What is BJT

BJT full form is a bipolar junction transistor that uses both electron and hole charge carriers. For their operation, BJT uses two junctions between two semiconductor-type such as n-type and p-type. 

BJTs are manufactured in two types, NPN and PNP, and are available as individual components, or fabricated in integrated circuits, in large numbers. The function of a BJT is to amplify current that can be used as amplifiers or switches. These functions offer a wide range of electronic equipment applications, including computers, TVs, mobile phones, audio amplifiers, industrial control, and radio transmitters. 

Meaning of BJT :
  • A bipolar junction transistor is a three layer, two junction NPN or PNP semiconductor device with one p-region sandwiched by two n-region and two p-region sandwiched one n-region. It has three terminal named collector (C), Emitter(E), and base(B). 
    meaning of BJT
    Figure of  BJT
  • The current flow in the device takes place due to movement both holes and electrons. 
  • An emitter is indicated by an arrowhead indicating the direction of emitter current. No arrow is associated with base or collector. 
Schematic diagram symbol of BJT :


diagram symbol of BJT
NPN                                                  PNP  
Types of  BJTs  :

There are two types of junction transistor :


1. NPN transistor 
2. PNP transistor 

This article gave brief details about NPN and PNP transistor like working principle, advantages and application to better understand this topic.
The working principle of NPN transistor : 
  • This circuit is NPN types of BJT transistor shown in the figure there are two types of current flow IC , Iis receptively known as collector and emitter current and VCB , VEB is collector-base voltage and emitter-base voltage.
  • Shown in figure current IC , IE  , Icurrent going into the transistor is and the sign is taken as positive and if current goes out sign is taken as negative.
NPN transistor

 NPN transistor Application:
  • Use as an amplifier
  • Use as a Darling-tone pair
  • Use as a switch
The working principle of PNP transistor :
  • In P-N-P junction transistor, emitter current enter through the emitter terminal shown in the figure. 
  • When using any BJT device, the junction of emitter-base is forward biased and the junction of the collector-base is in reverse biased.
So conclude that BJT can be operated in the different mode of BJT like cut off, saturated and active mode.
PNP transistor

PNP transistor application :
  • Used in darling-ton pair circuit
  • Used in heavy motors to control current flow
  • Used as switch
  • Used as a robotic workshop
  • Used in the amplifying circuit