A thin film transistor (TFT) is a type of field-effect transistor that is commonly used in electronic devices such as flat-panel displays, smartphones, and computer monitors. It is called a “thin film” transistor because it is made using thin layers of semiconductor material, usually amorphous silicon or polysilicon, which are deposited on a glass or plastic substrate. The TFT acts as an electronic switch, controlling the flow of current in the device.