NPN Transistor

Bipolar Junction Transistor

A bipolar junction transistor is a single component made of silicon where both electrons and holes are used as charge carriers. A bipolar junction transistor lets a small current be injected at one of its terminals to control large amounts of current flowing between the other two terminals. This makes the device capable of performing switching or amplification.

Bipolar junction transistor is of two types:

  • PNP transistors
  • NPN transistors

This bipolar PNP junction transistor is formed with three layers of semiconductor material, with two P-type regions sandwiched between one N-type region.

NPN Transistor

The NPN transistor consists of two n-type semiconductors that sandwich a p-type semiconductor. Here, electrons are the majority charge carriers, while holes are the minority charge carriers. The NPN transistor is represented, as shown below.

NPN Transistor Image 1

In the above figure, we can see an arrow pointing outwards from the emitter terminal. This indicates the direction of the flow of current through the device.

Construction Of PNP Transistor

NPN Transistor Image 2

The NPN transistor is made of semiconductor materials like silicon or germanium. When a p-type semiconductor material is fused between two n-type semiconductor materials, an NPN transistor is formed.

The NPN transistor features three terminals: emitter, base and collector. This transistor features two diodes that are connected back to back. The diode seen between the emitter-base terminal is referred to as the emitter-base diode. The diode between collector and base terminal is known as collector-base diodes. The emitter is moderately doped, the base is lightly doped, and the collector is comparatively more doped.

Working of NPN Transistor

npn transistor

When the emitter-base junction is forward biased, a small voltage VBE is seen. Reverse bias voltage VCE. Due to the forward bias, the majority charge carriers in the emitter are repelled towards the base. The electron-hole recombination is very small in the base region since the base is lightly doped. Most of the electrons cross into the collector region.

When the emitter is forward biased, electrons move towards the base and create the emitter current IE. Here, the majority charge carriers in the P-type material combine with the holes.

Since the base of the NPN transistor is lightly doped, it lets only a few electrons to combine and the remaining current is known as the base current IB. When the collector region is reverse biased, it applies a greater force on the electrons reaching the collector junction and hence attracts the electrons at the collector.

Read more about Transistor.

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Frequently Asked Questions on NPN Transistor

1. What are the three terminals of the transistor?

Emitter, base and collector.

2. What are the majority charge carriers in NPN transistors?


3. What are the majority charge carriers in PNP transistors?


4. Define emitter current.

Sum of the base current along with the collector constitutes emitter current.

5. What are the operative modes of a transistor?

  • Cut-off mode
  • Saturation mode
  • Active mode

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