ELECTRON-DRIFT MOBILITY MODEL FOR DEVICES BASED ON UNSTRAINED AND COHERENTLY STRAINED SI1-XGEX GROWN ON (001) SILICON SUBSTRATE

The electron mobility for unstrained and coherently strained Si1-xGex (SiGe) grown on a [001] silicon substrate is analytically obtained for Ge fractions less than 30%. The method is based on the following two assumptions: the conduction bands of the unstrained alloy are Si-like for Ge fraction less than 30% and in the case of the coherently strained alloy, strain-induced energy shifts occur in the conduction band valleys. The shifts in energy yield two different mobility values; one corresponding to the growth plane with a value larger than the unstrained mobility, and the other parallel to the growth direction and correspondingly smaller in value. In comparison to silicon, the results show a degradation of both the unstrained mobilities for doping levels up to 10(17) cm-3. Beyond this doping level, the strained mobility component parallel to the growth direction becomes slightly larger than the mobility of silicon.