A Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device used in electronic circuits for amplification and switching purposes. It comes in two main types: NPN (Negative-Positive-Negative) and PNP (Positive-Negative-Positive). Here, I'll focus on the NPN type, as it's more commonly used.
The NPN BJT has three layers of semiconductor material: two layers of N-type material (which has an excess of electrons) sandwiching a layer of P-type material (which has a deficit of electrons or excess of holes). The three terminals of the BJT are:
Collector (C): This terminal is connected to the heavily doped P-type layer and is responsible for collecting majority carriers (electrons in an NPN transistor).
Base (B): The middle terminal is lightly doped and acts as the control terminal. It controls the flow of current between the emitter and collector.
Emitter (E): This terminal is connected to the heavily doped N-type layer and is the source of majority carriers (electrons in an NPN transistor).
The operation of a BJT can be explained in two modes: the active mode and the cutoff mode.
Active Mode:
In the active mode, the BJT operates as an amplifier. To turn the transistor on in this mode, a small current is applied to the base-emitter junction (Ib). This current causes the base-emitter junction to become forward-biased, allowing electrons to flow from the emitter to the base region. These electrons then diffuse into the collector region because the base-collector junction is reverse-biased. As a result, a larger current (Ic) flows from the collector to the emitter, and this current is amplified compared to the base current (Ic = β * Ib, where β is the current gain of the transistor).
Cutoff Mode:
In the cutoff mode, the transistor is turned off. No current flows between the collector and emitter. This happens when there is no base current (Ib = 0), and both the base-emitter and base-collector junctions are reverse-biased. As a result, the majority carriers are unable to cross the junctions, and the transistor effectively stops conducting.
The operation of the transistor is primarily governed by the relationship between the base current (Ib) and the collector-emitter current (Ic). The ratio of Ic to Ib is known as the DC current gain (β) of the transistor. The transistor can be used as a voltage or current amplifier in various electronic circuits due to this amplification effect.
It's important to note that the BJT can also operate in saturation mode, where the transistor acts as a closed switch and allows a large current to flow from the collector to the emitter. This happens when the base-emitter junction is forward-biased enough to cause the base-collector junction to be forward-biased as well.
Overall, the bipolar junction transistor is a fundamental component in modern electronics and has widespread applications in various electronic devices and circuits.