High transconductance of 2.25 S/mm observed at 16 K for 195-nm-gate In0.75Ga0.25As/In0.52Al0.48As HEMT fabricated on [411]A-oriented InP substrate

We achieved a maximum transconductance (g_m) of 2.25 S/mm at 16 K for a 195-nm-gate In_0.75Ga_0.25As/In_0.52Al_0.48As pseudomorphic high-electron mobility transistor (PHEMT) fabricated on a [411]A-oriented InP substrate, which is the highest value ever reported for HEMTs. This PHEMT also showed a much enhanced cutoff frequency (f_T) of 310 GHz at 16 K, compared with its room temperature value (245 GHz). The significantly enhanced g_m and f_T at 16 K can be attributed to the higher saturation velocity in the region "under the gate," which is caused not only by suppressing the phonon scattering, but also by suppressing the interface roughness scattering due to the "(411)A super-flat InGaAs/InAlAs interfaces" (effectively atomically flat heterointerfaces over a wafer-size area).