PN Junction Diode (No Applied Bias)

Summary

This lecture introduces the concept of semiconductor diodes, focusing on the PN junction and its properties. It covers key topics like the formation of a PN junction, volt-ampere characteristics, and the difference between a PN junction and a PN junction diode. The lecture explains biasing (no bias, forward bias, and reverse bias) and details important phenomena such as diffusion current, depletion layer, and barrier potential. The discussion also touches on minority and majority charge carriers, drift current, and the balance between diffusion and drift currents in a steady-state PN junction diode.

Takeaways

• 🔌 PN junction diode is a two-terminal device formed by combining p-type and n-type semiconductors.
• 📈 The volt-ampere (VI) characteristics of a PN junction diode are critical for understanding its behavior.
• 🔋 Bias refers to applying external voltage across the terminals, with no bias, forward bias, and reverse bias as key conditions.
• ⚡ The diffusion process involves the movement of majority charge carriers from areas of high to low concentration, creating diffusion current.
• 🛡️ The depletion layer is formed due to the recombination of holes and electrons, leaving behind immobile ions that act as a potential barrier.
• 🔄 The barrier potential or built-in potential prevents further movement of free charge carriers, stabilizing the depletion layer's width.
• 🌡️ Minority charge carriers (electrons in p-type, holes in n-type) are influenced by the electric field, creating drift current.
• 🔗 Under steady-state conditions, the diffusion current (from majority carriers) equals the drift current (from minority carriers), resulting in zero net current in an open-circuited diode.
• 🧪 The behavior of the PN junction diode forms the foundation for understanding more complex devices like the bipolar junction transistor (BJT).
• 📏 Future discussions will focus on calculating the barrier potential and the width of the depletion region in a PN junction diode.

Q & A

What is a PN Junction diode?

A PN Junction diode is a two-terminal device made by combining P-type and N-type semiconductors. It allows current to flow in one direction and has three possible biasing conditions: no bias, forward bias, and reverse bias.

What is the difference between a PN Junction and a PN Junction diode?

A PN Junction refers to the boundary between a P-type and N-type semiconductor, while a PN Junction diode is the device formed by attaching metal contacts to the terminals of the PN Junction, making it a functional electronic component.

What does 'biasing' mean in the context of a PN Junction diode?

Biasing refers to the application of external voltage across the two terminals of a PN Junction diode. There are three types of biasing: no bias (no external voltage), forward bias (positive voltage on P-side), and reverse bias (negative voltage on P-side).

What happens in a PN Junction diode under no bias condition?

In the no bias condition, no external voltage is applied, and the PN Junction is in equilibrium. The diffusion and drift currents are equal and opposite, leading to no net current.

What is diffusion current in a PN Junction diode?

Diffusion current is caused by the movement of majority charge carriers (holes in P-type and electrons in N-type) from regions of high concentration to low concentration. It occurs due to the concentration gradient across the junction.

What is drift current in a PN Junction diode?

Drift current is caused by the movement of minority charge carriers (electrons in P-type and holes in N-type) due to the electric field across the depletion region. It is in the opposite direction to the diffusion current.

What is the depletion region in a PN Junction diode?

The depletion region is the area around the PN Junction where mobile charge carriers (electrons and holes) have recombined, leaving behind immobile positive and negative ions. This region is depleted of free charge carriers and has an electric field across it.

Why is the depletion region also called the space charge region?

The depletion region is also called the space charge region because it contains immobile ions that create an electric field across the junction, resulting in a region of charged particles without mobile carriers.

What is the barrier potential in a PN Junction diode?

The barrier potential, also known as the built-in potential, is the electric potential difference across the depletion region that acts as a barrier, preventing the further movement of majority charge carriers.

What happens to the diffusion and drift currents under steady-state conditions?

Under steady-state conditions, the diffusion current (due to majority charge carriers) and drift current (due to minority charge carriers) are equal and opposite, resulting in no net current flow through an open-circuited PN Junction diode.

Outlines

00:00

🔬 Introduction to Semiconductor Diodes and PN Junction

This section introduces the topic of semiconductor diodes, focusing on PN Junction diodes, which are two-terminal devices formed by combining p-type and n-type semiconductors. It explains how a PN Junction is created and the concept of volt-ampere (VI) characteristics of these diodes. The paragraph distinguishes between a simple PN Junction and a PN Junction diode, created by adding metal contacts to the two terminals. The main conditions of operation (no bias, forward bias, and reverse bias) are also introduced. The concept of 'biasing'—applying external voltage across the terminals—is defined.

05:02

⚛️ Formation and Behavior of PN Junctions

This section explains what happens when p-type and n-type semiconductors are combined. When the two types are brought together through methods like diffusion or implantation, charge carriers (holes in p-type and electrons in n-type) begin to move due to concentration gradients. This movement leads to the formation of diffusion current. The paragraph compares this behavior to a pressure difference between two regions, showing how charge carriers flow from high to low concentration, creating a diffusion current.

10:02

🌊 The Depletion Layer and Its Formation

This part details the formation of the depletion layer in a PN Junction diode. As holes from the p-side combine with electrons from the n-side, they leave behind immobile ions, creating a region devoid of mobile charges. This depletion region, also called the space charge region, forms due to the recombination of electrons and holes, leaving only fixed donor and acceptor ions. The section emphasizes the importance of this depletion region for understanding the diode's behavior.

15:04

⚡ Electric Field and Barrier Potential in PN Junctions

Here, the concept of an electric field across the depletion region is discussed. As the immobile positive and negative ions surface, an electric field is established from the positive side to the negative side of the depletion region, creating a barrier potential (also called built-in potential). This potential acts as a barrier for the movement of further free charge carriers, preventing holes and electrons from continuing to recombine.

Keywords

💡PN Junction

A PN junction is formed when a p-type and n-type semiconductor are combined. It is fundamental to understanding semiconductor devices like diodes. In the video, the PN junction is discussed as the foundation for creating a PN junction diode, which exhibits various characteristics based on how external voltage (bias) is applied.

💡PN Junction Diode

A PN junction diode is a two-terminal device created by attaching metal contacts to a PN junction. It allows current to flow in one direction under forward bias, and blocks it under reverse bias. The lecture emphasizes understanding the volt-ampere characteristics of this diode, which is crucial for learning about more complex devices like BJTs.

💡Bias

Bias refers to the application of external voltage across the two terminals of a semiconductor device. The video outlines three bias conditions for the PN junction diode: no bias, forward bias, and reverse bias. Understanding biasing is essential for analyzing how current flows through the device under different conditions.

💡Forward Bias

Forward bias occurs when the positive terminal of an external voltage source is connected to the p-type material and the negative terminal to the n-type material, allowing current to flow through the PN junction diode. The video promises future discussions on forward bias in later presentations, as it plays a critical role in the device’s operation.

💡Reverse Bias

Reverse bias is when the positive terminal is connected to the n-type and the negative terminal to the p-type, preventing current from flowing through the diode. The video will cover this condition in subsequent lectures, highlighting how it creates a high-resistance path that blocks current.

💡Diffusion Current

Diffusion current is the movement of majority charge carriers (holes in p-type and electrons in n-type) from high to low concentration across the junction. In the video, diffusion current is described as resulting from the concentration gradient of charge carriers and is crucial for understanding the current in a PN junction diode.

💡Drift Current

Drift current is caused by the movement of minority charge carriers (electrons in p-type and holes in n-type) due to the electric field in the depletion region. In the video, this current is created as minority carriers move in response to the electric field across the depletion layer, balancing the diffusion current under steady-state conditions.

💡Depletion Region

The depletion region is the area around the PN junction where mobile charge carriers have recombined, leaving behind immobile ions. This region is devoid of free charges and acts as a barrier to further charge movement. The video explains that the depletion region is responsible for the formation of a potential difference across the junction, which prevents the flow of majority carriers.

💡Barrier Potential

The barrier potential is the potential difference created by the immobile ions in the depletion region, which prevents further movement of majority charge carriers. In the video, it is also referred to as the built-in potential, and it acts as a barrier that must be overcome for current to flow in forward bias.

💡Bipolar Junction Transistor (BJT)

A BJT is a three-terminal device that operates based on the concepts of the PN junction diode. In the video, the BJT is introduced as an important topic that relies on the understanding of PN junctions. It consists of two PN junctions and can be considered as two diodes working together, where the current in one junction affects the other.

Highlights

Introduction to semiconductor diodes and PN junction diodes, highlighting the importance of understanding PN junctions for further electronic studies.

Explanation of the volt-ampere characteristics (VI characteristics) of the PN junction diode, a two-terminal device created by adding metal contacts to a PN junction.

Introduction of three biasing conditions for PN junction diodes: no bias, forward bias, and reverse bias.

Definition of 'bias' as the application of external voltage across the two terminals of a PN junction diode.

Explanation of the no bias condition, where no external voltage is applied, and the analysis focuses on the natural behavior of the PN junction diode.

Introduction to bipolar junction transistors (BJT), a three-terminal device that is heavily dependent on the concepts of PN junction diodes.

Comparison of p-type and n-type semiconductors, including the introduction of trivalent and pentavalent impurities, and how they affect charge carriers (holes in p-type and electrons in n-type).

Explanation of diffusion, the process by which majority charge carriers (holes and electrons) move from high concentration to low concentration across the PN junction.

Introduction of the concept of diffusion current, caused by the movement of majority charge carriers across the junction.

Formation of the depletion region at the junction due to recombination of holes and electrons, leaving immobile ions, and introduction of the term 'depletion layer'.

Clarification of the electric field formed in the depletion region, which prevents further movement of charge carriers, creating a barrier potential.

Introduction to the drift current, caused by minority charge carriers moving in response to the electric field in the depletion region.

Establishment of the steady state condition, where diffusion current (caused by majority carriers) is equal to drift current (caused by minority carriers), resulting in zero net current for an open-circuited PN junction.

Introduction to the barrier potential and the role it plays in preventing further charge carrier movement in the depletion region.

Preview of the next lecture's topics: deriving the expression for the barrier potential (VB) and analyzing the width of the depletion region (WD).

Transcripts

in this lecture we will start a new

chapter semiconductor diodes semi

conductor diodes this is the name of the

chapter and in this chapter we will

learn how a junction is formed between

p-type and n-type semiconductor we will

also learn volt-ampere characteristics

of the two terminal device obtained by

using PN Junction and the device is

called as PN Junction diode so I will

write this down we will learn

volt-ampere characteristics or VI

characteristics of PN PN Junction diode

PN Junction diode is a two terminal

device and there is a difference between

PN Junction and PN Junction diode we

obtain PN Junction diode by using the PN

Junction and we can do this easily by

attaching the metal contacts at the two

terminals so if I attach one metal

contact like this then we have PN

Junction diode by using the PN Junction

we will do the same thing for this

terminal also and because of this we

have three possibilities first

possibility is no bias condition no bias

conditions in second possibility we have

forward forward bias condition and in

third possibility we have reverse bias

condition in this lecture we will deal

with no bias condition and in the coming

presentations we will complete forward

and reverse bias condition of the PN

Junction diode now there is one question

what is the meaning of bias what do we

mean by biasing the two terminal device

so I will write this down the meaning of

bias

the term bias refers to the application

of external voltage across the two

terminals when we apply external voltage

across these two terminals we call it

biasing I will write this down

application application of external

external voltage across the two

[Music]

terminals is called as bias and if we

applied noise channel voltage it is

called as no bias condition so in this

lecture we are not going to apply an

external voltage to this PN Junction

diode and we will analyze the PN

Junction diode in this situation the

next thing is the BJT the bipolar

Junction transistor this chapter is very

important because we will use the

concepts and the theories developed in

this chapter for learning B JT

BJT stands for bipolar by polar Junction

transistor and this is very important

topic in analog electronics and it

depends on PN Junction diode so if you

have better understanding of PN Junction

diode you can easily understand the

bipolar Junction transistor BJT is a

three terminal device this is three

terminal device and you can consider

this as the device having two diodes

visit even consider as a device having

two diodes in which one diode is biased

by the current of other diode so you can

see how much it depends on the concept

that we are going to learn so without

wasting any time we will start with PN

Junction we already know what is p-type

and n-type semiconductor in case of

p-type semiconductor we add travel and

impurities in case of p-type

semiconductor we add travel and

impurities and because of this we have

holls holls as the majority charge

carriers on the other hand for n-type

semiconductor we add pentavalent

impurities and because of this we have

electrons as majority charge carriers

and if you talk about the minority

charge carriers then for p-type we have

electrons as the minority charge

carriers and for n-type we have whole

cells minority charge carriers and if

you remember the minority charge

carriers the minority charge carriers

depends only on temperature because

electron gains the thermal energy and

breaks the covalent bond so it only

depends on the temperature and it will

not depend on the external voltage that

we are applying so this is something we

already know and now we have to see what

will happen if we combine p-type and

n-type semiconductor together when we

combined p-type when we combined p-type

and n-type semiconductors together by

any means like diffusion or an

implantation these are the process to

combine P and n-type semiconductors we

dope one side of the semiconductor with

the trivalent impurity x' and we dope

other side of the semiconductor with

pentavalent impurities we will have

p-type on one side like this and we will

have n-type on the other side if we

consider the p-type if we consider the

p-type I will clear this portion if we

consider the p-type and then we have

positive immobile ions or we can say

acceptor ions these are the acceptor

ions and we have holes we have holes as

the majority charge carriers and on n

side we have donor ions donor ions and

electrons are the majority charge

carriers so this is what we have when we

join p-type semiconductor and n-type

semiconductor and this is the junction

this is the junction and if we talk

about holes then concentration of hole

is high

on the p-side has compared to the inside

and because of this we have

concentration gradients of holes from P

side to n side and similarly we have

concentration gradient of electrons from

n side to P side and if we try to

understand this thing by the help of

pressure difference let's say we have a

and B these are the two reasons and

pressure at a is PA and pressure at B is

P B and let's say PA is greater than PB

then air will flow from A to B right the

same thing will happen with holes on P

side concentration of hole is higher so

holes will move towards the N side like

this and if you talk about electrons

then concentration of electron is more

on n side

so electrons will move to P side like

this and as you can see we have movement

of charge through the cross sectional

area if we consider this Junction and

let's say the area of Junction is a then

there is movement of charged carriers

through this area and whenever this

happens there is current and as this

process the process of movement of

charge carriers from high concentration

to low is called as diffusion we call

this current diffusion current this

process is called as diffusion and the

current is called as diffusion current

this diffusion current is very important

and we have to use it a lot while

solving the numerical problems and

diffusion current is because of majority

charge carriers so we have one new thing

with us diffusion current so let us try

to understand it once more diffusion

current is because of the movement of

charge carriers the majority charge

carriers from high concentration to low

concentration we call it diffusion

current because the process of movement

is called as diffusion and this is

because of

already charged carriers right the next

important thing is depletion layer now

we have to understand how depletion

layer is formed in PN Junction diode

because of diffusion holes from P side

holes from P side combines with electron

on n side because we have electrons as

the majority charge carriers on n side

and in the same way electrons from n

side combine with the holes on P side

and as the recombination takes place in

mobile ions will surface out I will

write this point down immobile ions will

surface out because of what because of

diffusion now what do we mean by what do

we mean by immobile ions will surface

out what do we mean by surface out

surface out is nothing but uncovering of

immobile ions this acceptor ions and

this donor ions are immobile ions

because they are fixed they are not

moving and when we uncover them we call

immobile ions are surfaced out because

initially electrical neutrality was

maintained with acceptor ions we had

holes so electrical neutrality was

maintained in the same way with donor

ions we had electrons but as these holes

are combining with electrons we are left

with positive and negative ions because

of this depletion layer is formed in PN

Junction diode the reason with no mobile

charges but only uncovered immobile ions

is called as depletion region I will

write this down because this is

important

depletion reason in this reason there is

no mobile charges no mobile charges but

only but only uncovered

uncovered immobile ions are there and as

I have already explained why there is no

mobile charges because the holes and

electrons are combining with each other

and we are left with negative and

positive ions so I will try to draw this

quickly I will try to draw the depletion

layer quickly

the things are very important because

this will help you understand the basics

and after this you can understand any

concepts in analog electronics and if

you fail to understand this important

points then it will be very tough for

you to understand the coming

presentations first I will make P side

we have acceptor ions on the P side and

on the N side we have donor ions so we

have positive sign for them

holes are the majority charge carriers

in

p-site so we have holes as the majority

charge carriers and we have electrons as

the minority charge carriers in the same

wave for n side we have electrons as the

majority charge carriers and we have

holes as the minority charge carriers

now the electrons and holes have

recombined in this reason so we are left

with we are left with negative emotions

and positive immobile ions like this

because holes and electrons have already

recombined with each other and you can

clearly see because of this we have

layer of negative charges negative

immobile ions on this side and we have

layer of positive immobilise on this

side so we can say that this is acting

as the potential difference and because

of this potential difference there is an

electric field from right to left that

is from positive to negative there is an

electric field like this capital e is

the electric field from positive to the

negative this is the Junction I forgot

to make Junction here this is the

junction and this is the depletion

region now this depletion region this

depletion region is also called as space

charge region space charge region or

depletion layer depletion layer these

are the names for a depletion region now

there is one question why the width of

depletion reason why the width of

depletion region is fixed because this

hole can combine with this electron in

the same way all the holes on the P side

can combine with the electrons all the

electrons from the N side and the width

of depletion region is equal to the

width of the PN Junction dye

but this does not happen so what is the

reason behind this as I have already

told you we have electric field

developed because of the potential

difference and due to this electric

field there is no movement of charge

carriers because this positive layer

will repel the holes right because hole

is equivalent to positive charge and

this positive layer will repel the holes

so there is no movement of holes and in

the same way this negative layer this

negative layer will repel the electrons

from the inside so the width of

depletion region is fixed and as you can

see this potential is acting as the

barrier to the movement of holes and

electrons it is acting as barrier for

the further movement of free charge

carriers we call it barrier potential we

call it barrier potential or we call it

built-in potential built-in potential

right and we are calling it barrier

potential because it is acting as the

barrier for the further movement of free

charge carriers let's talk about

minority charge carriers in this

scenario we are done with the majority

charge carriers we have seen there is

diffusion current because of the

movement of majority charge carriers and

after the formation of depletion region

the majority charge carriers will not

combine with each other and hence there

is no movement of majority charge

carriers and as we already know in case

of p-type semiconductor electrons are

the minority charge carriers and for

n-type we have holes as the minority

charge carriers and because of this

electric field in density II electrons

will move to the right and the holes

will move to the left so there is

current and because of drift of

electrons towards the positive layer and

drift of holes towards the negative

layer we call this current drift current

and this drift current this drift

current having the direction opposite to

the diffusion car

and under steady state condition the

diffusion current is equal to the drift

current before that I will complete this

the drift current is due to the minority

minority charge carriers right and the

diffusion current was because of

majority charge carriers and under under

steady state diffusion current diffusion

current is equal to the drift current

this is very important point diffusion

current is equal to the drift current

under steady state condition and we can

say that we can say that the net current

is equal to zero in case of open

circuited PN Junction diode the net

current the net current is equal to zero

for open circuited PN Junction diode so

this is all for this lecture in the next

lecture

we will find out the expression for VB

that is the barrier potential and we

will also talk about width of depletion

region WD width of depletion region so

this is all for this lecture see you in

the next one