NPN – Transistor
A transistor is a semiconductor device which is used to switch or amplify electronic signals and electric power. It consists of semiconductor material mostly with at least three terminals for connection to an external circuit. In all the fields of electronics, transistors are used for almost all practical purposes. A semiconductor is mainly used to manufacture two devices, those are diodes and transistors. For amplifying a signal, junction diode cannot be used. For that, another type of semiconductor device came into account and these transistors are also known as bipolar junction transistors.
Bipolar Transistor or BJT, comes in two basic forms. An NPN (Negative-Positive-Negative) type and a PNP (Positive-Negative-Positive) type.
N – P – N Transistor: When a single p-type semiconductor layer is sandwiched between two n-type semiconductor layers, an NPN transistor is formed.
NPN Transistor Symbol: The circuit symbol and diode analogy of NPN transistor is shown in the below figure: The circuit symbol and diode analogy of NPN transistor is shown in the below figure.
In the above figure, it is shown that the electric current always flows from p-region to n-region.
NPN Transistor Construction: The NPN transistor is made up of three semiconductor layers: one p-type semiconductor layer and two n-type semiconductor layers. The p-type semiconductor layer is sandwiched between two n-type semiconductor layers.
The NPN transistor has three terminals: emitter, base and collector. The emitter terminal is connected to the left side n-type layer. The collector terminal is connected to the right-side n-type layer. The base terminal is connected to the p-type layer.
The NPN transistor has two p-n junctions. One junction is formed between the emitter and the base. This junction is called emitter-base junction or emitter junction. The other junction is formed between the base and the collector. This junction is called collector-base junction or collector junction.