Thin-film transistor

A thin-film transistor (TFT) is a switch made by stacking thin films on glass, rather than by building it into a silicon crystal like a computer chip. An LCD needs one for every subpixel.

Most TV transistors are built with the gate at the bottom. A strip of copper forms the gate, on a thin layer of molybdenum that helps it stick to the glass and stops copper atoms wandering into neighboring layers. Over it goes silicon nitride, the insulator, then a patch of amorphous silicon, the channel. Two more copper electrodes, the source and drain, are laid on top of the silicon with a small gap between them. Each layer is deposited and patterned in turn as the thin-film transistor array is built.

When a voltage is put on the gate, it draws charge into the silicon, and current can flow from source to drain to charge the subpixel. When the gate voltage drops, the switch turns off and the charge stays put.

The first amorphous-silicon TFT was reported in 1979 at the University of Dundee. Many newer panels use IGZO, an oxide of indium, gallium and zinc, which conducts roughly ten times better and allows smaller, faster transistors.