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. (C) 2003 Elsevier B.V. All rights reserved.