Undoped SiGe heterostructure field effect transistors

Undoped SiGe heterostructure MISFETs which are an analog of III-V HIGFETs and use a biaxially strained Si channel with a SiGe gate insulator are reported. The devices were grown on a buffer layer of relaxed SiGe, as were previously reported SiGe modulation-doped heterostructure FETs, but here no doping is used; instead, a gate field is used to form the electron channel. Undoped Si/SiGe heterostructure MISFETs show good FET characteristics with low gate leakage at 77 K. FETs with 1.5- mu m-gate-length and SiGe insulator thickness of 100 nm have a maximum transconductance greater than 100 mS/mm at 77 K. By reducing the insulator thickness to 50 nm, a maximum transconductance greater than 150 mS/mm was obtained for a 2. 5- mu m-gate-length device. A field effect mobility at 77 K of 16500 cm/sup 2//V-s, extracted from long gate-length device data, is comparable to that of modulation-doped high-mobility samples.