The invention of transistor led the invention of many other semiconductor devices including integrated circuits. And due to these integrated circuits, the modern-day computers and other electronic gadgets are possible. The bipolar junction transistor or BJT is a three-terminal semiconductor device that can act as a conductor or insulator based on the applied input signal. And due to this property, the transistor can be used as a switch in digital electronics or as an amplifier in analog electronics.
Nowadays, the field-effect transistors are widely used in electronics applications but still, these BJTs are quite extensively used and anyone who is interested in electronics should have some basic knowledge of this device.
Fig. Basic Structure and Symbol of Bipolar Junction Transistor (NPN Transistor)
BJT: Construction and Internal Structure
The bipolar junction transistor has three doped regions. The emitter, base, and collector.
Based on the doping of these three regions, it is known as either NPN or PNP transistor. In the case of the NPN transistor, both emitter and collector are doped with n-type impurity while the base is doped with a p-type impurity. On the other end, in PNP transistor, the base is doped with N-type impurity while the collector and the emitter is doped with a P-type impurity. The term bipolar indicates that both electrons and holes contribute to the current.
Fig. Bipolar Junction Transistor Basic Structure (PNP Transistor)
Inside the BJT, the two PN junctions are formed. One is between the base and emitter and the second is between the base and the collector. It appears as if two back to back diodes are connected in series.
Fig. Two PN junctions in Bipolar Junction Transistor
But actually, it won’t behave like that. Because when we connect two back to back diodes then we are assuming that there is no interaction between the two diodes. But in the case of the BJT actually, there is an interaction between these two regions.
In the internal construction of the BJT, the emitter is heavily doped. And the function of the emitter is to supply the electrons. The base region is lightly doped and the collector region is moderately doped. The doping concentration of the collector region is between the emitter and the base region. Among the width of these three regions, the base region is much narrower than the other two regions and the collector region is wider than the other two regions. Because the job of the collector region is to collect the electrons supplied by the emitter.
Fig. Doping Concentration and Width of Three Regions in BJT
BJT: Three Regions of Operation
Depending on the biasing, the BJT can be operated in three regions.
1) Active region,
2) Cut-Off region
3) Saturation region.
In case of the active region of operation, emitter-base junction is forward biased while the collector-base junction is reverse biased. While in case of the cut-off region, both emitter-base and the collector-base junctions are reverse biased.
And in the case of the saturation region of operation, both emitter-base and collector-base junctions are forward biased. Whenever BJT is used for the amplification then it is used in the active region and whenever it is used as Switch then it is used in the saturation and the cut-off region.
Where, FB – Forward Biased, RB – Reverse Biased
Different BJT Configurations:
As mentioned earlier, when BJT is used for the amplification of the signal, it is operated in the active region. And there are different ways to configure it.
- Common Emitter (CE)
- Common Base (CB)
- Common Collector (CC)
Depending on the requirement and the application, the BJT can be configured in any of the three configurations.
Common Emitter Configuration:
Fig. Common Emitter Configuration of BJT
In the case of the common-emitter configuration, the emitter terminal is common between the input and the output for the AC signal. That means in this configuration the AC input signal is applied between the base and the emitter while the output is measured between the collector and the emitter.
Common Base Configuration:
Fig. Common Base Configuration
In this configuration, for the AC signal, the base terminal is common between the input and the output. The AC input signal is applied between the emitter and the base terminal, while the output is measured between the collector and the base terminal.
Common Collector Configuration:
Fig. Common Collector Configuration of BJT
In this configuration, the base terminal is common between the input and the output. The AC input signal is applied between the base and the collector terminal, while the output is measured between the emitter and the collector terminal.
Working of BJT:
The figure shown below shows the operation of the BJT in the active region, where the base-emitter junction is forward biased, while the collector-base junction is reverse biased.
Fig. Operation of NPN Transistor in the active region
Voltage Notations for the BJT:
Voltage VBE – Voltage between the base and the emitter (VB – VE)
VB – Voltage between the base and the ground
VE– Voltage between the emitter and the ground
Voltage VCE- Voltage between the collector and the emitter
VCE = VC – VE
(Please note voltage VBE = – VBE and VCE = -VEC)
Voltage VBB and VCC are the supply voltages.
As mentioned earlier, in BJT, the emitter is heavily doped and has a large number of electrons as majority carriers. (for NPN transistor).As shown in the figure, when the voltage is applied between the base and the emitter then the negative voltage at the emitter pushes these electrons towards the base region.
Fig. Movement of electrons (majority charge carriers) from emitter to the base region
Once the electrons enter into the base region, there are two paths for the electrons. One is, the electrons can flow towards the positive terminal VBB through the base resistor. Or they can flow towards the collector region. (as shown in the figure)
Fig. Movement of electrons from the base region (two-paths)
Once the electrons enter the base region, they become a minority charge carrier. But because of the low concentration of holes in the base region (as the base is lightly doped) and narrow base width, their probability of recombination with the holes in the base region is very low.
And since those electrons act as a minority charge carrier in the base region, they get swapped into the collector region due to the strong electric field at the collector-base junction without any recombination. Only a few electrons recombine with the holes in the base region and get attracted towards the positive terminal of voltage VBB.
The same phenomenon is shown in the figure below.
Fig. Movement of electrons from the base to the collector region
Once the electrons enter the collector region, they get attracted to the positive terminal of the VCC. In the NPN transistor, the direction of the flow of electron is shown in the figure. The direction of the flow of holes will be exactly the opposite. And the direction of the conventional current is the same as the direction of the flow of holes.
Fig. The direction of flow of electrons and holes in the NPN transistor
Different Currents in the Transistors:
Because of the flow of electrons and holes, three different currents establish in the transistors.
The figure below shows the three different currents in the transistor.
- Base current (IB)
- Collector Current (Ic)
- Emitter Current (IE)
Fig. Different Currents in NPN Transistor
The relation between the Three currents in Transistor
Applying KCL, the relation between the three currents can be easily found.
∴ IE = IB + IC ———— (1)
The base current is very negligible. (i.e collector current is approximately equal to emitter current)
IC = α IE ————- (2)
From equation (1) and (2)
IE = IB + α IE ==> IE = (1- α) IB
Or IC = α / (1- α) IB ==> IC = β IB
Where β = α / (1 – α)
β is known as the current gain. And typically, its value varies from 20 to 400 from transistor to transistor.
So, far in the discussion, the current due to minority charge carriers (the reverse saturation current) is neglected. The reverse saturation current is very small, and typically it is in the range of tens of nA to few µA for the latest transistors.
As shown in the figure below, if the emitter terminal is open in the transistor circuit, then the only current which exists in the circuit is the reverse saturation current. (ICBO)
Fig. The reverse saturation current (ICBO) in the transistor when the emitter is open
ICBO is the reverse saturation current between the base and the collector when the emitter is open-circuited.
Including this reverse saturation current, the collector current
IC = β IB + ICBO
In the next article, the different BJT configurations will be covered.