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Transistor Working Basic

Transistor is 3 Terminal electronic semiconductor component/device.

It has three terminals named as BASE, Emitter And Collector.

Transistor name meaning lies in its own working.
Trans + istor = Transistor
The prefix trans tells about signal transfer from low resistance to high resistance part and istor means solid physical structure having property of resistance.

A transistor has a 3 sandwich layers of doped semiconductors.

Basically Transistors are 2 types.

1 . NPN Transistor

2. PNP Transistor

Transistor is the very important components in electronics system. This is the basic electronic component that uses vastly in the making  of electronic circuit.

Mainly transistors are used for the switching purpose. another uses of transistor is amplifier.

Transistor is made by using of 2 pn junction diode . 

NPN= negative -positive-negative. 

NPN transistor has both side material is n type and between of both a positive (Hole) type material. Electrons are the majority carriers and holes are minority carriers.

Base of NPN transistor is P-typeand Emitter is N-type and should connected with the negative supply.

A small Positive current  required at the Base terminal for the turning on the transistor.  By sending varying levels of current to the base, the amount of current flowing through the collector to emitter may be regulated.

When we apply a small Positive supply at base , then Current between emitter and collector will passed and we say that transistor is turn on.  A very small amount of current may be used to control a large amount of current, this property is known as amplifier . 

transistor symbol and basic structure


The base of the transistor is used to switch current through the collector and emitter. When the base is between 0V and 0.7V it is switched off and when above 0.7V it is switched on and allowing the current to flow from the collector to the emitter.

In PNP type transistor we need a Negative voltage at the base terminal for turning on the transistor. In case of pnp the only difference is due to p-type emitter and n-type base, holes are the majority carriers and electrons are minority carriers.

When we apply a small negative supply at the base of PNP then the current between collector an emitter will coduct.

PNP= Positive-negative-Positive

The following Animation shows the basic switching function using with Relay for high current switching. If You need small current switching then without relay, Directly you can connect load with Collector .

REES52 Electronics Project Starter Kit w/Breadboard, Jumper Wires, LED, Resistors, Motor for Arduino & Raspberry Pi 

npn pnp transistor working.

pnp transistor working as switch


npn transistor working as switch


Note – If use Relay in any circuit then a Diode (called as flyback diode) must connect with parallel of Relay input, This is essential for protection . Both with PNP and NPN transistor, Flyback diode is must in reverse bias. 

The current supply between collector and emitter controlled by the supply signal of Base terminal. If signal at Base increase then also current conduction between emitter and collector increased. Base terminal of transistor works as adjustable knob of any water tip/Bowl. That control the amount of water passing through it.

The current conduction in general transistor (npn and pnp) is done by both polarities , negative supply as well as positive supply because of both minority and majority carrier. So this is called bi-polar junction transistor (BJT).

 Bipolar Junction Transistors operate within three different regions

  • Active Region   –   the transistor operates as an amplifier 
  • Saturation   –   the transistor is “Fully-ON” operating as a switch and 
  • Cut-off   –   the transistor is “Fully-OFF” operating as a switch and 


In Electronics Transistor is widely used as the switching purpose .

In Logic gate designing in digital circuits transistors are also used.

About the Author


Hi, My name is Aman Bharti, I am interested in making and study of Electronics, circuit diagram, PCB designing and layout etc. I like to share the knowledge and all ideas with people which I get from My Experiment & from different sources. I try to provide any circuit details deeply with test results as possible. If you want to give a suggestion or comments on anything, please leave your comment in the comment box of the related page.

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