Based on simulations from a detailed Monte Carlo Poisson solver, we show that the effects of photoinduced screening of the bias field as a function of optical spot shape for ultrafast excitation of photoconductors to generate terahertz transients leads to alteration of the temporal shape of the terahertz transients related to the photoexcited carrier drift velocity. The results of the simulations demonstrate that given the same photoexcited carrier density, a line focus parallel to the direction of the bias field suppresses saturation at high optical fluence due to space-charge screening and enhances the peak terahertz power in the time domain.