Imagine two p-n junctions being joined back-to-back. This is the basic structure of the bipolar transistor. It is called bipolar because both electrons and holes carry current in the device. Bipolar transistors can occur in either n–p–n or p–n–p configurations. A schematic of a device in the n–p–n configuration is displayed below. Use the buttons to navigate through the animation.

Note: This animation requires Adobe Flash Player 8 and later, which can be downloaded here.

There are three contacts to the transistor in the above animation. These are (i) the emitter, (ii) the base and and (iii) the collector. The emitter and the collector are n-type doped material, while the base is p-doped material. These are often labelled E, B and C.

With forward biasing applied to the emitter-base junction, and reverse biasing applied to the base-collector junction, the bands deform as shown in the animation. The potential barrier for an electron to diffuse from the emitter into the base is relatively low. Electrons injected from the emitter then diffuse through the thin base region before being accelerated through the strong reverse biased field between base and collector. Note that while the electrons are in the base region, they are minority carriers. The base has to be thin, so that electrons diffusing through are not lost by recombining with a hole, i.e., a majority carrier within the base region. Not only will electron current reduced when recombination occurs, but heat is also dissipated in this process, which is also undesirable.

Varying the base voltage changes the size of the potential barrier for electrons to be transferred from the emitter. In this way, the emitter-base voltage can be used to modulate the flow of current from emitter to collector. The applied voltage can also be used to completely stop the current flowing through the device, in effect using the transistor as a switch. It is this switching function which is used in logic circuits, such as those used in computers. A switch closed is a binary 0 (no current flow from emitter to collector), while a switch open is a binary 1 (current flow occurs between emitter and collector).

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