Basic Structure of Bipolar Junction Transistor (BJT) - BJT Transistor - Working and Properties

In this post we will discuss the Bipolar Junction Transistor (BJT). The basic structure of BJT and its various properties will be explained here in detail.

Structure of Bipolar Junction Transistor (BJT)

The diagram shown below is the basic structure of Bipolar Junction Transistor-

Structure of Bipolar Junction Transistor (BJT)

The reason behind why it is known as bipolar junction transistor is; it has two types of charge carriers- electrons and holes. These electrons or holes may be present in majority or minority in BJT.

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While in case of Field Effect Transistors (FETs) and Metal Oxide Semiconductor Field Effect Transistors (MOSFETs), we have only one type of charge carriers in majority, either electrons or holes. Minority charge carriers are not present in FETs or MOSFETs. Therefore FETs and  MOSFETs are known as unipolar devices.
You can easily see the emitter region, base region and the collector region in the diagram of bipolar junction transistor shown above. These three regions form two junctions.
The junction formed by emitter and base regions is known as emitter-base Junction or emitter junction while the base and collector regions form the collector-base Junction (collector Junction).

For the transistor action it is desirable that large amount of charge carriers i.e. electrons or holes are emitted from the emitter region and they reach to the collector region through the base region. As the electrons or holes Pass through the base region while moving from emitter to collector, the recombination of charge carriers take place in the base region. 
Therefore the doping (adding external impurities to the pure form of semiconductor) and size of these three regions is selected in such a way that maximum number of charge carriers emitted by the emitter can reach to the collector region.

Now let's discuss some structural properties of bipolar junction transistor (BJT)-

Properties of Bipolar Junction Transistor (BJT)

Comparison of sizes of three regions
Collector > emitter > base
The size of collector is largest while the size of base is the smallest. 
Comparison of Doping 
Emitter > collector > base
Doping means adding external impurities in very small amount to the pure form of semiconductor to increase its conductivity. The pure form of a semiconductor is known as intrinsic semiconductor and after doping it is known as extrinsic semiconductor.

Now we will understand the reason behind relative sizes and the Order of doping in the three regions of the transistor.

*Base region has the smallest area to reduce the transit time (transit time is the time taken by the charge carriers to move from emitter to the collector)
In case of NPN transistors, the majority charge carriers are electrons and holes are in minority, while in PNP transistors the holes are in majority while electrons are in minority. 
As discussed already that for transistor action to work properly we need that maximum number of charge carriers that are emitted by the emitter should reach the collector region but some charge carriers are lost in the base region in the process of recombination of electrons and holes. So we want the base region area to be small to reduce the recombination. Therefore the base area is made smallest to reduce the chances of recombination of electrons and holes.

*The area of the collector region is made largest so that it can overcome heat dissipation. So with increased area the collector region can dissipate more heat.

*Emitter is doped highest, comparatively; to inject majority carriers into the base region. Because as the doping increases, the number of charge carriers increases. Therefore with increased doping the emitter can emit more number of charge carriers.

*The base region is doped lightest. This is done to reduce the recombination of charge carriers in the base region. Since as the recombination will decrease the maximum number of charge carriers emitted by the emitter will reach to the collector region.

Now let's discuss some other important facts and properties of the Bipolar junction transistor-

*Bipolar junction transistor was invented by William shockley, Bartain and Bardeen in 1947.

*It is called as a bipolar device as it has both majority and minority charge carriers. In case of NPN transistors, the majority charge carriers are electrons and holes are in minority, while in PNP transistors the holes are in majority while electrons are in minority. 
Other devices like Field Effect Transistors (FETs) and Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) are unipolar devices as they have only majority charge carriers (either electrons or holes).

*BJT is a low input resistance device

*BJT is a current controlled device. Because in BJT the base current controls the flow of current from emitter to the collector. While MOSFETs and FETs are voltage controlled devices. Because in these devices the flow of charge from the source to drain is controlled by the voltage applied at the gate terminal. 
In BJT the base current controls the collector current, therefore it is a current controlled current device. While in FETs and MOSFETs the gate voltage controls the drain current. Therefore it is a voltage controlled current device.
But in operational amplifiers (Op amps) the input voltage controls the output voltage, so we call it, voltage controlled voltage device.

*BJT is a temperature sensitive device. Because as the temperature increases the collector current of the BJT also increases. We know that beta is the ratio of collector current and the base current, so as the collector current increases, the beta of the transistor also increases. Therefore beta of the transistor is also temperature dependent. With increase in temperature the emitter-base voltage decreases. Minority charge Carriers are present in BJT, so with increase in temperature these minority charge carriers are generated in more numbers, which increases the reverse saturation current so the collector current increases as a result of it.

*Leakage current is present in BJT because of the presence of minority charge carriers. We have already discussed it in the previous point, that with increase in temperature the minority charge carriers are generated in increased number which is responsible for increase in the leakage current (reverse saturation current).

*Presence of minority charge carriers makes BJT a noisy device. Bipolar junction transistor is more noisy device in comparison to field-effect transistors because of the following two reasons-
1. BJT has two junctions- the emitter junction and the collector Junction, it creates noise when charge carriers cross two depletion regions. While FET is a unijunction device. So because of the presence of one junction only, the noise is less in comparison to BJT.
2.The another reason is- BJT is a bipolar device having majority and minority charge carriers both. These two types of charge carriers make the BJT transistor more noisy in comparison to FET which has only majority charge carriers.

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