THRESHOLD VOLTAGE MODEL FOR DEEP-SUBMICROMETER MOSFETS

The threshold voltage, V(th), of lightly doped drain (LDD) and non-LDD MOSFET's with effective channel lengths down to the deep-submicrometer range has been investigated. Experimental data show that in the very-short-channel length range, the previously reported exponential dependence on channel length and the linear dependence on drain voltage no longer hold true. We use a simple quasi-two-dimensional model, taking into account the effects of gate oxide thickness, source/drain junction depth, and channel doping, to describe the accelerated V(th) roll-off and the nonlinear drain voltage dependence. Relative to non-LDD devices, LDD devices have a smaller dependence of V(th) on channel length due to their lower drain-substrate junction built-in potentials. LDD devices also show less V(th) dependence on drain voltage because the LDD region reduces the effective drain voltage. Based on consideration of the short-channel effects, it is shown that the minimum acceptable length is equal to (0.0035 approximately 0.005)X(j)1/3T(OX) micrometer (X(j) in mum, T(OX) in angstrom).