A transistor is a device that acts as a switch or gate for electronic signals and regulates the flow of current and voltage. This device consists of three layers of semiconductors that carry current. In the year 1947, three scientists invented transistor in Bell Laboratories. It replaced the vacuum tube to serve the purpose of an electronic regulator. Moreover, in future, it paved a way for cheaper and smaller calculators, radios and computers. The transistor is a basic element in almost all electronic devices.

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There are two types of transistors – Junction transistor and Field-Effect transistor.


Functions Of Transistor-

(1) It is used as a switch for “large currents”

You can turn ON/OFF a large current flow by turning ON/OFF a small current using a transistor. Turning ON/OFF a large current flow directly without a transistor will be a difficult task and might need the person to manually cut off the line.

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(2) It is used for amplification

For example, If you can manipulate the “small current” wave shape to follow that of a sound signal (by converting sound to electrical signal), and then apply to the transistor, the “large current” will get the shape of that of the “small current” which in turns is the amplified sound signal shape. In short, sound signal (for that matter any signal) is amplified with the help of a transistor.

Working of a transistor-

The three different layers of a transistor are made of a different semiconductor material. Two out of three are doped in to give n-type or p-type, and one remaining layer is opposite of the other two layers. The layers are arranged in such a way that two similar layer sandwich the layer of opposite type. This gives two types of arrangement of layers in transistors namely P-N-P type and N-P-N type.

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The names for these three different layers are given below-

  • Emitter: It emits the charge carriers.
  • Collector: It collects the charge carriers.
  • Base: it is the region between emitter and collector.

In PNP transistors the sequence of doped regions is-

  • p-region collector
  • n-region base
  • p-region emitter


In NPN transistor the sequence of doped regions is-

  • n-region collector
  • p-region base
  • n-region emitter


A transistor can be considered as two P-N junctions that are placed back to back. One of these junctions is a base-emitter junction which is forward biased and the other base-collector junction is reverse biased. Here, considering the working of NPN transistor, the emitter and collector is n-type semiconductor material and the base is p-type.

The voltage of the base must be more positive than emitter and the voltage of collector must be more positive than the base. The batteries are connected in the circuit to supply the required voltages at different ends of the transistor. The emitter gives out the electrons and base pulls in these electrons because it has more positive voltage than the emitter. This transfer of electron creates a flow of electricity within the transistor.

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The current passes from emitter to collector through the base. The voltage of base varies to change the number of electrons in it and this modifies the flow of current. This small change makes a large change in the current flowing out of the collector.

A PNP transistor also works on the same principle as an NPN transistor. But it has one difference; the main flow of current in PNP transistor is controlled by changing the number of holes rather than the number of electrons in the base. Moreover, this transistor works well when the negative and positive connections are opposite of those in the NPN transistor.



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