Home Electronics Tutorials NPN Transistors & PNP Transistors : 5 Important Facts about Transistors as...

NPN Transistors & PNP Transistors : 5 Important Facts about Transistors as Amplifiers

What is a transistor?

Transistors definition: It’s a semiconductor device with three terminals mainly used for amplifying and switch electronic signals.

Who invented the transistor?

Transistors invented by John Bardeen, William Shockley and Walter Brattain. First, they invented Point contact transistor in 1947 and then in 1948, they invented bipolar junction transistor.

Bipolar junction transistors ( BJT )

There are two main types when we consider the transistor types.

  1. NPN transistors: Two N-type semiconductor layers are separated by one P-type layer.
  2. PNP transistors: Two P-type semiconductor layers are separated by one N-type layer.
NPN transistor PNP transistor
NPN Transistor & PNP Transistor Structure

Transistor symbol varies along with the point of the arrow. if the arrow points towards the emitter its an NPN transistor and if the arrow pointing towards the base, its a PNP Transistor.

Transistors NPN vs PNP

In the NPN transistors, the direction of the current is from collector to the emitter while in the PNP transistors the direction of the current is from emitter to collector.

NPN Transistors turns on when electrons enter into the base terminal while the PNP transistors turn on when holes enter into the base terminal.

In NPN transistors the majority charge carriers are electrons and in PNP transistors the majority charge carriers are holes.

There are 3 terminals in a transistor

  • C – collector terminal
  • B – base terminal
  • E – emitter terminal

NPN transistors are the Most widely using transistor types in the industry.

Actual construction and the inside view of a Bipolar junction transistor (BJT)

inside view of a transistor
Actual construction and the inside view of a Bipolar junction transistor (BJT)

Volume and doping density comparison of bipolar junction transistors (BJT)

The base of a BJT is having the thinnest layer with a small volume and then the Emitter having a medium volume while the collector occupies the largest volume within the bipolar junction transistor.

The doping density of a transistor varies inorder the collector having the lightest doping density and then the base having a medium dope density while the emitter of a BJT transistor having the heavy doping density

Modes of operation of a bipolar junction transistor

There are 3 main modes of operation in a bipolar junction transistor

  1. Cut off mode
  2. Active or linear mode
  3. Saturation mode

In the Cut off mode, emitter-base junction and the collector-base junction are reverse biased.

In the Active or Linear mode, emitter-base junction forward biased and collector-base junction reverse biased.

In the saturation mode, both emitter-base junction and the collector-base junction are forward biased.

(BJT) Bipolar junction transistors as amplifiers

An amplifier needs 2 terminals to connect the input signal and 2 terminals to connect the load.

So totally there needs 4 terminals but the NPN transistors and PNP transistors having only 3 terminals. Therefore it has to make one terminal common to both input and the output of the transistor.

So there are 3 common rails of a bipolar junction transistor (BJT)

  • Common Emitter ( CE )
  • Common Base ( CB )
  • Common Collector ( CC )

When transistors act as an amplifier, the emitter-base junction keeps as forward biased by applying a DC bias voltage.

The emitter current occurs due to the small voltage input signal contributes to the collector current and the collector current pass through the load resistor which makes a large voltage drop. 

A small input voltage turns into a large output voltage using the concept of transistors as amplifiers.

(BJT) bipolar junction transistors as a switch

Transistor switch operation mainly based on the region where the transistor operates in the I-V characteristic curve. The regions where a transistor can operate are active region, saturation region and the cutoff region. If the transistor operates in the saturation region, it acts as a fully ON state while the transistor operates in the cut-off region, it acts as a fully OFF state. Apart from that transistors act as an amplifier if it is operating in the active region.


DO YOU KNOW about Transistor radio ?

The transistor radio is a portable radio receiver which is small in size that mainly using transistor-based circuit within it. Regency TR-1 is the first transistor radio made in 1954 and then Sony TR-63 invaded the market. Then with the time, the use of transistor radio came to an end with boombox and Sony walkman and later with digital devices like mp3 players and mobile phones.

DO YOU KNOW How to find the 3 pins of a Transistor ? READ IT HERE >>>

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Priyan Bopitiyahttps://knovhov.com
BSc. Electronics Engineering | Managing Director - knovhov.com ™ | Marketing Consultant - Garbage disposal install ™

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