Bipolar Junction Transistor

Bipolar Junction Transistor or BJT is a semiconductor device which is constructed with 3 doped semiconductor Regions i.e. Base, Collector & Emitter separated by 2 p-n Junctions. It is a type of transistor that uses both electron and hole charge carriers. This transistor is commonly known as Bipolar transistor or BJT.

Bipolar transistors are manufactured in two types, PNP and NPN, and are available as separate components, usually in large quantities. The prime use or function of this type of transistor is to amplify current. This makes them useful as switches or amplifiers. They have a wide application in electronic devices like mobile phones, televisions, radio transmitters and industrial control.

Function of Bipolar Junction Transistor

BJTs are of two types namely NPN and PNP based on doping types of the three main terminals. An NPN transistor consists of two semiconductor junctions that have a thin p-doped anode region and PNP transistor also consists of two semiconductor junctions that have a thin n- doped cathode region.

The flow of charge in a Bipolar transistor is due to the diffusion of charge carriers between the two regions belong to different charge concentrations. Regions of BJT are known as the base, collector, and emitter.

The emitter region is highly doped when compared to other layers. Both collector and base layers have the same charge carrier concentrations. Among these junctions, the base-emitter junction is forward biased, and base-collector junction is reverse biased. Forward biased means p-doped has it contains more potential than the n-doped side.

Voltage, Charge Control and Current

The base-emitter current is controlled by collector-emitter current. This conclusion is drawn by the current-voltage relation of the base-emitter junction. Collector current has a base region where minority carriers are concentrated.

Transistor models such as Glenn poon model is responsible for the distribution of the charge which explains the behaviour of a transistor.

Bipolar Transistors Configurations

Since Bipolar Junction Transistor is a three terminal device, there are three ways to connect it within an electric circuit while one terminal is the same for both output and input. Every method of connection responds differently to the input signals within a circuit.

  • Common Emitter Configuration – has both voltage and current gain
  • The common Collector Configuration – has no voltage gain but has a current gain
  • The common base configuration – has no current gain but has a voltage gain

Characteristics of different transistor configurations are given in the following table:

Characteristics Common Base Common Emitter Common Collector
Power Gain low Very high medium
Current gain low medium high
Voltage gain High Medium low
Phase angle 0 180 0
Output impedance Very high high low
Input Impedance Low medium high

Bipolar Junction Transistor FAQs

Q1. What is BJT?

Ans: BJT stands for bipolar junction transistor and is defined as a device in a solid state which uses electron and hole charge carriers. It uses two junctions between two semiconductor types, p-type and n-type

Q2. What are the types of bipolar junction transistors?

Ans: There are two types:

  • NPN junction transistor
  • PNP junction transistor

Q3. Who invented BJT?

Ans: BJT was invented by W.H Brattin, Bardeen and William Shockley.

Q4. What are the operating regions of BJT?

Ans: The operating regions of BJT are:

  • Forward active or active region
  • Reverse active or inverted region
  • Saturation
  • Cut-off

Q5. What are the applications of BJT?

Ans: Following are the applications of Bipolar Junction Transistor:

  • It is used as an amplifier
  • It is used as an oscillator
  • It is used as a demodulator

Q6. What happens if the transistor is not biased properly?

Ans: Following is the list of consequences if the transistor is not biased properly:

  • The work efficiency of the transistor reduces
  • There will be a distortion in the output signal
  • The operating point may shift
  • Transistor parameters will change

Q7. Why is collector always reverse biased with respect to a base?

Ans: In order to remove the charge carriers from the base-collector junctions, the collector is always reverse biased.

Q8. Why is the emitter forward biased?

Ans: The emitter is always forward biased because most of the charge carriers are supplied to the base.

Q9. Why is collector larger than the emitter and the base?

Ans: The collector is larger because it dissipates much power.

Q10. How can a BJT be used as an amplifier?

Ans: BJT is used as an amplifier by transferring the current from low impedance loop to high impedance loop.