Working of Transistors

Summary

This lecture delves into the functioning of bipolar junction transistors (BJTs), highlighting their three-terminal nature and the significance of both electrons and holes in their operation. The video explains the active mode of BJTs, detailing how forward and reverse biases affect the resistance and current flow. It emphasizes the transistor's amplification capability, demonstrating how a weak input signal can be transformed into a stronger output. The script also discusses the movement of charge carriers, the role of barrier potential, and introduces the concept of reverse saturation current. The lecture concludes with a foundational understanding of the relationships between emitter, base, and collector currents in an NPN transistor.

Takeaways

• 😀 The lecture discusses the functioning of a Bipolar Junction Transistor (BJT), emphasizing its role in electronics.
• 🔬 The term 'BJT' stands for Bipolar Junction Transistor, highlighting the involvement of both electrons and holes as charge carriers.
• 🔋 The 'transistor' name is derived from 'transferred resistor,' referring to its ability to transfer a low resistance to a high resistance.
• 📡 In active mode, a BJT operates with one junction (J1) forward-biased and the other (J2) reverse-biased, affecting resistance levels.
• 🔌 The input voltage (VI) is measured across the low resistance, while the output voltage (VO) is across the high resistance, facilitating signal amplification.
• 🌐 The NPN transistor consists of an n-type emitter, a p-type base, and an n-type collector, with specific terminals for each.
• 🔄 To operate in active mode, the emitter-base junction (J1) must be forward-biased, and the base-collector junction (J2) must be reverse-biased.
• ⚡ The barrier potential at J1 decreases with forward bias, allowing electrons to cross from emitter to base and then to the collector.
• 🔝 Most electrons from the emitter bypass the base due to its thin and lightly doped nature, leading to a significant electron flow to the collector.
• 🔗 The collector current (IC) is primarily determined by the emitter current (IE) and a small reverse saturation current (Ico), with a relationship defined by the transistor's alpha (α).

Q & A

What does BJT stand for?

BJT stands for Bipolar Junction Transistor. It is called bipolar because it involves both types of charge carriers: electrons and holes.

What is the significance of the term 'transistor'?

The term 'transistor' is derived from 'transferred resistor'. It refers to the ability of the device to transfer a low resistance to a high resistance, which is key to its amplifying function.

How does the transistor achieve amplification?

Amplification is achieved by having a weak input signal across a low resistance and an amplified output signal across a higher resistance, with the same current flowing through both.

What is the composition of an NPN transistor?

An NPN transistor consists of an 'n' emitter region, a 'p' base region, and an 'n' collector region, with corresponding terminals for each.

What is the condition for operating an NPN transistor in active mode?

For an NPN transistor to operate in active mode, the Junction J1 must be forward biased, and Junction J2 must be reverse biased.

What happens to the barrier potential when a transistor is forward biased?

When a transistor is forward biased, the barrier potential at Junction J1 is reduced, allowing electrons to cross over from the emitter to the base.

Why does most of the electron flow go to the collector instead of recombining in the base?

Most of the electrons go to the collector because the base is very thin, lightly doped, and small, which results in minimal recombination of electrons with holes in the base.

What is the significance of alpha (α) in the context of a transistor?

Alpha (α) represents the ratio of electrons that are collected by the collector to the number of electrons that enter the base. It is a key parameter in determining the transistor's amplification capability.

What is the reverse saturation current and why is it important?

The reverse saturation current is the current that flows through Junction J2 when it is reverse biased. It is important because it represents the leakage current and is associated with the collector, denoted as IC_o.

How is the collector current (IC) related to the emitter current (IE) and the base current (IB)?

According to Kirchhoff's current law, the emitter current (IE) is equal to the sum of the base current (IB) and the collector current (IC), expressed as IE = IB + IC.

How does the direction of current relate to the movement of electrons in a transistor?

The direction of current is opposite to the movement of electrons. For example, in an NPN transistor, emitter current flows from right to left, while electrons move from left to right.

Outlines

00:00

🔬 Introduction to Transistors and BJT

This paragraph introduces the concept of transistors, specifically Bipolar Junction Transistors (BJT). It explains the significance of the term 'bipolar' due to the involvement of both electrons and holes as charge carriers. The paragraph also details the meaning behind 'transistor', derived from 'transferred resistor', highlighting the transistor's ability to transfer a signal from a low to a high resistance, thus amplifying the signal. The explanation includes the active mode of operation where one junction is forward-biased and the other is reverse-biased, leading to the amplification of the input signal at the output. The paragraph concludes with an introduction to the NPN transistor, describing its structure and the need to bias it correctly for operation.

05:03

🔍 Detailed Analysis of Transistor Biasing

The second paragraph delves deeper into the biasing of transistors, particularly focusing on the barrier potentials at Junction J1 and Junction J2. It discusses how forward biasing reduces the barrier potential at J1, allowing electrons to move from the emitter to the base and then to the collector. The paragraph emphasizes the thin and lightly doped nature of the base, which results in minimal recombination of electrons and holes, leading to a high electron transfer to the collector. It also introduces the concept of reverse saturation current or leakage current, which occurs when Junction J2 is reverse-biased. The paragraph concludes with a discussion of the movement of electrons and the resulting current flow, setting the stage for understanding the transistor's behavior in an electrical circuit.

10:05

🔗 Understanding Current Relations in a Transistor

The final paragraph focuses on the relationships between the emitter current (Ie), base current (Ib), and collector current (Ic) in an NPN transistor. It begins by establishing the direction of these currents based on the movement of electrons. The paragraph then uses Kirchhoff's Current Law (KCL) to establish the relationship Ie = Ib + Ic, which is crucial for understanding how current flows within the transistor. The discussion concludes with the formula for the collector current, Ic = α * Ie + Ico, where α represents the efficiency of electron transfer from the emitter to the collector, and Ico is the reverse saturation current. This paragraph provides a foundation for further exploration of transistor behavior and its applications in amplification and other electronic functions.

Keywords

💡Transistor

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is a key component in modern electronic devices. In the script, the transistor is central to the discussion as the lecturer explains its function and operation.

💡BJT

BJT stands for Bipolar Junction Transistor, a type of transistor that involves both electrons and holes as charge carriers. The script explains that 'B' stands for bipolar because both types of charge carriers are involved in its operation, and 'JT' stands for Junction Transistor, indicating the presence of two junctions.

💡Active Mode

Active mode refers to the operating condition of a transistor where it amplifies signals. In the script, the lecturer describes how a transistor operates in the active mode with one junction (J1) forward-biased and the other (J2) reverse-biased, which is essential for signal amplification.

💡Forward Bias

Forward biasing is the process of applying a voltage to a semiconductor junction in a direction that favors the flow of current. The script mentions that to operate a transistor in active mode, Junction J1 needs to be forward-biased to allow current flow.

💡Reverse Bias

Reverse biasing is the application of a voltage to a semiconductor junction that opposes the flow of current. In the context of the script, Junction J2 is reverse-biased to create a high-resistance path, which is necessary for the transistor's amplifying action.

💡Emitter

The emitter in a transistor is the region that emits charge carriers, typically electrons, into the base. The script explains that in an NPN transistor, the emitter is doped n-type and is connected to the negative terminal to forward-bias the transistor.

💡Base

The base is a thin layer in a transistor that separates the emitter and collector. It is lightly doped and plays a crucial role in controlling the current flow. The script emphasizes that due to its thinness, most electrons pass through to the collector rather than recombining in the base.

💡Collector

The collector is the region in a transistor where the majority of charge carriers are collected. The script describes that in an NPN transistor, the collector is n-type and is connected to the positive terminal to reverse-bias the transistor.

💡Alpha (α)

Alpha (α) is the common-base current gain in a transistor, representing the ratio of the collector current to the base current. The script uses alpha to explain how the current flowing into the emitter is related to the current flowing out of the collector.

💡Kirchhoff's Current Law (KCL)

KCL is a basic principle in circuit analysis stating that the sum of currents entering a junction is equal to the sum of currents leaving it. The script uses KCL to derive the relationship between emitter current (Ie), base current (Ib), and collector current (Ic) in a transistor.

💡Amplification

Amplification is the process of increasing the strength of a signal. The script explains how a transistor can amplify a weak input signal (Vi) to a stronger output signal (Vo) by utilizing the difference in resistance between the forward-biased and reverse-biased junctions.

Highlights

Explanation of how a transistor works, focusing on its bipolar nature involving both electrons and holes.

Definition of BJT, where B stands for bipolar and J for Junction, indicating two junctions in a transistor.

Origin of the term 'transistor' from 'transferred resistor', highlighting its function in resistance transfer.

Description of active mode operation where one junction is forward biased and the other is reverse biased.

Amplification process using a transistor by comparing input and output voltages across different resistances.

Introduction to the NPN transistor, detailing the roles of the emitter, base, and collector regions.

Procedure to bias the transistor in active mode with forward and reverse bias potentials.

Analysis of electron and hole movement within the transistor due to barrier potential changes.

Importance of the base's thinness and light doping in a transistor, affecting electron recombination.

Explanation of how most electrons from the emitter move to the collector due to high kinetic energy.

Concept of reverse saturation current or leakage current in a reverse-biased junction.

Calculation of collector current as the sum of electrons moving to the collector and the reverse saturation current.

Direction of emitter, base, and collector currents in an NPN transistor, explained with electron movement.

Kirchhoff's current law (KCL) application to determine the relationship between emitter, base, and collector currents.

Emphasis on the significance of alpha in the transistor's operation and its impact on current relationships.

Anticipation of further discussion on alpha in upcoming presentations to deepen understanding of transistor operation.

Transcripts

in the last lecture I introduced

transistors in this lecture I will

explain how transistor works I also

explained the meaning of named BJT BJT B

stands for bipolar B stands for bipolar

we have bipolar in the name because

there is involvement of both type of

charge carriers there is involvement of

electrons as well as holes in transistor

that's why we have bipolar in the name G

stands for Junction J stands for

Junction two junctions are formed in

transistor Junction j1 and Junction j2

two junctions are formed in transistors

so we have Junction in the name now the

most important thing is the meaning of

transistor why we call this three

terminal device transistor the name

transistor is coined from word

transferred resistor transistor is

coined from transferred resistor in

which we have trans

and store I sto R and this two makes the

word transistor now what is the meaning

of transferred resistor if we talk about

active mode if we talk about active mode

of operation then we already know

Junction j1 is forward biased and

Junction j2 is reverse biased when

Junction j1 is forward biased it will

offer a very low resistance or ideally

resistance should be zero when Junction

j2 is reverse biased it will offer a

very high resistance or ideally the

resistance must be infinity so this is

what we have in active mode and if we

have same current same current flowing

then initially it will flow through a

low resistance let's say the current is

I and after this at output it will flow

through high resistance let's say it is

capital R and the same current is

flowing through both these resistances

so somehow we have transferred the low

resistance to the high resistance that's

why we have the name transferred

resistor and from this we got transistor

there is very significant use of this

thing for example let's say VI is the

input voltage and V o is the output

voltage we are measuring VI across this

small resistance and we are measuring vo

across this large resistance so V I is

equal to I multiplied by r and vo is

equal to I multiplied by capital R the

current is same and the input resistance

is smaller than the output resistance so

we can say that VI is smaller than we

owe so we had a weak signal at the input

but we have an amplified signal at the

output so there is amplification

amplification by using the three

terminal device all these things will be

clear when I explain the working of NPN

transistor in active mode this is the

NPN transistor so the emitter region is

n the base region is P and the collector

region is n this is NPN transistor this

terminal is emitter terminal this

terminal is base terminal and this

terminal here is collector terminal and

we want to operate this transistor in

active mode we want to have proper eight

this transistor in active mode and we

already know in active mode Junction j1

is forward bias so two forward bias

Junction j1 we need to we need to apply

a forward bias potential and is

connected to the negative terminal and P

is connected to the positive terminal so

emitter is connected to the negative

terminal and base is connected to the

positive terminal let's say this forward

biasing potential is V EB

and we have to reverse bias Junction j2

so n is connected to the positive

terminal and P is connected to the

negative terminal collector is connected

to the positive terminal base is

connected to the negative terminal and

let's say this reverse biasing potential

is we see B now we have Junction j1

forward biased and Junction j2 reverse

bias now we will analyze the movement of

electrons and holes in this three

terminal device and let's say let's say

VB is the barrier potential for Junction

j1 in Junction j2 when the transistor

terminals are open circuited there is no

biasing potential in the transistor so

VB is the barrier potential for Junction

j1 and Junction j2 when there is no

biasing potential and we have to analyze

what will happen to the barrier

potential once we apply the biasing

potentials in the transistor this is

barrier potential of Junction j2 and it

is equal to VB this is barrier potential

of Junction j1 and it is also equal to

VB now Junction j1 is forward biased

after the application of V EB and the

barrier potential will now reduce and

let's say the new barrier potential is

this the new barrier potential the new

barrier potential is equal to VB VB

minus V EB forward biasing potential on

the other hand Junction j2 is reverse

biased so barrier potential will

increase and the new barrier potential

the new barrier potential is equal to VB

VB plus VC B plus we see be the reverse

biasing potential now we can easily

analyze the movement of electrons and

holes because we have idea about the

barrier potentials because of the

reduced barrier potential at Junction j1

the electrons on the N side that is the

emitter will cross the junction and move

to the base and recombine with the holes

in the base there is one very important

thing

that you always have to keep in your

mind base in case of transistor is very

small it's very small and it is also

lightly lightly domed and because of

this because base is thin and it is

lightly doped there is very small

recombination of electrons from the

emitter and most of the electrons from

the emitter pass over to the collector I

will repeat this point again barrier

potential at Junction j1 is reduced so

electrons from emitter will cross over

the junction and very small amount of

electrons from the emitter will

recombine in the base because base is

very small very thin and it is lightly

doped so most of the electrons will

cross the junction j2 because they have

high velocity and this implies they have

high kinetic energy so most of the

electrons emitted by the emitter will

find themselves in the collector a very

few electrons very few electrons will

recombine with the holes and this means

electrons will move to VEB the positive

terminal of the forward biasing

potential let's say n number of

electrons enter the base and number of

electrons enter the base out of which

one minus alpha n out of which one minus

alpha n electrons combined with the

holes in the base and alpha n alpha and

electrons move to the collector this is

what is happening in the transistor only

2 to 5% 2 to 5% electrons are combined

in the base and 95 to 98% electrons are

collected in the collector therefore

most of the electrons emitted by the

emitter moved to the collector so this

is what will happen when you forward

bias the junction j1 and junction j2 in

bipolar Junction transistor there is one

more thing

that we must not forget and it is the

reverse saturation current Junction j2

is reverse biased so there must be

reverse saturation current through

Junction j2 we have minority charge

carriers on n side and we have minority

charge carriers on P side on n side

minority charge carrier is hole on P

side minority charge carrier is electron

so this hole will move like this and

this electron will move like this so

there there is current called as a

reverse saturation current or leakage

current when the junction j2 is reverse

biased I am calling the reverse

saturation current reverse saturation

current or leakage current or leakage

current IC o---- because this current is

associated with the collector so there

is subscript C this o stands for open

circuit we measure this current when

emitter terminal when ammeter terminal

is open circuited so this is why I am

calling this current IC o---- and if I

if I want to find out the collector

current IC then it is equal to alpha I

alpha times the emitter current because

n is the number of electrons entering

the base and alpha n is the number of

electrons moving to the collector so

current will be alpha times ie plus IC

o---- so this is the value of collector

current the next thing is to find out

relation between the emitter current

base current and collector current for

this purpose first we have to find out

direction of the three currents you can

see electron is moving from left to

right so direction of current is like

this from right to left

this is i.e the emitter current when

electrons from emitter recombine with

the holes the direction of current will

be like this and the current is IB

because electron will move in this

direction to the positive terminal of

the battery so current will

flow in opposite direction and to find

out collector current we again have to

focus on the movement of electrons

electrons are moving from base to

collector like this in this fashion so

current will be like this from right to

left and to find out relation between ie

IB and IC we have to use Kirchhoff's

current law KCl and from KCl we know sum

of currents entering is equal to the sum

of currents leaving ie is leaving I II

is leaving IB and IC are entering so ie

is equal to IB plus IC this is what we

have in the NPN transistor in active

mode these two relations are very

important and we will talk more about

alpha in the coming presentations this

is all for this lecture see you in the

next one