p-type Ge-channel MODFETs with high transconductance grown on Si substrates

Modulation-doped FET (MODFET) structures with hole channels consisting of pure Ge were grown by molecular beam epitaxy (MBE) on Si substrates. To overcome the relatively large lattice mismatch, between Si and Ge, a relaxed Si/sub 1-x/Ge/sub x /buffer layer with linearly graded Ge concentration and a final x value of around 70% was grown first. Hall mobilities of up to 1300 cm/sup 2//V-s at room temperature and 14000 cm/sup 2//V-s at 77 K were measured. Devices with and without gate recess were fabricated, which result in enhancement- and depletion-type FETs. Maximum extrinsic transconductances of 125 and 290 mS/mm at room temperature and 77 K, respectively, were found for gate lengths L/sub G/ around 1.2 mu m.<
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