In this case, both positive and negative voltages can be applied on the gate as it is insulated from the channel. The voltage at gate controls the operation of the MOSFET. The following figure shows the construction of a MOSFET. Depending upon the substrate used, they are called as P-type and N-type MOSFETs. In the construction of MOSFET, a lightly doped substrate, is diffused with a heavily doped region. This oxide layer acts as an insulator (sio 2 insulates from the substrate), and hence the MOSFET has another name as IGFET. An oxide layer is deposited on the substrate to which the gate terminal is connected. The construction of a MOSFET is a bit similar to the FET. The following figure shows how a practical MOSFET looks like. The FET is operated in both depletion and enhancement modes of operation. This is also called as IGFET meaning Insulated Gate Field Effect Transistor. MOSFET stands for Metal Oxide Silicon Field Effect Transistor or Metal Oxide Semiconductor Field Effect Transistor. To overcome these disadvantages, the MOSFET which is an advanced FET is invented. FETs have a few disadvantages like high drain resistance, moderate input impedance and slower operation.