How Does a Bipolar Junction Transistor Work?

Background

Developed in the late 1940s, the transistor has literally transformed our world. The transistor's reliability, low price and size have made electronics common and available to everyone. These devices can range from a single transistor that can be used as a switch or amplifier to an Integrated Circuit (I.C.) that may contain millions of transistors on a single chip, and which powers a computer. A bipolar junction transistor, hereafter referred to as BJT, is a kind of transistor that uses two types of a doped semiconductor material. The BJT will have two PN junctions and will use three connections to control the transistor. These connections are called the base, the emitter and the collector. This material is known as P material or N material. The two materials are sandwiched together in two different configurations to build a PNP or NPN transistor. The current flow through the transistor will depend on the transistor configuration.

BJT Theory

Transistors can be thought of as two different configurations of diodes. A PNP BJT will function like two diodes that have both cathodes facing each other, and the NPN will have the two diodes with the anodes facing each other. Silicon is the most common type of doped material. The N doped material works by letting electrons become the majority carriers throughout the material. The P doped material uses holes, which are spaces without an electron occupying them. Both styles of BJTs function by letting a small current input to the base control an amplified output from the collector. The result is that the transistor makes a good switch that is controlled by its base input. The BJT also makes a good amplifier, since it can multiply a weak input signal 100 times its original strength. Strings of transistors are used to make powerful amplifiers with many different applications.

BJT switch

The attached drawing shows how an NPN BJT could be wired to function as a switch for a light-emitting diode, or LED. Closing the switch S1 will put a small input from the 9-volt supply on the transmitter's base. This input will turn on the BJT, allowing the LED to light when it receives the full input from the power supply. Notice how the base control circuit uses a 10,000-ohm resister to limit the base input so the transistor will not be destroyed.

BJT amplifier

The circuit shown here depicts what is known as a Darlington pair of transistors. This configuration functions as one big amplifier--although the signal is increased by the first transistor, which then feeds this to the second transistor, which in turn amplifies the signal even more. This circuit illustrates just how well a Darlington pair can work. Complete the circuit by touching the S1 contacts with your finger, and the Darlington pair will amplify the small current enough to turn on the LED.