The dependence of channel current in subthreshold operation upon drain, gate, and substrate voltages is formulated in terms of a simple model. The basic results are consistent with earlier approaches for long-channel devices. For short-channel devices, the variation of current with drain voltage up to the punch-through voltage is accurately described. The threshold voltage of a short-channel device as a function of applied voltages follows as a natural result of the derivation. Results are presented which confirm the theory over a wide range of drain and gate voltages. With the application of substrate bias it is concluded from the data and the theory that two-dimensional effects can cause dramatic increases in the drain conductance.