SiGe-Channel Heterojunction p-MOSFET’s

The advances in the growth of pseudomorphic silicon-germanium epitaxial layers combined with the strong need for high-speed complementary circuits have led to increased interest in silicon-based heterojunction field-effect transistors. Metal-oxide-semiconductor field-effect transistors (MOSFET’s) with SiGe channels are guided by different design rules than state-of-the-art silicon MOSFET’s. The selection of the transistor gate material, the optimization of the silicon-germanium channel profile, the method of threshold voltage adjustment, and the silicon-cap and gate-oxide thickness sensitivities are the critical design parameters for the p-channel SiGe MOSFET. Two-dimensional numerical modeling demonstrates that n+ polysilicon-gate SiGe p-MOSFET’s have acceptable short-channel behavior at 0.20 µm channel lengths and are preferable to p+ polysilicon-gate p-MOSFET’s for 2.5 V operation. Experimental results of n+-gate modulation-doped SiGe p-MOSFET’s illustrate the importance of the optimization of the SiGe-channel profile. When a graded SiGe channel is used, hole mobilities as high as 220 cm2/V.s at 300 K and 980 cm2/V.s at 82 K are obtained.