hardware//electronics//semiconductor//transistor

2026-10-03

A transistor is a valve for current controlled by a voltage, and it is the part every chip is built from: driven fully on or fully off it is a switch, held in between it is an amplifier. Logic, memory, multiplexers that pick one input among many and the amplifiers that lift a sensor's microvolts are all transistors, used one way or the other.

In the MOSFET, the kind in almost every chip, current flows between two terminals, the source and the drain, through a channel of semiconductor. A third terminal, the gate, sits over the channel behind a thin layer of insulating oxide, so no current flows into it; the voltage on it attracts or repels charges in the channel and decides how easily current passes. As a switch the gate voltage is either high or low and the channel either conducts or does not; as an amplifier a small change of gate voltage produces a larger change of current, which a resistor or a load turns back into a larger voltage.

The same transistor is a switch when it is driven hard and an amplifier when it is driven gently.

A chip chooses per circuit: digital logic and selection switches use the first mode, the front end that reads a sensor uses the second.

A switch costs almost nothing to keep. Once its gate is set, a transistor used as a switch draws almost no current, so a chip can carry hundreds of them, each connecting one sensor to a shared line, and set them once from a few configuration bits.

An amplifier wants larger transistors. Small transistors are noisier at low frequencies and less alike from one to the next, so circuits that amplify faint analog signals use devices larger than the smallest the process can make, which is one reason analog chips stay on older process nodes.

Many transistors in two complementary types make the CMOS process, and their arrangement on a die is a chip.