The TiN HEMT demonstrated a power added efficiency (PAE) of 53.4%—a record for N-polar GaN at <tex-math notation="LaTeX">W</tex-math> -band. The associated output power density was 3.7 W/mm, and the linear power gain (G <tex-math notation="LaTeX">_p</tex-math> ) was an exceptional 10.2 dB.