Welcome to professor MAD.
Electronics for beginners.
Have you ever heard
the single most important invention of the 20th century,
was the transistor?
Transistors critical as a control source in just about every modern circuit.
In small discrete quantities,
transistors can be used to create simple electronic switches,
digital logic and signal amplifying circuits.
In quantities of thousands, millions and even billions,
transistors are interconnected and embedded into tiny chips,
to create computer memories, microprocessors and other complex IC s .
In this video we'll introduce you to the
basics of the most common transistor around.
The bipolar junction transistor or BJT.
It's slightly easier to understand.
There are actually two versions of BJT s.
NPN and PNP.
Transistors are fundamentally three terminal devices.
Those pins are labeled as collector, base and emitter.
The circuit symbols for both are as like this.
The only difference between an NPN and PNP symbols
is the direction of the arrow.
On the emitter, the arrow on the NPN points out.
And on the PNP it points in.
Let's talk about transistor construction.
Transistors rely on semiconductors to do their magic.
A semiconductor is a material that's not quite a pure conductor.
But also not an insulator.
The conductivity of a semiconductor depends on variables like,
temperature or the presence of more or less electrons.
Let's talk about P and N type semiconductors.
The conductivity of a semiconductor may easily be modified by,
introducing impurities into their crystal lattice.
The process of adding controlled impurities to a semiconductor
is known as doping.
The concentration of the dopant used,
affects the materials' charge carrier concentration.
There are two types of charge carriers.
Electrons and Holes.
We know electrons are negatively charged.
The hole is the absence of electron.
Since electrons are negatively charged,
we call "The absence of electron", is a positive charge"
This is Mr. Electron.
He is sitting on a chair.
Since his negative charge is balanced by the chair,
he is comfortable there.
Let's force him to stand up and move away.
Now he is a free electron.
He don't like to be this way.
So he is searching somewhere to sit down.
So this free electron is movable.
Not like the electron that has a chair.
Now pay attention to the chair.
Earlier there was an electron.
Now it's a free chair that has a positive charge.
The chair is looking for an electron.
Because it also don't like to be alone.
This chair actually represents a hole,
the absence of electron.
This is a line of chairs.
One electron can sit one chair.
One chair can only have one electron.
Electrons always prefer to sit on chairs.
Even if we apply a voltage, electrons have no free chairs to move.
They all are in comfortable positions.
So no charge carriers.
That means it's like an insulator.
If we introduce a few new electrons here,
they have no chairs to sit on.
They are not comfortable like that.
Free electrons are always searching for chairs to sit.
If we guide them using a voltage, they will obey us and move.
Movement of free electrons make a current.
Introducing new electrons makes this a conductor.
Since electrons are negatively charged,
we call this an N-type semiconductor.
Let's go back.
Again we have an insulator.
Let's add a new chair here.
Electrons have an extra place to be now.
They can move a little, if they want.
So if we ask them politely to move,
they all move right to accommodate the hole.
Actually it seems like a chair moving from one end to another,
not the electrons.
The electrons are moving in the opposite direction of the positive hole.
Movement of holes make a current.
Introducing new holes make this a conductor.
Since holes are positively charged, we call this a P-type semiconductor.
Now we know about the N-type and P-type semiconductors.
But keep in mind,
that semiconductors are electrically neutral.
The number of total negative charges are
always equal to the number of positive charges.
N-type or P-type means that what is the major charge carriers of the material.
Each positive or negative carrier belongs to a,
fixed negative or positive charge dopant.
Let's see what happens in the P-N junction.
P-N junction is a interface between,
P-type and N-type semiconductor materials.
At the junction,
free electrons in the N-type are
attracted to the positive holes in the P-type region.
The region near the P-N interface lose their neutrality,
and most of their mobile carriers,
forming the depletion layer.
The depletion region has no charge carriers.
and act as an insulator.
If we connect the P-side to the negative terminal of a battery,
and N-type to a positive terminal,
more electrons are attracted to the P side.
The region near the P-N interface lose more of their mobile carriers,
making the depletion layer more stronger.
Now it's like two conductors are,
separated by an insulation layer.
No current can go through the reverse biased P-N junction.
If we connect the P side to the positive terminal of a battery,
and N-side to negative terminal,
the moved electron will again back off to their original positions.
This causes to vanish the depletion layer.
Now it's like a normal two conductors connected together.
It can conduct the current.
There is an important fact that we should keep in mind.
To vanish the depletion layer,
it takes a small amount of voltage.
Normally around 0.7 V.
To flow the electrons,
the forward voltage should be more than 0.7V.
Those are the basic concepts
that help us to understand about the transistors.
Transistors are created by stacking
three of those N and P layers top of one another.
It may be NPN or PNP.
For ease of understand,
let's only focus into NPN transistors.
This has two P-N junctions.
So two depletion layers.
Its emitter region is heavily doped and wider.
Hence the number of free electrons is large here.
The collector region is also wider,
but it is moderately doped.
Hence the number of free electrons is not as much as the emitter region.
The base region is diffused in between the
wider emitter and collector region.
But the base region is quite thin,
compared to the outer emitter and collector region.
But also it is very lightly doped.
So the number of holes is quite small.
Now we connect a battery in between emitter and collector.
The emitter terminal of the transistor
is connected to the negative terminal of the battery.
Hence the E-B junction becomes forward-biased.
The B-C junction becomes reverse biased.
We know the applied voltage is working across both junctions.
The sum of the V(eb) and V(bc) is equal to the V(ce).
And we also know that V(eb) junction is forward biased.
But no current goes through it.
Because the forward voltage is only enough to remove the depletion layer.
But not to make the electron flow.
B-E junction is only 0.7 V.
So what should we do to pass electrons through the E-B junction?
The V(eb) should be larger than 0.7 V.
If we increase the V(ce) furthermore,
it only goes to increase the reverse bias of V(bc).
Whatever be the voltage across the device,
the forward barrier potential across the E-B junction,
always remains 0.7 V.
And the rest of the source voltage is dropped across the B-C junction.
So the only way to increase the V(eb) is
adding a separate voltage source to E-B junction.
A very strange and interesting thing happens now.
Let us recall the constructional and doping details of npn transistor.
Here the emitter region has high concentration of free electrons.
The base region on the other hand is a very thin and lightly doped.
So as a result,
there is a very tiny number of holes present in this region.
If we apply a separate power source to B-E junction,
the B-E junction gets forward voltage individually and certainly.
If the voltage of B-E is more than 0.7 V,
it can overcome the forward potential barrier,
and hence the free electrons in the emitter region will
cross the junction and come in the base region.
Where they get very few number of holes to recombine.
But due to the electric field across the junction,
the free electrons migrating from the emitter region get kinetic energy.
The base region is soo thin,
that the free electrons coming from the emitter,
do not get sufficient time to exit through the base.
And hence cross the reverse bias depletion region,
and ultimately come to the collector zone.
The current through the emitter is called emitter current.
The current through the collector is called collector current.
And the tiny current flowing through the base terminal is called base current.
Hence here emitter current is the sum of the
base current and collector current.
But compared to the I(c) the I(b) current is very low and negligible.
So we can assume I(b) is equal to 0.
And the I(e) is almost equal to the I(c).
Now we know applying a small voltage to base,
the current through the collector and emitter can be controlled.
The NPN transistor is designed to pass the electron from the emitter to the collector.
The emitter emits electron into the base,
which control the number of electrons the emitter emits.
Most of the electrons emitted are collected by the collector.
Which sends them along to the next part of the circuit.
The construction and circuit symbols for both PNP and NPN bipolar transistors are like this.
The arrow in the circuit symbol,
always showing direction of conventional current flow.
The transistor is kind of like a switch.
We can ON and OFF a light bulb,
using a mechanical switch like this.
But in an electronic circuit we must need a switch,
that can be controlled using electrical signals.
That is where transistor is come in.
If the base voltage is zero,
then the transistor is like an open switch.
To turn ON the bulb, we should make it a closed switch.
We can set the base voltage is higher than 0.7 V.
Now the transistor acts like a close switch.
That is the very basic of transistor.
See you in another video.