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<inf>0.7</inf>Ga<inf>0.3</inf>As Metamorphic HEMTs (MHEMTs) with a record value in f<inf>T</inf>. The devices feature a Pt gate sinking process to effectively thin down the In<inf>0.52</inf>Al<inf>0.48</inf>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<inf>g</inf> = 40-nm exhibits V<inf>T</inf> = 0.05 V, g<inf>m,max</inf> = 2.7 mS/µm, f<inf>T</inf> = 688 GHz and f<inf>max</inf> = 800 GHz. In addition, we have developed an analytical model of f<inf>T</inf> in a III–V HEMT based on a small-signal equivalent circuit, which provides an excellent agreement with measured f<inf>T</inf>. This in turns guides a realistic way to further improve f<inf>T</inf> beyond THz.