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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