0.15-$muhboxm$-Gate InAlAs/InGaAs/InP E-HEMTs Utilizing Ir/Ti/Pt/Au Gate Structure

High-current 0.15-
$muhboxm$
-gate enhancement-mode high-electron mobility transistors utilizing Ir/Ti/Pt/Au gate metallization were fabricated using a new process including a high-temperature gate anneal that is required for Schottky-barrier height enhancement for the Ir-based gate contact.
$hboxSiN_x$
encapsulation was employed to prevent thermal degradation of device layer during the high-temperature gate anneal. Excellent enhancement-mode operation, with a threshold voltage of 0.1 V and
$I_ DSS$
of 2.1 mA/mm, was realized. Both the annealed and unannealed devices exhibited high
$g_m,max$
and
$I_D,max$
of 800 mS/mm and 430 mA/mm, respectively. A unity current-gain cutoff frequency
$f_T$
of 151 GHz and a maximum oscillation frequency
$f_ MAX$
of 172 GHz were achieved. From the dc and RF characteristics, it can be deduced that there was no degradation of the gate contact and the heterostructure due to gate annealing. Furthermore, it was found that the gate diffusion during gate annealing was negligible since no increase in
$g_m,max$
was observed.