Scalability of hole mobility enhancement in biaxially strained ultrathin body SOI

The effect of biaxial strain on the valence band structure of ultrathin body (UTB) silicon-on-insulator (SOI) is studied using an sp³d⁵s^* tight binding model. In contrast to bulk silicon, where biaxial tensile strain improves hole mobility via band splitting and decreasing the effective mass, moderate values of tensile strain degrade the mobility in UTB SOI. Strong confinement cancels out some of the band splitting and effective mass reduction offered by biaxial tensile strain in ultrathin SOI. Consequently, higher levels of strain are needed in order to get a band splitting similar to that observed in bulk strained silicon. Alternatively, [100] channel orientation can be employed to avoid an excessive increase in the effective mass in the [110] direction.