Transistors are semiconductor devices used to switch electronic signals, amplify, or electrical power. Transistors are fundamental in modern electronic circuits. A transistor is made constructed of semiconductor materials and typically has three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.

Macro shot of power transistors on the b

Bipolar Junction Transistor Symbols

There are two types of transistors, NPN and PNP.

  • The NPN transistor has two blocks of N-type semiconductor material and one block of P-type semiconductor material.

  • The PNP transistor has two P-type and one layer of N-type material. The symbol of NPN of both NPN and PNP is shown in the figure below.

npn trans diagram2.png
pnp trans diagram2.png

The difference between NPN and PNP transistors is in the direction of the current. The arrow in the above diagram indicates the direction of flow of typical current in the emitter with forward biasing applied to the emitter-base junction.

Transistors have three terminals as shown above:

  • Emitter

  • Collector

  • Base

Emitter - The emitter segment supplies a large portion of the charge carrier. The emitter is constantly connected in forward biased in respect to the base, it supplies the bulk charge carrier to the base. The emitter-base junction inserts a large amount of majority charge carrier into the base because it is heavily fixed and moderate in size.

Collector The collector accumulates the major percentage of the charge carrier supplied by the emitter. The collector-base junction is constantly in reverse bias. Its function is to remove the preponderance charges from its junction with the base. The collector segment of the transistor is moderately fixed but greater in mass so that it can collect most of the charge carrier supplied by the emitter.

Base – The middle section of the transistor is the base.


The base forms two circuits:

  • The input circuit with the emitter.

  • The output circuit with the collector.

The emitter-base circuit is forward biased and offers low resistance to the circuit. Whereas the collector-base junction is in reverse bias and offers higher resistance to the circuit. The base of the transistor is lightly incapacitated and extremely thin as it submits the majority charge carrier to the base.

working of transistor white2.png

Silicon is generally used to make transistors for their high voltage rating, greater current, and less temperature sensitivity. The emitter-base section kept in forward biased establishes the base current that flows through the base region. The magnitude of the base current is exceedingly small. The base current causes the electrons to move into the collector area or creates a hole in the base region.

The base of the transistor is extremely thin and lightly fixed because of this it has fewer electrons compared to the emitter. The limited electrons of the emitter are combined with the hole of the base region and the remaining electrons move toward the collector region and constitute the collector current.