Transistors

Overview

A transistor is a three-terminal semiconductor device used mainly as an amplifier or an electronic switch. It can let a small electrical input control a much larger current or voltage. That ability made it possible to replace many vacuum tubes with devices that were smaller, cooler, more rugged, and far more energy efficient.

The first successful transistor was demonstrated at Bell Laboratories in December 1947 by John Bardeen and Walter Brattain, working in William Shockley's research group. Their first device was a point-contact transistor made from germanium. Semiconductor transistors rapidly changed radio, telephone, instrumentation, computing, and nearly every other branch of electronics.

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From the diode to the transistor

The diode section introduced semiconductor doping, P-type and N-type material, electrons, holes, depletion regions, and PN-junction bias. A transistor builds on those ideas but adds a third terminal so the motion of charge carriers can be controlled.

Different transistor families accomplish that control differently:

Family	Control idea	Common terminals
Bipolar junction transistor (BJT)	Carrier injection through two closely related PN junctions.	Emitter, base, collector.
JFET	A reverse-biased junction changes the width of a conductive channel.	Source, gate, drain.
MOSFET	An electric field through an insulated gate creates or controls a conductive channel.	Source, gate, drain.

Amplifier or switch?

A transistor used as an amplifier operates in a region where a small change at its input produces a controlled larger change at its output. A transistor used as a switch is driven between conditions intended to represent “off” and “on.” The same physical device can often do either job, but the design rules are different.

Transistor packages

The outside package does not tell you the exact transistor type or pin order. Three-leaded packages such as TO-92 may contain NPN or PNP BJTs, JFETs, MOSFETs, sensors, voltage references, and other devices. Even two transistors in the same package style can have different pinouts.

Never assume a transistor pinout from package shape alone. Check the exact part number and manufacturer datasheet before connecting it.

What transistors made possible

Compact radios and hearing aids.
Low-power telephone and communication equipment.
Electronic switching without moving contacts.
Reliable digital logic and computers.
Audio and radio-frequency amplification.
Motor drives and power conversion.
Integrated circuits containing millions or billions of transistors.

Continue through the transistor pages

The Basics page introduces switching, amplification, terminals, bias, and safe testing. BJT follows directly from the PN-junction diode. JFET and MOSFET introduce field-effect control. Circuits shows the most common circuit building blocks, and Power Transistors covers higher-current switching. The History page follows the invention from Bell Labs into silicon and MOS technology. The Advanced page adds device equations and small-signal models, while To ICs explains how individual transistors become integrated circuits.

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Electronic Parts

Discrete Components