A model transistor is defined and used to understand steady-state electron transport ill nanoscale silicon transistors. For this model device, the electric field profile is fixed and not computed self-consistently from the carrier density profile. The current versus voltage (I-V) characteristics of the model device as well as the internal carrier density and velocity profiles are computed by solving the Boltzmann transport equation. We find that the I-V characteristics of the model transistor can be explained by simple, physical arguments even when the critical regions are of the order of a mean free path and strong off-equilibrium transport occurs. This analysis provides insight into the current-limiting mechanisms of a nanoscale transistor. It also provides a basis For testing some commonly used transport models and shows that many of them are seriously unphysical at nanoscale dimensions.