High fmax n-type Si/SiGe MODFETs

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 Si ₆₀Ge₄₀ buffer shows high mobility (μ=1190 cm ²/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 (R_S⩽0.2 Ωmm) and high carrier densities (n_s=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 f_T=46 GHz and a record of f_max=81 GHz at low supply voltages around 1.5 V