fT = 688 GHz and fmax = 800 GHz in Lg = 40 nm In0.7Ga0.3As MHEMTs with gm_max > 2.7 mS/µm

We have demonstrated 40-nm In
_0.7
Ga
_0.3
As Metamorphic HEMTs (MHEMTs) with a record value in f
_T
. The devices feature a Pt gate sinking process to effectively thin down the In
_0.52
Al
_0.48
As barrier layer, together with dual Si d-doping in the barrier to lower the potential barrier in the S/D access region. The fabricated device with L
_g
= 40-nm exhibits V
_T
= 0.05 V, g
_m,max
= 2.7 mS/μm, f
_T
= 688 GHz and f
_max
= 800 GHz. In addition, we have developed an analytical model of f
_T
in a III-V HEMT based on a small-signal equivalent circuit, which provides an excellent agreement with measured f
_T
. This in turns guides a realistic way to further improve f
_T
beyond THz.