Organic FET transistors (OFETs) were fabricated on silicon substrates using pentacene as active organic layer and gold source and drain contacts. OFETs were used as test structures in order to study carriers photogeneration effect in organic layers. Photogeneration process was induced by exposing organic layers to a laser beam with 625 nm wavelength and an applied power varying in the range 10-40 mu W. As an effect of light exposure, OFET drain current is increased by three-to-six orders of magnitude, depending on devices operation conditions and their geometry. It is suggested that both electrons and holes generated in pentacene layer contribute to current increase. We put in evidence that electrons trapped at the pentacene-dielectric interface can act as a negative biased gate, demonstrating new operating capabilities of OFETs as well as light sensing functionality.