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Large differences in the experimentally observed strain-induced threshold-voltage shifts for uniaxial and biaxial tensile-stressed silicon (Si) n-channel MOSFETs are explained and quantified. Using the deformation potential theory, key quantities that affect threshold-voltage (electron affinity, bandgap, and valence band density of states) are expressed as a function of strain. The calculated threshold-voltage shift is in agreement with uniaxial wafer bending and published biaxial strained-Si on relaxed-Si1-xGex experimental data , and explains the technologically important observation of a significantly larger (>4x) threshold-voltage shift for biaxial relative to uniaxial stressed MOSFETs. The large threshold shift for biaxial stress is shown to result from the stress-induced change in the Si channel electron affinity and bandgap. The small threshold-voltage shift for uniaxial process tensile stress is shown to result from the n+ poly-Si gate in addition to the Si channel being strained and significantly less bandgap narrowing.
Date of Publication: Nov. 2004