Semiconductors

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What is a semiconductor and what is the basis of this classification?

A semiconductor is a material that intermediates between a conductor and an insulator.

Electrical conduction represents the ability of a material to conduct electric charges. Some materials are very good at it and are called conductors; the materials that are hopeless at it are called insulators.

Semiconductors (aka semi-insulators) are not very good at it. But under certain conditions, they can almost be a conductor.




What are the electron shells in an atom?

Electrons ‘revolve’ around the nucleus but they do not have the same orbit. The organization of electrons within an atom is actually a very difficult problem, which is solved by a discipline called quantum physics.

Electrons occupy orbits that are called energy levels. The first orbit closest to the nucleus in the K-shell (or shell 1); the second orbit, further away is the L-shell (or shell ); the third shell is the M-shell (or shell ), etc.

However each level can only carry a maximum no. Of electron which is n^ (n is the level no.)

What is the distribution of electrons in the Silicon atom?

Silicon (Si) atomic no. Are 14

· electrons go into the level 1

· 8 electrons go into the level

· 4 electrons go into the level

How are the ‘energy levels’ and electron shells in an atom related?

What are ‘orbitals’?

Orbitals are finer subdivisions of the energy levels.

Where are the lowest energy states for electrons found?

Energy dictates how the energy levels arrange themselves in the atom. The lowest energy states are the most stable. They are also closer to the nucleus.

What are the valence electrons and why are they so important?

If we look at how the electrons fill the orbitals, we notice that the elements that belong to the same column of the periodic table (e.g. sulfur and oxygen) have the same no. of outermost electrons. These are called the valence electrons they drive the chemical, electronic and optical properties of the elements.

What happens when two atoms of the same species approach each other?

As they move closer together, the energy levels broaden into bands to accommodate more electrons. In a material, which contains many atoms, the energy levels form a continuous band. The gap between the bands represent energies that no electron can possess. They are forbidden states.

What happens in materials, which contain many atoms?

In a material that contains many atoms, the energy levels form a continuous band.

What is significant about the gap between electron bands in such materials?

The gap between electron bands and such materials is significant because it represents energies that no electrons can posses. It is a forbidden state.

The highest energy band that is occupied by one or more electrons is called the valence band. The next higher vacant level is called the conduction band. Conduction and valence band are separated by a gap, which represent a set of energies that electrons cannot have.

What do we term the highest energy band occupied by one or more electrons?

In a material, which contains many atoms, the energy levels form a continuous bands.

What so we call the band above this (normally with no electrons in it)?

The gap between the bands represent energies that no electron can possess, they are forbidden states.

What separates the valence and conduction bands?

The highest energy band that is occupied by one or more electrons is called the valence band. The next higher vacant level is called the conduction band.

The conduction and valence band are separated by a gap, which represent a set of energies that electrons cannot have.

Two atoms of the same species, as they move closer together, the energy levels broaden into bands to accommodate more electrons. In a material, which contains many atoms, the energy levels form a continuous band. The gap between the bands represents energies that no electrons can posses, forbidden states.

What are the three possible positions of the mezzanine level that the creatures can jump to?

You can adjust the height of the mezzanine level to see how the creatures jump. There are three positions available low, medium and high.

What happens when the heat is turned on?

The creatures get a little bit excited they move faster and jump higher; the mezzanine level is occupied from time to time.

What happens when the mezzanine level is raised to the high level and the heat is turned up?

When the level of the mezzanine is raised. Our creatures want to find some space but they cannot jump high enough to reach the mezzanine level, they don’t have enough energy. When the heat is turned on the creatures get excited but they do not have enough energy to jump on the mezzanine, which remains unoccupied.

What happens when the heat is on and the mezzanine has been lowered a little?

Which variations of the experiment model the behaviour of the electrons in

i.) Conductors?

ii.) Insulators?

iii.) Semiconductors?

Describe an insulator in terms of the energy bans of electrons in a material

In an insulator the valence band is completely filled with electrons. The energy between the gaps and the conduction band is large. Electrons do not have enough energy to jump to the conduction band.

Describe a conductor in terms of the energy bands of electrons in a material. What is the relationship between the valence band and the conduction band in a conductor?

In a conductor, the valence band is partially filled with electrons. The electrons can move freely inside the valence band throughout the solid. They don’t need to jump the energy gap. The valence band is the conduction band.

Describe a semiconductor in terms of the energy bands. How big is the gap between the valence and conduction bands? How can electrons gain enough energy to move between valence and conduction bands?

In a semiconductor the valence band is completely filled with electrons just like in the insulator. However the energy gap between the valence and the conduction band is small. Electrons can get enough energy to jump to the conduction band. The surplus of energy can be brought by different sources temperature and illumination.

What are two examples of semiconductors?

Silicon and germanium.

What are some uses of semiconductors?

Until the invention of the transistor, semiconductors were just a scientific curiosity. Since then, they have become the key material in electronics. They can be used to amplify signals, to detect radiation, to emit light, etc.

Semiconductors are present in our every day life.

What are the useful properties of semiconductors based on?

Semiconductors have many useful properties that conductors and insulators do not possess. These properties are based on the fact that an electron can jump from the valence band to the conduction band and vice versa.

How is a HOLE generated and where is it created?

The hole is created when an electron in a semiconductor jumps from the valence band to the conduction band.

However the scientists decided to give a scientific name to this empty space and call it the hole.

Which are the important electrons?

Some electrons and holes play an important role in the electrical conduction in semiconductors

· These electrons are the ones that jumped in the conduction band.

· These holes are the ones that are created in the valence band.

What charges do holes have? Can they move? How fast do they move compared with electrons? In which direction do they move compared to electrons?

Electrons have a negative charge. Holes have a positive charge.

Electrons and holes are not static they can move. Holes move more slowly than electrons. When electrons move in one direction, holes move in the opposite direction.

What does it mean to say that an electron or hole is ‘free’?

A solitary electron in the presence of a solitary hole will recombine.

Only electrons and holes that are free, and hence have not recombined, play a role in electrical conduction.

What are two ways that an electron-hole pair can be created? What is the phenomenon called caused by light?

· Temperature can give this little extra energy to an electron and make it jump to the conduction band thus creating a hole in the valence band.

· Light can also give this energy boost and create what scientists call an electron-hole pair a free hole and a free electron this phenomenon is called absorption.

What is photoconductivity?

Photoconductivity is the increase of current in a semiconductor due to the absorption of photons.

What causes photoconductivity?

Light has a dual nature it behaves as a wave and a particle.

The particle associated with light is called a photon. Photons can have different energies.

When light illuminates a semiconductor

· The photons with the right energy are absorbed by the material.

· The electrons from the valence band have enough energy to jump to the conduction band.

· The conductivity increases due to the higher number of electrons in the conduction band.

The movies show photons of red light failing to cause photoconductivity while those of blue light do cause it. How is this explained in terms of the band gap energy required by electrons and Planck’s equation (E=hf)?

The electrons need a minimum energy to jump to the conduction band. This minimum energy is the energy gap between the valence band and the conduction band. Photons with energy greater than the energy bandgap of the material will be absorbed.

What is electroluminescence?

Electroluminescence is the conversion of electrical energy into light.

How does electroluminescence work?

Let’s consider electrons in the conduction band. These electrons are in an excited state they have gained some energy to jump to the conduction band.

Such electrons eventually fall into the valence band in a lower energy state

· They release the extra energy that they have.

· This energy is emitted as a photon

Why is the resulting light called incoherent?

Photons emitted by electroluminescence come out in random directions this type of light is called incoherent light.

How is stimulated emission different from electroluminescence?

Stimulated emission is a bit like electroluminescence except that it is not a spontaneous process the excited electron is forced into jumping back to the valence band and emitting a photon.

What does it mean to say that electrons are in an ‘excited state’?

Let’s suppose that we sent photons

· These photons stimulate the electrons in the conduction band.

· The electrons fall back into the valence band.

What stimulates the emission of photons?

Photons emitted by stimulated emission have the same energy as the incident photons and are in step with each other this type of light is called coherent light.

What does it mean to say that the light is coherent? What sort of device causes light amplification by stimulated emission of radiation?

Stimulated emission and electroluminescence convert information and energy from an electronic to an optical form.

However in the case of stimulated emission, we have a coherent light emitting device. Or a LASER

Changing the Properties of Semiconductors - DOPING



This section is about creating p and n type semiconductors by doping elements like germanium and silicon with other elements like phosphorus and boron.

An n-type semiconductor has electrons in the conduction band as the majority carriers of charge. The ‘n’ type comes from the fact that electrons are negative.

A p-type semiconductor has holes in the valence band as the majority carriers of charge. The ‘p’ comes from the fact that holes are opposite of electrons and can be thought of as the equivalent of positive charge.

What is doping?

In material sciences, doping refers to the introduction of foreign impurity atoms in a material in order to change or alter its performance.

Doping is very important and extensively used in the semiconductor industry to change the electrical properties of a semiconductor.

How many valence electrons does silicon have and what shell are they in?

Atomic number 14

It has three shells in the first, 8 in the second and 4 in the third.

What bonding is present in the lattice of silicon?

Silicon atoms are bound to each other by what is called a covalent bond.

The silicon atoms co-operate and lend each other their outer electrons in order to have eight in the outer shell, which is a stable configuration.

How do you dope silicon to get a p-type semiconductor?

With boron.

How doe you dope silicon to get an n-type semiconductor?

With phosphorus.

Compared to silicon how many valence electrons does phosphorus have?

Atomic number 15

Compared to silicon how many valence electrons does boron have?

Atomic number 5

How is it that, if silicon is doped with phosphorus, you have an extra electron which is free to move within the crystal? I.e. How has the phosphorus created a donor site?

Replace the central silicon atom in lattice with a phosphorus atom. 4 out of the 5 outer electrons are the phosphorus are shared with the other silicon atoms. However, we have an extra electron which is free to move in the crystal.

Phosphorus has created a donor site because it is able to donate one of its outer electrons.

How does doping silicon with boron create an acceptor site (a hole)?

Replace the central silicon atom by a boron atom. The outer electrons of the boron are shared with the other silicon atoms. However, it is one electron short.

Boron has created an acceptor site because it is ready to accept an electron to complete the bond.

Define an n-type semiconductor.

In the case of silicon

· If we have a lot of phosphorus in a silicon crystal

o We have a lot of free electrons.

· If we have a lot of boron in a silicon crystal

o We are short of electrons or in other words, we have a lot of free holes.

When the majority carriers are electrons, the semiconductor is said to be of n-type.

Define a p-type semiconductor.

When the majority carriers are holes, the semiconductors are said to be of p-type.

http//www.vosislab.usyd.edu.au/photonics/devices/index.html

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pros and corns of coputer

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I srongly believe that computer has a great role in todays education.it helps studens in many ways.

Computers saves a lot of time rather than wasting on writing and storing that.Iformations are readily accessible as it can be stored in datas.The websites helps in finding out vast variety of material for researches rather than searching in libraries for material.

Some courss like computer aplication cannot be taught without the computer.Distance learning is also possible throuh computers .Its an excellent Type writer.

But it has got some bad effects on children. bad typing spped may spoil time.Children get addicted to it then they may forget their other work and become socially withdrawn,.They may develop some medical problems such as vision problem,Backache due to over use.Also they watch unsensored programme which is not healthy.It also decreases the cognitive and the mental development of the child. The practical aspects of the study is totally not possible eg chemistry experiments , dissections.So children should be given practical experiences also. More over they should be closely observed while on coputers.they should be given proper gidance.Vision shoul be checke devery yearly.Children can be in a fatasy world so they should be made aware of the fact that it;s not real.Children should be encouraged with study material than the games.It is not allways possible due to the lack of enoug telephone lines. The connections are too expensive. Some schools doesnt have good wiring system and lack good supporting system if theh system gets down. so these all thing to be managed more over there should be enoug teachesrs who are trained




In conclusion it has both positive and neative values

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Effective Communication

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This lesson will cover what effective communication

is, the key elements of the process, the various ways we communicate,

and a brief look at barriers that get in the way.

Examples of internal




barriers are fatigue, poor listening skills, attitude toward the sender or

the information, lack of interest in the message, fear, mistrust, past experiences,

negative attitude, problems at home, lack of common experiences,

and emotions. Examples of external barriers include noise,

distractions, e-mail not working, bad phone connections, time of day,

sender used too many technical words for the audience, and environment.

Barriers keep the message from getting through.When communicating,

watch out for barriers. Monitor the actions of the receiver.Watch

her body language; check to make sure the message the receiver received

is the one sent�ask questions and listen.

Here are the various components of the communication process in detail.

• Input. The sender has an intention to communicate with another person. This intention makes up the content of the message.

• Sender. The sender encodes the message, e.g. the idea of piece of furniture to sit on = . Thus he gives expression to the content.

• Channel. The message is sent via a channel, which can be made of a variety of materials. In acoustic communication it consists of air, in written communication of paper or other writing materials.

• Noise. The channel is subjected to various sources of noise. One example is telephone communication, where numerous secondary sounds are audible. Even a solid channel such as paper can be crushed or stained. Such phenomena are also noise in the communicative sense.

• Receiver. The receiver decodes the incoming message, or expression. He translates it and thus receives the

• Output. This is the content decoded by the receiver.

• Code. In the process, the relevance of a code becomes obvious The codes of the sender and receiver must have at least a certain set in common in order to make communication work

᠃Sender The person who initiates the dialogue.

Attempts to find words, phrases and non-verbal

language that is mutually understood by the receiver.

This process is called encoding.

᠃Receiver The person who attempts to translate the

sender’s message into meaningful information. This

process is called decoding.

᠃Message Has many forms, including written,

electronic, verbal and non-verbal. The best form to

use depends on many variables including importance,

urgency, and goal of the communication.

᠃Reverses the process. Receiver of the original

message becomes the sender. Feedback may

have to be solicited from the receiver.

᠃Confirms the message was heard and understood.

In most cases, the greater the feedback, the more

effective the communication!

᠃This process creates a feedback loop of

messages passed back and forth between the

sender and the receiver.



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A life of Richard Corey's companion

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Richard Cory Monologue

(Jane Pemberly, newly appointed headmistress of the local orphanage, talking to God about the recently deceased Richard Cory. Experiencing somewhat wild mood swings.)

Why did I have to become the headmistress now? Did I do something wrong…I didn’t, I don’t deserve this. Blast that Mr. Cory, oh but the poor boy, I can’t but help feel sorry for him even though there must have been something wicked about him. I have no clue how I’m going to keep this orphanage running without his generous donations. Stupid, foolish boy, how am I going to explain this to the children? I just don’t know, I JUST DON’T KNOW ANYMORE! I’ve just got to find something to do. I must keep myself from thinking these horrible thoughts about the late Richard Cory; he did help us a lot in his day.

He must have been exceedingly troubled to go and do such a horrid thing and to himself no less. I can’t for the life of me imagine what was wrong with him, he always seemed such a stable, kindhearted man. Although over his last few weeks I had noticed that he looked a little lonelier that I’ve ever seen him look. Yet, he still managed to be as polite and well mannered as ever.




Oh dear…I wonder if I should have done more. If I should have approached him and asked if he wanted a friend. However he most likely would have declined, he doesn’t seem the type to go accepting help or charity from anyone and most definitely not me. All the better I say; I’ve seen the kind of company he keeps. All those scandalous people and parties. I guess it’s better that the children have not been exposed to that.

Sigh. I just don’t know what to think of Mr. Cory. When he’s alone he’s the very essence of politeness and courtesy, but when he is with company he is most definitely the most vulgar and disobliging person I have ever met.

Having said that I can only hope that whatever demon was tormenting his soul has been evicted from his heart so that he may now rest in peace.

Amen.

By Ellen Sinclair

Teacher Robert

Subject Drama

Due Date October 7, 00



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