An analytic characterization of weak-inversion drift current in a long-channel MOSFET

A weak-inversion drain-current model that accounts for both drift and diffusion of carriers in a long-channel MOSFET is derived. The drift current increases exponentially with surface potential, and for typical silicon MOSFET's the drift-diffusion current ratio is about 0.2 at threshold. From the model a simple analytic threshold current expression is obtained that facilitates a quick experimental determination of the threshold voltage.