Transistor

A transistor is an electrical switch with no moving parts. Current can flow between two regions, the source and the drain, through a channel. A third terminal, the gate, sits just above the channel. Put a voltage on the gate and the channel conducts; remove it and the channel shuts off. Switching billions of these, billions of times a second, is how chips compute.

Transistors aren't made one at a time and assembled; they are built in place, all at once, on the wafer. The source, drain and channel are regions of doped silicon. Over the channel, a film of hafnium dioxide only a few nanometers thick is laid down one atomic layer at a time; it insulates the gate from the channel. On top goes a gate of titanium nitride and other metals. Then tiny holes are etched through an insulating layer above and filled with tungsten, making plugs that connect each transistor to the copper wiring overhead. In the newest chips, the channel is a thin fin or stack of sheets that the gate wraps around.

Linked by wiring, these switches make up a silicon die. The first transistor was built at Bell Labs in 1947.