Physics semiconductors project

1 | p h y s i c s p r o j e c t .
Semiconductor
s…
Physics Project..
-Aashirwad
jindal

to – heema mam
“Ifwe knew what it was we were doing, it would not be calledresearch,
would it?”
― Albert Einstein

Welcome to this basic
tour of semiconductor
physics! Two of our
most excellent guides,
Sally Con and Jerry
Manium, will take you
through.
Sally and Jerry explain
things in different ways.
Sally tries to be correct,
and likes to stick to the
facts. Jerry is easy-going,
and uses examples from
the everyday world
around us. let Sally and
Jerry explain a few
things!
Introduction…

Let's begin this journey into the world of semiconductors with a look at
the history books. In the early 1900s, not much was known of the world
at an atomic level, and even less so at the subatomic level. Physics, to a
large extent, still calmly followed classical rules. But new discoveries
like Röntgen's x-rays, Thomson's electron and Rutherford's discovery
of the atomic nucleus made it clear that new rules were needed. Scientists
like Planck, Einstein, Bohr,Pauliand Heisenberg, to name a few, all
contributed to the development and understanding necessary for the
creation of the new paradigm of quantum physics. The development of
quantum physics also laid the ground for 'Solid State Physics' which is a
discipline explaining the internal atomic structure and the electronic
properties of the materials that we see in our everyday life such as
metals, plastics, glass, etc.
History..

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History..

Before we start, it would be a good idea to clarify what
electricity is. Electricity can be seen as a stream of
electrons. Electrons are tiny particles with a negative
charge. So, roughly explained, electricity is a stream of
electrons flowing from one point to another.
A good way to explain an electric current passing through a
cable would be to imagine a pipe filled with marbles that
exactly fit the pipe. If we push a marble into the pipe in one
end, the motion would be distributed, each marble pushing
its neighbor, so that almost instantly a marble in the other
end would be pushed out of the pipe.
Electricity

Conductivity..!!

Why semiconductors..???

Semiconductors can be made of a single material or
a combination of several different materials. In
early semiconductor devices germanium was often
used. However in today's semiconductor industry,
silicon is commonly used.
Silicon is very easy to find in Nature. Ordinary
sand, like on the beach or in the desert for example,
is nothing more than one silicon atom combined
with two oxygen atoms. However, if you want
silicon in its pure form suitable for the production
of, for instance, computer chips it has to be purified
in a carefully monitored process.
One of the main reasons for the popularity of silicon
is that it is stable and can be heated to a rather high
degree without loosing its material characteristics.
This means that engineers can be sure it will
perform according to their plans, even under quite
extreme conditions.
Semiconducting materials..!!

Just to make sure we avoid misunderstandings,
when we talk about silicon, we don't mean silicone
spelled with an "e" at the end, 'cause that is a
material mostly known for its use in human
implants.
To understand the principles of semiconductors, it is
good to first understand the basics of atoms and
energy levels within atoms. So that's where we're
going to start.
Semiconducting materials cont….

If we look at the solid material of Silicon we will see
that it is built from a huge number of Silicon atoms
that are brought together. When the atoms interact
with each other, the atomic shells of each atom
interacts with the atomic shells of neighboring atoms.
On an energy scale, the overlapping energy shells of
all the separate atoms form energy bands that are
similar to the energy shells in the single atom.
Between the bands no electrons are allowed. In a
simplified way, it is almost as if the solid material is
an enlargement of the single atom.
Silicon and its molecules..

To continue our journey, you don't need to
fully understand what Sally just said. But
what you do need to understand is, that the
highest energy band that is occupied by
electrons in a material is called the valence
band, just like in the single atom where the
highest shell occupied by electrons is called
the valence shell. The band with energy
one step higher than the valence band is
theconduction band. The energy gap
between these two bands, where no
electrons are allowed, is called the band
gap.
If you think of the energy bands as steps in
a staircase then the band gap is the area
between the steps. You can put your foot on
the first step of the stairs and you can put it
on the second, but you can never put it
Bands and their theories...

somewhere between the first and second.
Bands and their theories... cont…

A very important feature of the semiconductor material is
the electron-hole pair. To get a semiconductor to conduct
a current, we must make an electron jump from an
occupied to an unoccupied energy level. When it does
this it leaves a hole (an empty state). This hole can be
filled by another electron, which itself leaves a new hole.
Therefore, we could say that both the hole and the
electron contribute to the conductivity as they move
around in the material. The hole is like a positive charge
(lack of negative), the electron is negative.
It's a little bit like this simple puzzle game where you
move pieces around to form an image. The moving pieces
correspond to the electrons, of course.
A electron-hole pair..!!

As mentioned earlier, the semiconductorhas a conducting
capacity somewhere between the conductorand the insulator. If we look
closer at the materials we can see why they behave like this. Before we
go on, note that contrary to what its name may suggest, the conduction
band is not the only band where conduction of a current may occur.
Conduction is equally possible in the valence band.
In a good conductor like a metal, the highest energy band with electrons
(valence band) is only partially filled. This means that the electrons can
accelerate. In other words, they gain energy so that they can transfer to
Conduction in different types of materials…

higher energy levels that are empty. Simply put, in a conductor there is
plenty of room for the electrons to jump from an occupied state to an
empty one.
If you felt that Sally's explanation of the conduction
properties in different materials was crystal clear, you
can skip the following part. But if you're still a little
unsure of how it works, I will try to show you another
way of looking at this phenomenon. To help my
explanation, I am going to use the unrealistic cup with
the water-filled compartments again. The compartments
equal the energy bands of the material and the water

equals the electrons. This time the cup only has two
compartments, one for the valence band and one for the
conduction band.
In a conductor, the valence band is only partially
filled. This means that, in our cup, we are going to
have the valence compartment half-filled with water.
If we tip the cup from side to side, we will see that it
is easy for the water to move back and forth, just as it
is easy for the electrons to move within the
conductor.
A semiconductor at low temperature is an insulator
because there is no place for the electrons to go to.
The valence compartment is filled and no matter how
we tip the cup there is no room for the water to move
into. At room temperature, the heat (energy) makes
the atoms vibrate slightly, enough for a few of the
electrons to break their bonds and jump into the

conduction band. If we take some water (electrons)
from the valence band and move it to the conduction
band, we will have place for the water (electrons) to
move in both bands. If we tip our cup, water will
move both in the valence and conduction band. Thus,
in a semiconductor at room temperature, a small
current will flow.
In an insulator, the valence band is completely filled,
and as a result no electrons can move. In the cup, no
water will move no matter how we tip it. The band
gap between the valence and the conduction band is
huge. To move water (electrons) from our valence
compartment to the conduction compartment, we
would need to add such an amount of energy that our
cup (material) would be close to breaking before any
water (electrons) would begin to move between the
compartments.
Conduction in different types of materials cont…

Now we are going to talk about doping. Maybe the word
makes you think of athletes taking illegal drugs to perform
better. Although doping in sports is outrageous, the parallel
between that and doping of semiconductors is not too far-
fetched. In both cases you have something pure, like an athlete
or a semiconducting material, and add something foreign to it
to change its performance.
So, in the process of doping you add a tiny amount of atoms
from another material to the pure semiconductor. By doing so,
you can drastically increase its ability to conduct a current.
There are two forms of doping, p and n. p stands for positive
and n for negative. Finally, two words that are good to know:
a pure non-doped semiconductor is called intrinsic, while a
doped semiconductor material is called extrinsic.
Doping …
Doping..cont..

Before we look at examples of doped semiconductors, let's
look at how the silicon atoms in pure silicon interact to form
the crystal structure of the material. In pure silicon, each
atom has four valence electrons and these are shared with
four neighboring silicon atoms to make four double bonds.
Now each atom will have a completely filled valence shell
of eight electrons. At low temperature this bond is very
stable, completely filling the valence band and thus making
conduction impossible. Here is a model of the structure of
pure silicon:
In a pure semiconductor at low temperature, the
valence layer is completely filled with electrons and the
conduction band is empty. That would be equal to one
filled and one empty compartment in my cup. The
water (electrons) can't move because there is no
empty space.
Pure semiconductors..

p-doping is when you add atoms with less valence electrons to
the semiconductorso that the material gets a shortage of
electrons in the crystal bonds.This way positive holes that can
transport current are formed.The materials that add holes are
called acceptors because they accept electrons from the
surrounding atoms. In a p-type semiconductorthe major
carrier of current are the holes, not the electrons.
The p in p-doping stands for positive.This is because
compared to the atoms in the semiconductormaterial the
added atoms have fewer negative valence electrons.In the p-
doped semiconductor the higher conduction band is empty,
but there will be holes in the valence band.
In the cup, this means that we remove some water from the
valence compartment.In other words, we form air bubbles
(positive holes) in the water. Now if we tip the cup, there is
room for the water (electrons)to move in one direction and for
the created holes (lack of electrons)to move in the opposite
direction (just like bubbles would do in water).
P doping

In the processof n-doping you add atoms with one extra
valence electronto the pure semiconducting material. This
creates a situation where there are extra electrons that are just
looselybound in the crystal. The amount of energy needed to
get these electrons to jump to the conduction band so that a
current may pass is very small. The materials that add
electrons are called donors. This is simply because they
donate electrons to the semiconductor.In the n-type
semiconductorthe major carrier of current is the negative
electrons.
The n in n-doping stands for negative. This is because
compared to the atoms in the semiconductormaterial the
added atoms have more negative valence electrons.In the n-
doped semiconductor,the valence band is full so there is no
room for the electrons to move there. Instead, the extra
electrons move into the conduction band.
In our cup, we can see that no water will move in the full
valence compartment.Instead, the extra water (electrons)
added will move within the conduction compartment.
N-doping…

In a world where computers become faster and faster
each year, semiconductorcomponents, like chips and
transistors, must be made smaller and smaller. This
means that we will eventually reach a limit on how much
faster and more effective the Silicon based technique can
be made (in fact, devices operating with just a single
electron have already been demonstrated)."What
happens then?" you might ask yourself. Well we don't
know for sure, but today's scientists are working hard to
find new materials or to improve old ones. In the future,
large molecules might do the work that transistors do
today. This field is called Molecular Electronics. So
hopefully (if you like information technology, that is)
computers can continue to evolve for a long time to
come.
Semiconductors-the future

Characteristics of transistor..
Transistor as amplifier.
Transistor as switch
Circuit diagram…

Full wave rectifier.
half wave rectifier.
Circuit diagrams..

logic gates…
Integrated circuit
Circuit diagram…

Physics semiconductors project

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