A thin film transistor (TFT) is a special kind of field effect transistor made by depositing thin films of a semiconductor active layer as well as the dielectric layer and metallic contacts over a supporting substrate. A common substrate is glass, since the primary application of TFTs is in liquid crystal displays. This differs from the conventional transistor where the semiconductor material typically is the substrate, such as a silicon wafer.

TFTs can be made using a wide variety of semiconductor materials. A common material is silicon. The characteristics of a silicon based TFT depend on the crystalline state. That is, the semiconductor layer can be either amorphous silicon, microcrystalline silicon, or it can be annealed into polysilicon. Other materials which have been used as semiconductors in TFTs include compound semiconductors such as cadmium selenium (CdSe) and metal oxides such as Zinc Oxide. TFT's have also been made using organic materials (referred to as an Organic TFT or OTFT).

By using transparent semiconductors and transparent electrodes, such as indium tin oxide (ITO), some TFT devices can be made completely transparent.

Because the substrate cannot stand for the high annealing temperature, the deposition process has to be completed under relatively low temperature. Chemical vapor deposition, physical vapor deposition (usually sputtering) are applied. Also the first solution processed transparent TFTs (TTFTs), based on zinc oxide were reported in 2003 by researchers at Oregon State University. Wager, John. OSU Engineers Create World's First Transparent Transistor. College of Engineering, Oregon State University, Corvallis, OR: OSU News & Communication, 2003. 29 July 2007.