We present an analysis of an undoped semiconductor strongly pumped by a coherent electromagnetic radiation field. The free-carrier dynamics and the incident and scattered fields are described classically and self-consistently. The analysis predicts the existence of a collective instability which simultaneously generates a strong free-carrier grating along the propagation direction and a coherently backscattered radiation field. An example is given showing scattered intensities approaching 100% of the pump intensity. A realistic experiment to enable observation of this phenomenon is described.