High f(max) n-type Si/SiGe MODFETs

The authors report on the fabrication of 0.18 mu m gale length n-type Si/Si60Ge40 modulation-doped field-effect transistors (MODFETs) with improved RF performance. The 2D electron gas channel, a strained Si layer grown on a relaxed 2 mu m thick graded Si60Ge40 buffer shows high mobility (mu = 1190cm(2)/Vs at room temperature). High DC transconductances (up to 270mS/mm at room temperature), high saturation currents (260mA/mm) due to reduced parasitic series resistances (R-S less than or equal to 0.2 Omega mm) and high carrier densities (n(s) = 1.9 x 10(12)cm(-2)) are achieved. Very good RF characteristics have been found for the non-recessed device structure with cutoff frequencies up to f(T) = 46GHz and a record of f(max) = 81GHz at low supply voltages around 1.5V.