Field effect transistors have been made, the channels of which consist of conducting, iodine-doped Langmuir-Blodgett (LB) films. In the chosen configuration. the modulation effect is shown to arise-mostly from the source channel contact, which behaves as a reverse-biased Schottky diode. The gate voltage modulates the height of the diode barrier by controlling the surface charge density of the oxide LB film interface states. This model explains qualitatively and quantitatively the behaviour of the transistor and gives rise to remarkably good fits with the experimental transistor characteristics. The model is also supported by a counterexperiment in which the source and drain contacts are ohmic, as expected this leads to almost no drain current modulation.