Velocity saturation limitations of lightly doped drain transistors

Carrier velocity saturation in the lightly doped drain (LDD) region of an n-channel transistor decreases the saturated transconductance. This effect is modeled by inclusion of a gate-voltage-dependent source resistance in the expression for the saturated drain-source current. Experimental results are given that are consistent with the model. Velocity saturation diminishes transconductance more severely as the temperature is reduced from room temperature. Higher saturated channel velocity and lower critical electric field for the onset of LDD velocity saturation are obtained at reduced temperatures and magnify the effect of LDD velocity saturation. As the transistor is cooled to near the boiling point of nitrogen, surface roughness scattering also affects the saturated transconductance