

The Function and Applications for Bipolar Junction Transistors


BJT Layers

A bipolar transistor consists of a three-layer “sandwich” of doped (extrinsic) semiconductor materials, (a and c) either P-N-P or N-P-N (b and c ). Each layer forming the transistor has a specific name, and each layer is provided with a wire contact for connection to a circuit. The schematic symbols are shown in the figure (a) and (c).

BJT transistor: (a) PNP schematic symbol, (b) layout (c) NPN schematic symbol, (d) layout.


Bipolar transistors work as current-controlled current regulators. In other words, transistors restrict the amount of current passed according to a smaller, controlling current. The main current that is controlled goes from collector to emitter, or from emitter to collector, depending on the type of transistor it is (NPN or PNP, respectively). The small current that controls the main current goes from base to emitter, or from emitter to base, once again depending on the kind of transistor it is (NPN or PNP, respectively). According to the standards of semiconductor symbology, the arrow always points in the direction of current flow.

The direction of the small, controlling current and the large controlled current for (a) a PNP and (b) an NPN transistor.

Bipolar Transistors Contain Two Types of Semiconductor Material

Bipolar transistors are called bipolar because the main flow of current through them takes place in two types of semiconductor material: P and N, as the main current goes from emitter to collector (or vice versa). In other words, two types of charge carriers—electrons and holes—comprise this main current through the transistor.


No current through the base of the transistor shuts the transistor off like an open switch and prevents current through the collector. A base current turns the transistor on like a closed switch and allows a proportional amount of current through the collector. Collector current is primarily limited by the base current, regardless of the amount of voltage available to push it. The next section will explore in more detail the use of bipolar transistors as switching elements.


  • • The three leads of a bipolar transistor are called the Emitter, Base, and Collector.
  • • In order for a transistor to properly function as a current regulator, the controlling (base) current and the controlled (collector) currents must be going in the proper directions: meshing additively at the emitter and going in the direction of the emitter arrow symbol.


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