Observations of a directional asymmetry in the sub-ps THz radiation generated from ultrafast excitation of biased GaAs are presented. This asymmetry is inconsistent with the long standing and widely accepted surface-layer current-surge description of the THz emission process. A model based on propagating carrier excitation fronts during spectral-hole-burning is proposed to explain these observations. This model introduces new roles for the semiconductor optical and carrier transport properties in determining the efficiency of the THz generation process.