How Do Transistors Work?

How Do Transistors Work?

The transistor is one of the most important and versatile electrical components in modern electronics. It's the building block of microchips and computers, as well as MP3 players, digital cameras and other electronic gadgets. The tiny semiconductor device can be switched on and off with a small electric current. Some consider the transistor to be one of the most important inventions of the 20th century, because it opened the door to so many other technological advancements.

The basic working principle of the transistor is that a small current flowing through the base terminal can control a larger flow of current between the emitter and collector terminals. This is what makes the transistor work as an amplifier, amplifying a relatively small input current into a much larger output current. The transistor also can act as a switch, switching on or off depending on the state of the gate electrode -- usually a few volts or less, but sometimes up to 100 volts or more.

In a basic bipolar junction transistor, or BJT, a thin layer of P-type silicon (yellow) is placed on a thicker layer of N-type germanium orange). The three contacts are labeled as the emitter (E, red), the collector (C, blue) and the base (G, green). When no current flows through the transistor, it's "off." When we apply a small voltage to the gate electrode, it becomes the control terminal -- it can either allow or prevent current from passing through the transistor. The transistor can stay in one of these two stable states, "on" or "off," until another current with the same base current again flows through it.

The base of the transistor is called the P-type side because it has atoms doped with more electrons than required, creating free electrons that are "holes." When we connect a battery across this atomic junction, the negative end of the battery ("cathode") pushes electrons from the n-type semiconductor into the p-type semiconductor, where they fill holes and allow current to flow through the circuit.

This simple action illustrates the fundamental physics that's behind diodes, transistors and many other logical electronic devices. In fact, all electronic devices can be categorized as logical components that act on electrical impulses. These components may be small and enclosed in a resin case or large and mounted on a metal body to help remove heat generated by the constant interaction of electricity and matter. The higher power transistors can be so hot that we typically see them attached to a heat sink to disperse the heat.