The NPN transistor is done by taking a P-type semiconductor material and sandwiching that in between two N-type semiconductor materials. In reality, the P-type layer is thin. This transistor has there terminals known as emitter, base, and collector.
The schematic diagram of an NPN transistor is shown in the above figure. The voltage between the base and emitter terminals is called VBE and is more positive at the base than the emitter because, for NPN transistors, base terminals should be higher in potential than the emitter.
A transistor has three sections of doped semiconductors. With one side emitter and another is the collector while the base lies in between. The middle portion is called the base. It forms the two aforementioned PN junctions between the emitter and collector.
Construction of NPN Transistor
Emitter: One section which supplies charge carriers is called the emitter. To supply a large number of charge carriers, the emitter is always forward-biased when compared to the base.
Base: The middle section of the transistor, which forms the two PN junctions between the emitter and collector, is called the base. The base-emitter junction is forward-biased which allows a low resistance for the emitter circuit.
Collector: The section other than the emitter which collects the charges is called the collector. The collector is always reverse-biased.
Working of NPN Transistor
The working of an NPN transistor is more complex than that of the PNP transistor. The transistor is in an OFF state if; the voltages at the base and emitter are the same. As the base voltage increases above the emitter voltage, the device shifts to its ON state.
In the ON state, there is sufficient voltage difference with the base terminal being the higher one, there is a flow of electrons generated from the collector to the emitter which in turn causes a current to flow from the emitter to the collector. In this transistor, based on the working we can divide the three terminals it has into input and output ports. The input terminal is the base terminal, and the output terminal is the collector-emitter region.
The voltage at the collector terminal should be greater than the emitter and must be positive with respect to the emitter voltage of the transistor, this ensures the current flow between the emitter and the collector. There is also a positive potential drop across the base-emitter junction, which is to no surprise the voltage drop across a silicon diode, i.e. 0.7V.
To use the NPN transistor as a signal amplifier, it has to be operated in the active region. In a configuration where the emitter has a common connection, the transistor total current flow is defined as the ratio of the collector current to the base current, and this ratio is called DC current gain.
Operation Regions of NPN Transistor
- Active Region
- Cutoff Region
- Saturation Region
- Breakdown region
NPN Transistor Applications
- Used for switching purposes.
- Used in the Darlington pair configuration to amplify weak signals.
- In circuits where we need to sink the current