The role of ballistic mobility and saturation velocity in performance evaluation of a nano-CMOS circuit

The mobility and saturation velocity are the two important parameters that control the charge transport in a channel. The mobility is ballistic as channel length is scaled down to length smaller than the mean free path of carriers in the channel. It is found that the ballistic saturation velocity does not sensitively depend upon low-field mobility or scattering interactions. In an infinite electric field and in the absence of quantum emission, the saturation velocity is limited to the thermal velocity in the nondegenerate realm and Fermi velocity in the degenerate realm. It may be lowered by the emission of a quantum. However, the drain electric field is always finite and hence the drain velocity is always smaller than the saturation velocity at the onset of quasi current saturation. Expressions for voltage VDsat and current IDsat at the onset of current saturation transform dramatically in a nanoscale circuit. The modeling applied to 80-nm and 45-nm CMOS processes give exceedingly good comparison to the experimental data.