Semiconductor in Electronics: Electron Theory, Valence Band, Conduction Band

(Last Updated On: October 11, 2020)

Semiconductor in Electronics:

25 years ago vacuum tubes were used all over world. Vacuum tubes are heavy and large in size, while semiconductors are light and small in size. The construction of vacuum tubes is complicated, while the construction semiconductor is simple. There is no need of a heater in semiconductor for this emission of electrons from cathode. Semiconductors are cheaper as compared to the vacuums tube. Semiconductors are durable while vacuum tubes are not. Semiconductors have best efficiency than vacuum tubes.

Due to these facts vacuum tubes are replaced with semiconductors. But inspite of these, vacuum tubes are still used in some places, like they are used in picture tubes, they are also used in computer technology. They are also used in different industries and are also widely used in space technology and war technology. Before knowing the semiconductor diode and integrated circuits (IC’s), first we must know about the electron theory and semiconductor theory.



Electron theory:

Anything occupies space and have a mass is called matter. Matter can be found in three states.

  1. Solid
  2. Liquid
  3. Gas

In gas, the atoms or molecules are comparatively at large distance. In liquid state of the atoms or molecules are close than the gas molecules or atoms. While in solid state the atoms or molecules are closest.

Anything in this universe is made up of matter, and matter is composed molecules, while molecules are composed of millions of atoms.

The substances whose molecules are made up of same atoms are called elements.118 elements have been discovered till now. Like oxygen, iron, silver, copper, etc.

The substances whose molecules are composed of different atoms are called compounds. Compounds are uncountable.

As I have told earlier that molecules are composed of small particles called atoms. While atom further consists of small particles called Electrons, Protons, and Neutrons.

An atom has two parts:

(1) Inner part

(2) Outer part

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The inner part of atom is called as Nucleus. It consists of two particles protons and neutrons. Proton has (+) Positive charge on it while neutron has no charge.

The outer part of atom consists of electrons revolving in different orbits or shells. Electron is the lightest particle of atom. It is 1/1840 times lighter than proton or simply we can say proton in 1840 times heavier than electron. Electron has mass of 9.11X10-28  and have radius of 1.9X10-15 m .An electron carries a unit of negative charge that is -1.6X10-19.The electrons revolve in the circular orbits or elliptical.

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As the number of electrons and protons are equal and both have opposite charges. So they will balance each other, due to this fact electrons cannot go out of the outer part and also cannot get inside the inner part that we call Nucleus. So we can say that atom is electrically neutral.


Semiconductor theory:

Semi means in between or intermediate or partial. Materials are divided into three classes, namely

  1. Insulator
  2. Conductor
  3. Semiconductor

The materials are defined on the basis of resistivity.

Insulator:

The materials which produce maximum resistivity (106Ω-m) for the flow of current are called insulators.

Conductor:

The materials which produce minimum resistivity for the flow of current (10-6Ω-m) are called conductors.

Semiconductor:

The materials whose resistivity lies in between insulator and conductor are called Semiconductor (102Ω-m or cm).

Electronic industry depends on the semiconductor materials, and all electronic equipments are made up of semiconductor materials, commonly used semiconductor material is the silicon, although germanium, carbon, and calcium arsenide are also used. Silicon was discovered in 1823 and germanium was discovered in 1886.

Materials on the basis of electrons:

Note that the insulators, conductors, and semiconductors are defined in terms of the electrons having energies in the valence and conduction bands.

Insulators:

Insulators are those materials in which valence electrons are very tiedly bound to the concerned atom. Thus, requiring a strong electric field to remove them from the attraction of their Nucleus. Hence the materials having no free electrons are called insulators.

Conductors:

The materials having plenty of free electrons and due to these free electrons conduction will take place. For such materials the electrons can move easily from one atom to another. For example silver being the best one and copper being second best.

Semiconductors:

A semiconductor material is one whose electrical properties lie in between these insulators and conductors. For example silicon and germanium.

Electron shell and their structure:

We know that electrons revolve around the nucleus in orbits. These orbits are designated by alphabetical letters (K, L, M, N, O…) starting from the nucleus outward.

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The orbit closest to nucleus is called K shell. The next farther one is called L shell and so on. The maximum number of electrons a shell cam have is 2n2.

For example K shell is the first shell for which n=1 so the number of electrons for K shell will be

2n2=2

Similarly, for “L” shell the number of electrons will be “8” and so on.

Note that an atom can have a maximum of seven orbits. For their electronic Configuration, the electronic distribution in all atoms will be subjected to the following four rules.

  • The formula 2n2 is not valid for the outermost orbit.
  • The maximum number of electrons in the outermost orbit of an atom cannot exceed its electrons.
  • The shell just before outermost shell will not exceed 18 electrons.
  • In any shell the number of electrons cannot exceed 32.

Every atom have specific radius, for their first orbit that is K orbit, similarly, the second orbit that is “L” orbit. The radius of “L” orbit will be bigger than the radius of “K” orbit. No electron can revolve in between any two orbits. Every atom have the specific orbits called as the allowed orbits. The orbit in between the allowed orbits is called forbidden orbit.


Valence electrons: 

The number of electrons in the outermost shell are called valence electrons.

Stable Atom:

A stable atom of an element is that whose outermost shell is completely filled. That is it will have 8 electrons.

Unstable atom:

An unstable atom of an element is that whose outermost shell is incomplete. That is it will have less than 8 electrons.

The tendency of an atom to give up its valence electrons depends on the chemical stability. When an atom is stable then it resists to give electrons. If an atom is unstable, its tends to give up electrons. If an atom valence shell is more than half filled, then atom tends to fill its shell and if the atom having less than half filled, it tends to give up electrons. As atom is electrically neutral. If an atom gains extra electrons it will have a negative charge and will become negative ion which is called anion. If an atom lost electron it will become a positive ion and will called cation.

On the basis of number of valence electrons: conductors, insulators, and semiconductors.

Conductors:

The atom of an element, if having less than four electrons in their outermost shell are called conductors.

For example copper, aluminum, sodium, etc.

Insulator:

If an atom having more than four electrons in their outermost shell are called insulators.

For example Nitrogen, Sulfur, etc.

Semiconductors:

If an atom having four electrons in their outermost shell are called semiconductors.

For example Carbon, Silicon, and germanium.

Electronic structure of Copper, Aluminum, Na, Nitrogen, sulfur, Neon, Carbon, Silicon, and Germanium.

The atom of an element have their own atomic number (Z) due to this atomic number we know the number of protons. As the number of protons equal to the number of electrons.

Conductors:

Copper:    (Atomic Structure of CU)

Since atomic number represents the number of proton and CU atomic number=29. It is also the number of orbiting electrons which are distributed as:

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As the outermost shell have only one electron and the atom of an element having less than four electrons in the valence shell is called conductor.

Al = 13:

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As the outermost shell consists of 3 electrons and we know that the atom of an element having less than four electrons are called conductor. So, Al is an conductor.

Na: Atom number is 11.

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It is a conductor.

Insulators:

Nitrogen:

Its atomic number is 7.

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As the outermost shell consists of 5 electrons and as we know that the atoms of elements having more than 4 electrons in the outmost shell is called insulator.

Sulfur:

Atomic number = 16

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Sulfur is an insulator as it has more than 4 electrons in the outmost shell.

Ne: Atomic number = 10

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Semiconductors:

Carbon: Its atomic number is 6.

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Silicon: Its atomic number is 14

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Germanium: Its atomic number is 32.

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Silicon itself is an element and is most commonly found in rocks and sand. Sand is silicon dioxide(sio2).Germanium is recovered from the ash of certain coal. The oxide of both germanium and silicon are removed chemically to produce the element (Ge and Si) in the pure form. Then we will add the desired impurities to these pure semiconductors. Generally silicon is used for all most all semiconductors devices. Almost 90% of electronic equipments are made up of silicon.



Bound and Free electrons:

The electrons in the  inner shells of an atom do not normally leave the atom, because they are under the strong influence of nucleus. And these electrons are remained bound to the nucleus and are called as bound electrons.

And the electrons which are in the outermost shell are under less influence of nucleus. So they do not confined to the same atom. Some of these valence electrons move in random manner and they may travel from one atom to another atom. These electrons are called as free electrons. These free electrons causes for the conduction and conduction is the flow of electrons, and the flow of electrons is called current. The atom having less electrons (1 to 4) or less four in their outmost shell, require less energy for the dischargement (detaching) of these electrons. When an atom discharge its electrons, it will get +ve charge. these types of atoms are called +ve ions. If an atom gets electrons it will have –ve charge and these type of atoms are called –ve ions.

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Energy levels:

Every orbit has their own energy level. If an object is raised above earth we exert some force and that object will have its P.E at certain point. More we take the object to the height more we will use the force. The energy which is applied on the object will be stored in that object in the form of potential energy (P.E). When this object is released to the ground, then on striking the ground the stored energy will be released. Note that the object at more height will have more energy.

Now come to the electrons orbiting in their particular shells. In first shell that is K-shell, the electrons are orbiting very close to the nucleus, so they are tiedly bound to the nucleus and they poses very small amount of energy. The electrons which are at the greater distance will least bound to the nucleus and these electrons will have greater its total energy. By total energy we mean its P.E and K.E. The equivalent energy level of an electron of single isolated atom is shown as:

Isolated Atom: Isolated Atom is that atom which do not interact with other atoms.

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Atoms of gases can be considered isolated because of their greater spacing under normal pressure and temperature. Hence energy level of such atoms are not affected by other distant atoms.

Now consider the solid material for which atoms are very close to each other. If there are hundred atoms in a single piece of solid material. Then there will be hundred levels of slightly different energies and the hundred energy level diagram would be super imposed on each other.

Now see figure given above, the first orbit is called first energy level. The 2nd orbit is called 2nd energy level and the 3rd orbit is called 3rd energy level. The three orbits shown having radius R1, R2,  and R3 from nucleus outward respectively. Energy is required for bringing an electron from the first orbit to the 2nd orbit. If an electron move towards its upper orbit, it will gain potential energy to keep control on the energy of nucleus. If this electron is allow to come to its original orbit it will release the absorb energy. If an atom is given some energy from an external source, then the electron absorb energy and they jump to the higher energy level. Now it’s clear this valence electron will have maximum energy due to this they can take a part in chemical reaction and in bounding the atoms. An electron energy is expressed as electron volt = ev = 1.6X10-19J.


Energy Bands:

Energy Band is the combination of large number of energy levels.

These energy levels are very close to each other but they are separated by very very small distances.

As we know that electrons in the outermost shell are called valence electrons. They have maximum orbiting energy. Energy bands are of two types

  • Valence band
  • Conduction band

Valence band:

The energy band which occupied by valence electrons is called valence band. This may be filled completely or partially with electrons. But can never be empty. The electrons  which left the valence band are called conduction electrons. They partially leave the atom. The band occupied  by these electrons (conduction electrons) is called conduction band. This band lies next to the valence band. These two bands are separated by some gape as shown in the figure given below.

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The conduction band may either be empty or partially filled with electrons. In conduction band electrons move freely and conduct current through that material. The gape by which valence band and conduction band are separated is called forbidden energy gape. energy is required to left an electron from the valence band to the conduction band. The required energy will be used in crossing the forbidden energy gap.

 In terms of energy gap insulator, conductor and semiconductors are defined as:

Insulator in the term of energy:

Those materials which have full valence band and an empty conduction band. It have large energy gap (5ev or more) between valence band and conduction band.

Conductors:

Conductors are defined as those material which have over-lapping valence band and conduction band. In fact there is no physical distinction b/w the two bands and there is no forbidden energy gap.

Semiconductors:

Semiconductors are designed as those materials which at room temperature partially filled valance band and partially filled conduction band. Semiconductor have very narrow forbidden energy gap (1ev).

For silicon, eg=1.2ev and for germanium eg=0.72ev

Eg=energy gap.

Note that at 0K(absolute zero temperature)  no electrons are there in the conduction band, and the valence band will be completely filled. Hence at 0k (absolute zero temperature) A piece of Ge or Si acts like a perfect insulator. However with increase in temperature width of the forbidden energy gap will decrease and some electrons are then liberated into the conduction band. Hence it is concluded that conductivity of Semiconductors increases as the temperature increases.

Filled Band:

(The very first band)

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The bands filled by electrons are called filled band. No electrons in a filled band can move because they are tiedly bound to the nucleus. Hence electrons in the filled band cannot contribute to the conduction.

Consider the case of silicon (semiconductor) having atomic number 14 its electronic configuration is given as

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As K-shell and L-shell are completely filled so they are known as filled bands.

The energy band diagram is given as:

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first two shells are completely filled and are called as filled bands and the third band is incomplete and it’s known as valence band. This band will take part in conduction.

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Breakdown of an Insulator:

An insulator does not conduct at room temperature, because there are no conduction electrons in it. It will conduct when temperature is raised or if high voltage is applied. This conduction through an insulator is called breakdown of an Insulator.

 

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My name is Shahzada Fahad and I am an Electrical Engineer. I have been doing Job in UAE as a site engineer in an Electrical Construction Company. Currently, I am running my own YouTube channel "Electronic Clinic", and managing this Website. My Hobbies are * Watching Movies * Music * Martial Arts * Photography * Travelling * Make Sketches and so on...

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