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.