Definition: transistor

In the analog world of continuously varying frequencies, a transistor amplifies a weak electrical signal to drive a speaker. In the digital world, a transistor is an on/off switch and the fundamental building block of computer circuitry. Like a light switch on the wall, a transistor either prevents or allows current to flow through.

The Foundation of All Activity
Transistors, wired in patterns, enable a computer to follow instructions to calculate, compare and copy data. All instruction execution and data processing take place because transistors are wired together in digital circuits (see 3 C's and Boolean logic).

A chip can contain from thousands to billions of transistors, and the elements making up the transistors are measured in billionths of a meter (see process technology and transistor density).

Silicon Is Switchable
The active part of the transistor is silicon, which changes its electrical state when pulsed. A transistor may normally allow or impede current flow, but when voltage is applied, it flips its state. See chip and active area.

From Transistors to Systems

Transistors make up logic gates. Gates make up circuits. Circuits make up electronic systems (see Boolean logic and Boolean gates).

Current Is Always at the Source

There is always current at the transistor's input side. When the gate is pulsed, the semiconductor element becomes conductive and current flows to the output side (drain).

Building the Transistor
Chips are made using photolithograpy. Through multiple stages of photomasking, etching, and chemical implantation (diffusion), the chip's sublayers are created. The final stage lays the top metal layers that connect the transistors mostly to each other, but also to the outside world.

A Hundred Million on the Head of a Pin
As you review the following stages in the building of a transistor, think about the fact that today's chip making creates and interconnects more than a hundred million of these transistors in one square millimeter, the same size as the head of a pin! In addition, state-of-the-art transistors are even more complicated than the bipolar examples below (see FinFET). See EUV machine and photolithography.

At the Same Time

Most of the transistors in every chip on the wafer are created at the same time. A 300mm wafer like this can hold hundreds and thousands of dies (chips), which means billions and trillions of transistors are fabricated simultaneously. See wafer. (Image courtesy of Intel Corporation.)

The First Silicon Transistor

In 1954, Texas Instruments pioneered production of discrete transistors on a commercial scale. About a quarter inch square, this amount of space can hold trillions of transistors today. (Image courtesy of Texas Instruments, Inc.)

IBM "Solid Logic"

Instead of only one transistor per package, IBM's advanced engineering placed three transistors on a single module for its System/360 family in 1964. With the cover removed, the three are plainly visible. See active area. (Image courtesy of IBM.)

Transistor Evolution

Computers in the 1940s and mid-1950s used vacuum tubes (left) for on/off digital switches, but by the late 1950s, discrete transistors (middle) replaced them. Integrated circuits emerged in the 1960s and evolved into chips with millions and billions of transistors; for example, the chip (right) has more than 35 billion (see Versal). The pencil is added for size comparison. See integrated circuit. (Image courtesy of Xilinx.)