P-TYPE GE-CHANNEL MODFETS WITH HIGH TRANSCONDUCTANCE GROWN ON SI SUBSTRATES

Modulation-doped FET structures with hole channels consisting of pure Ge were grown by molecular beam epitaxy on Si substrates. To overcome the relatively large lattice mismatch between Si and Ge, a relaxed Si1-xGex buffer layer with linearly graded Ge concentration and a final x value of around 70% was grown first. Hall mobilities of up to 1300 cm2 / V . s at room temperature and 14 000 cm2 / V . s at 77 K were measured. Devices with and without gate recess were fabricated, which result in enhancement- and depletion-type FET's. We found maximum extrinsic transconductances of 125 and 290 mS / mm at room temperature and 77 K, respectively, for gate lengths L(G) around 1.2 mum.