A bipolar junction transistor, abbreviated BJT, is a type of transistor with three terminals that is made from a doped semiconductor material. Its primary purpose is for use in electrical devices to amplify or change electrical signals. To understand the bipolar junction transistor, it's best to understand the basic function of transistors and what the terms "doped" and "semiconductor" mean.
Transistors simply take an electrical signal that passes through them and either amplify it or alter it. The electrical current enters into the transistor and comes out stronger, thus amplified, when it leaves. Transistors are one of the basic units that make up all electrical devices and help control and manipulate the electric currents that pass through them. A bipolar junction transistor is simply a specific kind of transistor used to send currents in two different directions.
In electronics, the terms “doped” and “semiconductor” refer to specific qualities of transistors. Doped means that the material the transistor is made from is altered slightly to purposefully have imperfections in it. This manipulation of the material alters the charge slightly from the charge it normally carries. Semiconductor material refers to any type of material that can conduct an electrical current but not as strongly as a true conductor. Semiconductors are most commonly used in electronic devices.
Bipolar, in the case of the bipolar junction transistor, describes how the transistor uses both electrons and holes to function. When an electrical charge enters the transistor, it spreads out across two areas that carry different charges. One section of the bipolar junction transistor is forward-biased and the other half is reverse-biased.
The forward-biased section of the transistor is formed by connecting a type of semiconductor known as a P-type semiconductor to a positive terminal of a battery and connecting another semiconductor known as an N-type semiconductor to the negative battery terminal. To create the reverse-biased section of the transistor, the process is simply reversed so the P-type semiconductor connects to the negative terminal and the N-type semiconductor connects to the positive terminal. These different connections create the two opposite sections inside the transistor.
Two common functions for bipolar junction transistors are temperature detection and performing mathematical functions to solve logarithms and anti-logarithms. The ability of the bipolar junction transistor to carry two different currents allows it the flexibility to solve these types of math problems. This property also allows it to detect changes in temperatures by comparing two voltages and calculating the difference.