BJT

BJT is a 3 terminal semiconductor device used as a switch or to amplify, rectify and control electrical signals. It is mainly used as a switch in digital electronic applications and as an amplifier in analog electronic applications. The 3 terminals of BJTs are emitter, base, and collector. Each terminal has different doping concentrations. Based on this doping concentration it is divided into 2 types NPN and PNP. In NPN transistor both emitter and collector are doped with n-type impurity and base with p-type impurity similarly in the case of PNP transistor both emitter and collector are doped with p-type impurity and base with an n-type impurity. Here as the name suggests i.e., Bipolar junction indicates that both holes and electrons are responsible for the flow of current. The below figures show the representation and symbols of both NPN and PNP transistors. Here as you can see

Representation and symbol

Emitter region — Highly Doped
Base region — Lightly Doped
Collector Region — Moderately Doped

— -Now, based on the biasing the BJT can be operated in 3 regions:

1] Active region:

Active Region

To operate BJT in the active region the base to emitter junction must be forward bias and the base to collector junction must be reverse bias. To obtain the above condition the emitter voltage must be less than the base voltage and the base voltage must be less than the collector voltage. Consider emitter voltage as VE, base voltage as VB, and collector voltage as VC. So, to operate it in the active region we must satisfy the below condition

VE<VB, VB<VC,
VE<VB<VC,
Similarly, if we consider the PNP transistor then,
VE>VB, VB>VC,
VE>VB>VC,

2] Cut off Region

Cut off Region

To operate BJT in the cut-off region both base to emitter junction and base to collector junction must be reverse biased. To obtain the above condition base voltage must be less than both emitter and collector voltage. Consider emitter voltage as VE, base voltage as VB, and collector voltage as VC. So, to operate it in the active region we must satisfy the below condition

VE>VB, VB<VC.

3] Saturation region

Saturation Region

To operate BJT in saturation region both base to emitter junction and base to collector junction must be forward biased. To obtain the above condition base voltage must be more than both emitter and collector voltage. Consider emitter voltage as VE, base voltage as VB, and collector voltage as VC. So, to operate it in the active region we must satisfy the below condition

VE<VB, VB>VC.

Apart from these 3 regions, BJT can be also operated in the 4th region i.e., the Reverse active region where the base to emitter junction is reverse bias and base to collector junction is forward bias. But the gain is very low while operating in this region hence it is not widely used.

BJT can be used as an amplifier when operated in the active region and can be used as a switch when operated in cut off and saturation region.

— -BJT can be configured in 3 ways:

1] Common Emitter: Here, the emitter terminal is common between the input and output terminals. The input is applied between base and emitter and output is measured between collector and emitter terminal.

Common Emitter

2] Common Base: Here, the base terminal is common between the input and output terminal. The input is applied between base and emitter and output is measured between collector and base terminal.

Common Base

3] Common Collector: Here, the collector terminal is common between the input and output terminal. The input is applied between base and collector and output is measured between collector and emitter terminal.

Common Collector

— -Working of BJT

Consider an NPN transistor as shown in the figure below:

Working

Here,

VBB =source voltage for base connected.
VCC =source voltage for collector connected.
VBE=Voltage between base and emitter.
VCE =Voltage between collector and emitter.
VBE = VB -VE = -VEB
VCE = VC-VE

BJT is operated in such a way that base to emitter is forward bias and base to the collector is reverse bias. As the emitter is highly doped and has electrons as its majority charge carriers and once, we apply the biasing voltages the electrons from the emitter region will be pushed towards to base region. Once these electrons from the emitter enter the base region there are 2 paths: 1] towards the positive terminal of VBB to the left and 2] enter the collector region. But most of the electrons will enter the collector region as the base region is lightly doped i.e., the number of holes in the base region is very less as compared to the electrons entering from the emitter region. And also, the width of the base region is small hence the electrons from the emitter region can easily escape the base region and enter the collector region. Hence fewer electrons will be combined with the holes in the base region and get attracted to the positive terminal of VBB and the rest will enter the collector. Now, once these electrons enter the collector region, they will get attracted towards the positive terminal of VCC. The flow of electrons will be from emitter to collector and some electrons from the base will flow towards VBB and the flow of holes will be exactly opposite and the flow of current will also be in the same direction.

Now, let us consider the relation between these currents,
Applying KCL,
IB + IC = IE,
As very less amount of current will flow through the base IB is negligible,
Therefore,
IC ~= IE ,
Or
IC = α IE ,
α= IC / IE ,
IB + α IE = IE ,
IB = (1 — α) IE ,
IB = (1 — α) (IC / α),
IC = (α / (1 — α)) IB ,
IC = β IB ,
β = current gain or amplification factor of BJT,
IE = (1 + β) IB ,

As base current i.e., the input current can control the collector current i.e., output current BT is a current-controlled current source device.

By connecting a resistor between the emitter and collector or output side we can amplify the input signal.

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