Physical mechanisms of electron mobility enhancement in uniaxial stressed MOSFETs and impact of uniaxial stress engineering in ballistic regime

The physical mechanisms of mu_e enhancement by uniaxial stress are investigated. From full band calculations, uniaxial-stress-induced split of conduction band edge, DeltaE_C and effective mass change, Deltam*, are quantitatively evaluated. It is experimentally and theoretically demonstrated that the energy surface of 2-fold valleys in Si (001) FETs is warped due to uniaxial lang110rang stress, resulting in lighter m_T of 2-fold valleys parallel to the stress. By using calculated DeltaE_C and Deltam*, experimental (ie enhancement is accurately modeled for biaxial, uniaxial lang100rang, and uniaxial lang110rang stress. The limits of mu_e enhancement and the effectiveness of uniaxial stress engineering in enhancing nFET ballistic I_d,sat are also discussed