Strained-Si based SiGe MODFETs on ultra-thin SiGe virtual substrates prepared by molecular beam epitaxy (MBE) and helium implantation are presented and compared to similar devices on thick, compositionally graded virtual substrates grown by low energy plasma enhanced CVD (LEPECVD). MBE grown 100 nm thick pseudomorphic Si0.67Ge0.33 layers have been relaxed to about 70% by using He+ ions implanted with a dose of 2 x 10(16) cm(-2) at an energy of 18 keV approximately 100 nm below the hetero-interface, followed by an annealing step at 850 degreesC for 600 s. Then another Si0.67Ge0.33 layer and a modulation doped strained Si QW are grown on top of the virtual substrate having a final degree of relaxation of about 80%. A cut-off frequency of f(max)(MAG) = 121 GHz was achieved with RF devices having a gate length of 70 nm. Self-heating effects in devices on thick, graded, and on thin buffer layers are addressed by experimental DC and RF data and 2-D numerical simulations of the device temperature. (C) 2004 Elsevier Ltd. All rights reserved.