The device characteristics of the nanoscale Schottky-barrier tunnel transistor (SBTT) are investigated by solving the self-consistent two-dimensional Poisson-Schrodinger equations and treating the ballistic transport with the nonequilibrium Green's function formalism. A main focus lies in the assessment of the device performance of the SBTT as the channel length is gradually reduced down to a few nanometers. Due to the assumed ballistic transport, the device characteristics are almost the same if the channel length is greater than about 20 nm, but the device performance starts to degrade below L = 20 nm. By examining the device performance in terms of the voltage gain, transfer characteristics, and the threshold voltage behavior, we suggest that the channel length of the SBTT can be reduced to approximately 10 nm. Discussions on how scattering affects the simulation results and how to control on- and off-currents by varying the Schottky-barrier height and the gate dielectric constant are also presented.