Evidence for Silicon Bandgap Narrowing in Uniaxially Strained MOSFETs Subjected to Tensile and Compressive Stress

Experimental evidence is presented for silicon bandgap narrowing in uniaxially strained MOSFETs subjected to both tensile and compressive stress. For both n-channel MOSFETs with n+polysilicon gate and p-channel MOSFETs with p+polysilicon gate, the strain-induced threshold voltage shift (ΔVth) can be explicitly approximated from the changes in silicon bandgap (ΔEg) and carrier mobility (Δμ). From the measurements of both ΔVthand Δμassociated with the strain ε, a simple ΔEgmodel at low strain is found to exactly match a tight-binding band structure calculation, clearly indicating the presence of uniaxial strain-induced silicon bandgap narrowing, regardless of strain or carrier type.