Semiconductor Materials

Semiconductor Materials 

Definition:

A material that is neither a good conductor of electricity nor a good insulator, but has properties of electrical conductivity somewhere between the two.

Semiconductor is a material that is neither a good conductor or a good insulator 

but that conducts more electricity when heat, light or voltage is added.

Semiconductors are employed in the manufacture of various kinds of electronic 

devices, including diodes, transistors, and integrated circuits. Such devices have 

found wide application because of their compactness, reliability, power efficiency, 

and low cost.

Elements of semi conductor materials:

The elemental semiconductors are those composed of single species of atoms, such as silicon (Si), germanium(Ge) and Tin(Sn) in column IV and selenium (Se) and tellurium (Te) in column VI of the periodic table.

There are, however, numerous compound

Semiconductors, which are composed of two or more elements. 

•Gallium arsenide (GaAs)

• cadmium selenide(CdSe)

Electrical Nature:

• Semiconductors in their natural state are poor conductors because a currentrequires the flow of electrons, and semiconductors have their valence bands filled, preventing the entire flow of new electrons.

• Several developed techniques allow semiconducting materials to behave like

conducting materials, such as doping or gating.

• These modifications have two outcomes: n-type and p-type. 

These refer to the excess or shortage of electrons respectively. An unbalanced number of electrons

would cause a current to flow through the material.

Band structure, Energy Gap:

• From the band theory of solids we see that semiconductors have a band gap between

the valence and conduction bands. The size of the band gap has implications for the

types of applications that can be made. A low band gap implies higher intrinsic

conduction, and a high band gap implies a larger possible photon energy associated

with a transition across the gap in light emitting diodes.

Energy Gap:

• The gap between the valence band and the conduction band is referred to as forbidden

gap.

• As the name suggests, the forbidden gap doesn’t have any energy and no electrons stay

in this band. If the forbidden energy gap is greater, then the valence band electrons are

tightly bound or firmly attached to the nucleus. We require some amount of external

energy that is equal to the forbidden energy gap.

Formation of electron and hole:

An electron hole is one of the two types of charge carriers that are responsible for

creating electric current in semiconducting materials. A hole can be seen as the "opposite" of an electron. Unlike an electron which has a negative charge, holes have a positive charge that is equal in magnitude but opposite in polarity to the charge an electron has.

Holes can sometimes be confusing as they are not physical particles in the way

that electrons are, rather they are the absence of an electron in an atom. Holes can move from atom to atom in semiconducting materials as electrons leave their positions.

An analogy may be helpful. Imagine people standing in a line, on a set of steps. If

the person at the front of the line goes up one step, that person leaves a hole. As everyone steps up one step the available step (the hole) moves down the steps.

•Holes are formed when electrons in atoms move out of the valence band (the outermost shell of the atom that is completely filled with electrons) into the conduction band (the area in an atom where electrons can escape easily), which happens everywhere in a semiconductor.

• Both electrons and holes are vital to the creation of current in semiconductors.

Under the influence of some external voltage, both electrons and holes can move through a semiconducting material.

Electrical conduction in semi conductors:

A semiconductor has two types of charge carriers i.e. holes and electrons.The cause of electrical conduction in semiconductors is due to the movement of the holes in the valence band and the movement of the electrons in the conduction band.When electric field is applied then as a result the electrons will start moving in the conduction band in the direction opposite to that of the electric field. The current produced at that time is known as hole current.

In other words we can say that two types of currents are mainly working in the semiconductor. One is the hole current that is due to the movement of the holes in the

valence band and other is the electronic current which is caused by the movement of

the electrons in the conduction band. The total resultant current in the semi conductor is the sum of the both the currents.

The conductivity of a semiconductor is sum of conductivities of holes and electrons

i.e. σ = eni(μh +μe )

where,

eni= intrinsic concentration(no. of electrons in conduction band or holes in

valence band)

 μh = mobility of holes

 μe = mobility of electrons

Therefore, electrical conduction in a semiconductor is due to holes and electrons both.

Intrinsic and Extrinsic Semiconductor:

The pure form of the semiconductor is known as the intrinsic semiconductor and the semiconductor in which intentionally impurities is added for making it conductive is known as the extrinsic semiconductor.

The conductivity of the intrinsic semiconductor becomes zero at room temperature while the extrinsic semiconductor is very less conductive at room temperature.

Intrinsic semiconductor

Intrinsic semiconductor definition is, a semiconductor that is extremely pure is an

intrinsic type. On the energy band concept, the conductivity of this semiconductor will become zero at room temperature.Examples are Si & Ge.

In the above energy band diagram, the conduction band is empty whereas the valence band is filled totally. Once the temperature is increased, some heat energy can be supplied to it. So the electrons from the valence band are supplied toward the conduction band by leaving the valence band.

The flow of electrons while reaching from valence to the conduction band will be random. The holes formed within the crystal can also flow anywhere freely. So, the behavior of this

semiconductor will show a negative TCR (temperature coefficient of resistance). The TCR means, when the temperature increases, the material’s resistivity will be decreased & the conductivity will be increased.

Extrinsic semiconductor

To make a semiconductor like conductive, then some impurities are added which is called extrinsic semiconductor. At room temperature, this kind of semiconductor will conduct a small current; however, it is not helpful in making a variety of electronic devices. Therefore, to make the semiconductor conductive, a little quantity of appropriate impurity can be added to the material through the doping process.

Doping:

• The process by which an impurity is added to a semiconductor is known as Doping. The amount and type of impurity which is to be added to the material have to be closely controlled during the preparation of extrinsic semiconductor.

•Generally, one impurity atom is added to 108

 atoms of a semiconductor.

• The purpose of adding impurity in the semiconductor crystal is to increase the

number of free electrons or holes to make it conductive.

 If a Pentavalent impurity, having five valence electrons is added to a pure semiconductor a large number of free electrons will exist.

If a trivalent impurity having three valence electrons is added, a large number of

holes will exist in the semiconductor.

Depending upon the type of impurity added the extrinsic semiconductor may be

classified as n type semiconductor and p type semiconductor.

N type semiconductor:

When a small amount of Pentavalent impurity is added to a pure semiconductor providing a large number of free electrons in it, the extrinsic semiconductor thus formed is known as n-Type Semiconductor. The conduction in the n-type

semiconductor is because of the free electrons denoted by the pentavalent impurity atoms.

These electrons are the excess free electrons with regards to the number of free electrons required to fill the covalent bonds in the semiconductors.

In n-type semiconductors, electrons are the majority carriers and holes are the minority carriers.

The following points are important in case of the n-type semiconductor:

• The addition of Pentavalent impurity results in a large number of free electrons.

•When thermal energy at room temperature is imparted to the semiconductor, a hole-electron pair is generated and as a result, a minute quantity of free electrons are available. These electrons leave behind holes in the valence band.

•Here n stands for negative material as the number of free electrons provided by

the Pentavalent impurity is greater than the number of holes.

P type semi conductor:

The extrinsic p-Type Semiconductor is formed when a trivalent impurity is added to a pure semiconductor in a small amount, and as a result, a large number of holes are created in it. A large number of holes are provided in the semiconductor material by the addition of trivalent impurities like Gallium and Indium.

In p-type semiconductors, holes are the majority carriers and electrons are the

minority carriers.

Difference Between Intrinsic Semiconductor & Extrinsic semiconductor