The metal-oxide-semiconductor field effect transistor, MOSFET, is the most widely studied and understood IGFET, and is used here to describe key aspects of IGFET behaviour. The topic is introduced by a physical description of MOSFET operation, which identifies the linear and saturation operating regimes. A simple analytical model is developed from this, yielding expressions for important parameters such as threshold voltage, saturation voltage and carrier mobility. These are extensively used in TFT analysis. A more rigorous analysis is presented to explicitly include the role of substrate doping, and its effect upon saturation voltage. These descriptions are valid for the on-state regime, in which the gate bias, VG, is greater than the threshold voltage, VT. A further operating regime is described for VGVT, which is referred to as the sub-threshold regime.In this regime, the current increases exponentially with gate bias, and is characterised by the sub-threshold slope. This is another concept, and parameter, which is widely used in the analysis of TFT behaviour. Finally, the role of film thickness, in thin film devices, in modifying the standard expressions for threshold voltage and saturation voltage is presented.