The main mechanisms of electron-hole generation in a semiconductor induced by absorption of a sub-band gap photon are considered. Electron-hole generation with sub-band gap light involves optical and thermal transitions between delocalized valence or conduction band states and localized band gap states. A semiconductor device with a retrieval layer in which electron-hole pairs are effectively separated is considered. For each mechanism, the steady-state photocurrent quantum yield and the photocurrent response to a harmonic modulation of the intensity of sub-band gap light is calculated. It is shown that sub-band gap intensity modulated photocurrent spectroscopy makes it possible to resolve different contributions to the sub-band gap photocurrent and to study the kinetics of free carrier generation induced by sub-band gap light absorption. Experimental results obtained with nanoporous GaP photoanodes are analyzed; two different mechanisms are responsible for sub-band gap photocurrent flow. The potential of sub-band gap intensity modulated photocurrent spectroscopy for the characterization of (surface) localized band gap states is discussed and illustrated with results obtained with nanoporous GaP.