It is a transistor that uses both electrons and electrons holes in the form of charge carriers. This word transistor is derived from the words “transfer” and “resistor”. This device transistor was invented in by William Shockley, John Bardeen and Walter Brattain at the “Bell Telephone Laboratories”.
Definition
Bipolar Junction Transistor is a transistor which allows injection of a small current through one of its terminals so that the current which is large and flowing between the terminals can be controlled which turns the device fit to use for the purpose of amplification or switching.
It’s a semiconductor device of three terminals having two p-n junctions. It is a device being controlled through the current. Emitter, Base and Collector are the three terminals of the BJT. If input in the form of a signal of small amplitude is given to the base then output of a signal having magnified amplitude will be available in the collector.
It is a point to focus on that further, a source of D.C. current is required for this amplification.
Construction of BJT
This semiconductor device has three regions which are doped differently and these regions are Emitter, Base and Collector.
There are two ways through which this device Bipolar Junction Transistor can be made and
i.e., PNP and NPN, and generally they both are available as two different components but they are present in quite good quantities
The base is placed between the emitter and collector , and is made up of lightly doped materials but they are highly resistive in nature whereas the emitter is surrounded by the collector which
creates a situation of impossibility to the electrons which are injected to the base to escape without being collected. They are being used very widely in televisions, radio, mobile phones etc.
Operation of BJT
There are three regions in which a Bipolar Junction Transistor operates and these regions are mentioned below:
i. Active Region : In this region, the transistor acts as an amplifier.
ii. Saturation Region: In this region, the transistor acts as a switch while being fully on and it operates in a way that collector current is maintained as equal as the saturation current.
iii. Cut-off Region : It is the region in which the transistor is in off mode, and the magnitude of collector current equals zero.
Types of BJT
As mentioned above, there are two types of BJT:
- PNP BJT
- NPN BJT
PNP BJT
In this type of transistor, on both the sides there are p-type of semiconductor whereas in the middle there is n-type of semiconductor. The middle one which is n type acts as a base and the p-type which is situated on the either side act as emitter and collector respectively.
Entrance of the current is done through the emitter side and it is done in such a way that the emitter base is forward biased and the collector base is reverse biased.
NPN BJT
In this type of transistor, both the n-type of semiconductors and p-type of the semiconductor are fixed between these n-type of semiconductors. Both the n-type of semiconductors act as emitter and collector whereas the p-type of semiconductor acts as a base. Positive sign is assigned to the current which enters through the emitter in the transistor and negative sign is assigned to the current which leaves the transistor.
Function of BJT
In these transistors, these p-types of semiconductors are anode regions and the n-types of semiconductors are cathode regions. In (emitter, base and collector) these regions, as the concentration of charges vary so the diffusion of charges happens which results in the flow of the charge. Among all the three regions of emitter, base and collector, emitter is heavily doped and concentration of charges in base and collector are same. Base-emitter junctions are forward biased whereas the collector base is reversely biased in nature among these 2 junctions.
To get it better, forward bias can be understood in a way that p doped materials are potentially better than n doped materials in that region.
Applications of BJT
The different applications of the BJT are mentioned below:
- It is used as the amplifier, switch and sometimes as an oscillator.
- When it is used as a detector then it is known as a demodulator.
- BJT are also used as logic circuits and switching circuits.
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1947 There are few applications of BJT mentioned below: i. used as amplifier and switch ii. used as a demodulator. The different regions of the transistors are emitter, base and collector. In active region, transistors act as an amplifier.y Bipolar Junction Transistor FAQs
In what year the bipolar junction transistors are discovered?
List a few applications of BJT.
What are the different regions of the transistors?
In what region, transistors act as an amplifier?
