How a Transistor Works as a Switch and an Amplifier
A transistor lets a small signal control a larger current. Learn how that makes amplifiers and digital switches, from the 1947 invention to the modern MOSFET.
In this article
A transistor is a small semiconductor part in which a weak signal controls a larger electric current. Used one way it is an amplifier. Used another way it is an on-off switch, and that is how it is used inside computer chips. This article covers the idea, the 1947 invention, how the common MOSFET type works, and why one design came to dominate.
The basic idea
According to Wikipedia, a transistor is a semiconductor device with at least three terminals. A voltage or current applied to one pair of terminals controls a larger current flowing through another pair. Because the output power can be greater than the input power, a transistor can act as an amplifier. Because it can be driven fully on or fully off, it can also act as an electronic switch, which is how it is used in digital logic.
The first transistor, 1947
The Computer History Museum describes the first one: in 1947, John Bardeen and Walter Brattain, working under William Shockley at Bell Labs, demonstrated the point-contact transistor. It was a sliver of germanium touched by two closely spaced gold contacts held in place by a plastic wedge. A small change in the current through one contact caused a larger change in the current at the other, so the device amplified current.
Wikipedia adds that Shockley went on to develop the bipolar junction transistor, and that the three men shared the 1956 Nobel Prize in Physics.
How a MOSFET works
The most widely used type today is the MOSFET, short for metal-oxide-semiconductor field-effect transistor. It has three main terminals: the source, the drain and the gate. Current flows between source and drain, and the gate decides whether it can.
The gate is separated from the semiconductor channel by a thin insulating layer, traditionally silicon dioxide. Because that layer blocks steady current, the gate draws almost none. Applying a voltage between the gate and the source creates an electric field that pulls charge carriers toward the surface and forms a conducting path between source and drain, which Wikipedia calls an inversion layer.
- Below the threshold voltage there is essentially no channel, and only a tiny leakage current flows. The switch is off.
- Above the threshold the channel conducts, and the more gate voltage, the more current. The switch is on.
- When used as a digital switch, the aim is to be fully on or fully off.
Two flavours: NMOS and PMOS
An n-channel device (NMOS) conducts using electrons. A p-channel device (PMOS) is the mirror image and conducts using positive charge carriers called holes. Pairing one of each gives CMOS logic. Wikipedia explains that CMOS circuits draw significant power mainly while they are switching, which is why the approach dominates processors and memory.
Amplifier or switch?
The same part can do both jobs, depending on how it is biased. In what Wikipedia calls the saturation mode, the current depends mainly on the gate-to-source voltage, which suits amplification. In the linear mode it behaves like a resistor controlled by voltage. For switching, the device is pushed fully on or fully off. Switching speed is limited by how quickly the gate, which behaves like a small capacitor, can be charged and discharged.
From one part to billions
Wikipedia reports that Mohamed Atalla and Dawon Kahng at Bell Labs proposed the silicon MOS transistor in 1959 and demonstrated a working device in 1960. It says MOSFETs now account for about 99.9% of all transistors made. Most transistors are built into integrated circuits, and a single advanced processor chip can hold tens of billions of them. The page cites a microprocessor with as many as 92 billion transistors on one piece of silicon as of 2023.
The field-effect idea is older than the working device. The same source credits Julius Edgar Lilienfeld with proposing it in 1925, though a working version was not built then.
Why it matters
Every digital choice a computer makes comes down to billions of tiny switches turning on and off. The transistor also amplifies weak signals from antennas and microphones. Replacing earlier bulky parts with something this small made today's phones and computers possible. General information only; for circuit design, use the manufacturer's datasheet.
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