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 (ΔV_th) can be explicitly approximated from the changes in silicon bandgap (ΔE_g) and carrier mobility (Δ_μ). From the measurements of both ΔV_th and Δ_μ associated with the strain ε, a simple ΔE_g model 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.