High fmax n-type Si/SiGe MODFETs

Abstract

The authors report on the fabrication of 0.18 µm gate 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 µm thick graded Si60Ge40 buffer shows high mobility (μ = 1190 cm2/Vs at room temperature). High DC transconductances (up to 270 mS/mm at room temperature), high saturation currents (260 mA/mm) due to reduced parasitic series resistances (RS ≤ 0.2 Ωmm) and high carrier densities (ns = 1.9 × 1012 cm-2) are achieved. Very good RF characteristics have been found for the non-recessed device structure with cutoff frequencies up to fT = 46 GHz and a record of fmax = 81 GHz at low supply voltages around 1.5 V.