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At a glance of BJT

In today’s blog, we will discuss BJT( bipolar junction transistor), Construction of BJT, Operation of BJT such as Active mode, Saturation mode, Cut off mode, Reverse Active mode, and BJT -current control device.

The following block diagram will show the map of the blog

BJT( bipolar junction transistor), Construction of BJT, Operation of BJT such as Active mode, Saturation mode, Cut off mode, Reverse Active mode, and BJT -current control device.

History of BJT

The transistor is first invented in December 1947 at the bells lab in the USA. It was invented by John Borden, who got the Nobel Prize in physics in 1972 for BCS theory, Walter Brattain, and William Shockley, who was the head of this team and invented the junction version of transistors 1951. All three physicists got the Nobel Prize in physics in 1956 for this remarkable research.

Introduction

The transistor is a three-terminal semiconductor device that amplifies weak signals.

And they are also used in switching operations for both analog as well as digital electronics.

The bipolar junction name suggests two junctions formed in the device contain two polarities due to holes and electrons. And the name transistor is a combination of two analogies transfer + resistor. As the name suggests, it transfers the low resistance into high resistance. Hence the name bipolar junction transistor indicates that it is two junction devices that amplify a weak signal.

When the junction J1 forward bias and junction J2 is reverse biased, then J1 has the low resistance means it contains a weak signal which is then amplified using high resistance at junction J2 and output is taken from the junction J2.

Construction of BJT

According to construction, the transistor is divided into two types

PNP transistor
NPN transistor

NPN transistor

As the name suggests, it combines two PN junctions at the junction J1 N- the region is called the emitter region, which emits electrons during the operation.

The P-type semiconducting lightly doped material is sandwiched between two N-type semiconducting material type semiconducting material, and this P- region is known as base.

At junction J2, the largest region is made from N-type semiconducting material, heavily doped, and called a collector. The name suggests it collects the electrons emitted by the emitter.

Junction J1 is an emitter-base junction.

Junction J2 is the base-collector junction.

Junctions are formed because of two regions (P -region and N- region).

The width of the collector region is more because it collects the electrons. In this process, heat is generated, and if the width is more, the heat will dissipate.

The width of the collector is more significant than the base and emitter (C>E>B)

And the doping concentration of the emitter is more significant than the collector and base (E>C>B).

PNP transistor

The construction of the PNP transistor is similar to the NPN transistor, except the N-type semiconducting material is sandwiched between two P-type semiconducting materials.

Symbol of BJT

You might have always wondered why there is a shifting of the arrow direction between symbols of NPN and PNP transistors.

For the NPN transistor, the arrow is downwards because the current flow is from the Base – Emitter.

And for the PNP transistor, the arrow is upward because the current flow is from emitter to base.

Operation of BJT

The transistor is operated in four regions. The following table will give you a clear picture of this.

J1	J2	Mode of operation
Forward Bias	Reverse Bias	Active mode ( as an amplifier)
Forward Bias	Forward Bias	Saturation mode( logic on/close switch)
Reverse Bias	Reverse Bias	Cut off mode ( logic off/open circuit)
Reverse Bias	Forward Bias	Reverse Active mode ( rarely used ) Emitter and collector switch their role.

Active mode

In the active mode of operation, junction J1 is forward biased however, it will offer ideally zero resistance, and junction J2 is reverse biased. For this condition, it will provide infinite resistance.

In the active mode, the transistor operates as an amplifier. You might have to wonder how this will happen if we have the same current flowing.

Let’s say the current is I, and after this, at the output, it will flow through a high resistance. Let’s say R and the same current are flowing through both the resistances, so somehow, we have transferred the low resistance to the high resistance. That’s why we got the name transferred resistance.

Vi is the input voltage, and Vo is the output voltage. We measure VI across small resistance r, and we measure Vo across this large resistance R.

Vi = I×r and Vo = I×R

The input current is the same. The input resistance is smaller than the output resistance. We can say that we have a weak signal at the input, but we have an amplified signal at the output, so we have amplification by using a three-terminal device.

You’ll get a clear picture of this once I explained to you the simple working of the NPN transistor in active mode.

NPN transistor in active mode

The emitter region is N; the base region is P, and the collector region is N; we already know that inactive mode junction J1 is forward biased. However, to do that, we apply a forward bias potential. N is connected to the negative terminal, and P is connected to the positive terminal. So emitter is connected to the negative terminal, and the base is connected to the positive terminal.

Let’s say this forward biasing potential is VEB, and the reverse bias potential is VCB. Now we have junction J1 forward biased, and junction J2 reverse biased.

Before analyzing the movement of holes and electrons, consider Vb as the barrier potential across the junction J1 and J2 when the transistor is unbiased and in open condition.

Barrier potential at junctions

As shown in the above fig., there are two barrier potentials for both the junctions. Now, junction J1 is forward biased after the application of VEB. The barrier potential will now the new barrier potential is equal to the VB – VEB on the other hand, junction J2 is reverse biased, so barrier potential will increase and become VB + VCB. Now you got the general idea about barrier potentials, let’s analyze the movements of electrons and holes.

When junction J1 is forward biased due to reduced barrier potential, the electrons on the n- side will cross the junction, move towards the base, and recombine with the holes in the base.

Lets N no. of electrons enters the base out of which 1-αN electrons combine with the holes in the base and αN electrons will move to the collector. Inside the transistor, only 2—5 % electrons are incorporated in the base, and 90-95% electrons travel towards the collector, so most of the electrons will pass towards the collector, so this will happen when we forward bias the junction J1.

At the same time junction, J2 is reverse biased, so at the junction, J2 reverse saturation current will flow through the device due to minority charge carriers from both the regions ( holes from the N region and electrons from the P region)

If you want to find out the collector current IC then,

IC = αIE + ICO

Because N is the number of electrons entering the base and αN is the number of electrons moving to the collector.

Why base is lightly doped?

The base in the transistor case is minimal and lightly doped, and because the base is thin, there is very small recombination of electrons and holes, so there are two paths for electrons to circulate.

Through the base and return to the supply (no. of electrons returning to the supply, after attracted by positive terminal of the battery is significantly less)
Pass the base and travel towards the collector.

Relationship between emitter, base, and collector current.

To find this relation, we need to understand the direction of the three currents.

The electrons are moving from emitter to collector, therefore the direction of emitter current will be from collector to emitter (that is opposite to the flow of electrons).when the electrons from the emitter recombine with the hole, the direction of current will be (base current) towards the base. for the collector, current electrons are moving base to the collector, so the direction of current will be from collector to base

To find the relationship between the three currents, we have to use Kirchhoff’s current law, and we know that this law states the sum of current entering is equals to the sum of current leaving.

So IE is leaving, and IB and IC are entering so,

IE= IB +IC

From the above equation, we can say that IB is approximately equal to IC.

IC = αIE

Here the α represents a fraction of the emitter current flowing through the terminal.

IB + αIE = IE

From the above equation, if we take emitter current common then,

IB= (1-α)IE

Putting the value of IE= IC /α

IB= [(1-α)/ α] IC

Similarly, IC becomes,

IC = [α/(1-α)] IB

Let suppose α/(1-α) = β

Where β= current gain of BJT and it is 50-400.

IC = β. IB

We know that

IE= IB +IC

IE= IB + β. IB

IE= (1+ β). IB

Saturation mode

Saturation is where the transistor is on. in saturation mode transistor acts as a short circuit between collector and emitter.

In saturation mode, both junctions are forward biased. In which VE>VC and VB>VE, in other words, VB is more significant than VC and VE.

To operate transistor in saturation mode, VBE must be greater than a threshold voltage, .and the voltage drop at threshold point is 0.6V, so the voltage applied should be above 0.6V for operating in saturation mode.

In this region, the transistor tends to behave like a closed switch. The collector and Emitter currents are maximum in saturation mode of operation.

Cut off mode

Cut-off mode is also known as ‘off state’ or ‘0 states’. In this mode, both the junctions (emitter-base junction and collector-base junction) are reverse biased. Due to which transistor is cut off mode works as an open circuit switch.

The transistor acts as an open switch in the cut-off mode of operation.

In saturation mode, the transistor is operated under two modes

Forward saturation mode
Reverse saturation mode

The transistor is called under the forwarding saturation region if the emitter junction voltage is greater than the collector junction voltage.

But we can call the transistor under reverse saturation region when collector junction voltage is greater than emitter junction voltage.

Reverse Active mode

The working of the transistor in reverse active mode is just opposite to the active mode because, in this mode of operation, the collector-base junction is forward biased. In contrast, the emitter-base junction is reverse biased.

As the gain is negligible in this mode of operation, we avoid transistors in this region.

BJT -current control device

The device’s base current is known as the input current, and the collector current is known as the output current.

IC = β. IB and IE= (1+ β). IB

Therefore From the above two equations, you can see that by controlling the current from the base side, it is possible to control the current of the collector side. The collector current is the device’s output current; hence, the BJT is known as a current control device.

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