The reported observation of SERS on semiconductors has confirmed the feasibility of distinguishing the charge-transfer mechanism from the electromagnetic one responsible for the enhancement of the signal in metal nanoparticles Experimental investigation of the well characterized dopamine-TiO2 system revealed an unexpected dependence on coverage and size We propose here a theoretical model applicable to SERS on semiconducting substrates that explains this remarkable behavior The model is based on a competition mechanism arising from the formation of an electron gas in the conduction band of the semiconductor due to the photoexcitation of a charge-transfer complex Taking into account the two competing effects, a linear increase in the Raman intensity arising from increasing coverage and a quenching effect due to the photon absorption by the electron gas provides excellent agreement between our model and the experiment for 5 nm nanoparticles Discrepancies for the case of 2 nm nanoparticles are attributed to quantum confinement, an effect that is investigated elsewhere