Extremely high transconductance Ge/Si/sub 0.4/Ge/sub 0.6/ p-MODFET's grown by UHV-CVD

Ge-channel modulation-doped field-effect transistors (MODFET's) with extremely high transconductance are reported. The devices were fabricated on a compressive-strained Ge/Si/sub 0.4/Ge/sub 0.6/ heterostructure with a Hall mobility of 1750 cm2/Vs (30,900 cm2/Vs) at room temperature (77 K). Self-aligned, T-gate p-MODFET's with L/sub g/=0.1 μm displayed an average peak extrinsic transconductance (g(m/sub ext/)) of 439 mS/mm, at a drain-to-source bias voltage (V/sub ds/) of -0.6 V, with the best device having a value of g(m/sub ext/)=488 mS/mm. At 77 K, values as high as g(m/sub ext/)=687 mS/mm were obtained at a bias voltage of only V/sub ds/=-0.2 V. These devices also displayed a unity current gain cutoff frequency (fT) of 42 GHz and maximum frequency of oscillation (fmax) of 86 GHz at V/sub ds/=-0.6 V and -1.0 V, respectively.