SEMICONDUCTORS,CONDUCTORSAND INSULATORS

All materials are made up of atoms. These atoms contribute to the electrical properties of a material, including its ability to conduct

current.

CONDUCTORS• A conductor is a material that easily conducts electrical current. The best conductors are single element materials such as Copper, Silver, Gold, Aluminium & nickel

• Alloys are also good conductors:– Brass & steel

• Good conductors can also be liquid:– Salt water

These elements are charachterized by atoms with only one valence electron very loosely bound to the atom.

These loosely bound valence electrons can easily break away from their atoms and become free electrons. So a conductive material has many free electons that,when moving in the same direction make up the current.

INSULATORS

An insulator is a material that do not conduct electrical current under normal conditions. Most good insulators are compounds rather than single element

materials.

Valence electrons are very tightly bound to the atoms therefore, there are very few free electrons in an insulator.

SEMICONDUCTORS

A semiconductor is a material that is in between conductor and insulator in its ability to conduct electric current.

A semiconductor in its pure intrinsic state is neither a good conductor nor a good insulator.

The most common single element semiconductors are • Silicon• Germanium • Carbon

The single element semiconductors are charachterized by atoms with four valence electrons.

Uses of semiconductors

Semiconductors are the foundation of modern electronics, including radio, computers, and telephones. Semiconductor-based electronic components include transistors, solar cells, many kinds of diodes including the light-emitting diode (LED), the silicon controlled

rectifier, photo-diodes, and digital and analog integrated circuits.

ENERGY BANDS

Recall that the valence shell of an atom represent the band of energy levels and when an atom acquires additional energyit can leave the valence shell, become a free electron, and exist

in what is known as conduction band.

The differnce in energy between the valence band and the conduction band is called an Energy Gap.

This is the amount of energy that a valence electron must have in order to jump from the valence band to the conduction band.

The following figure shows energy diagrams for insulators, semiconductors, and conductors.

In part (a) of the fig the insulators shows a wide energy gap. Valence electrons do not jump in to the conduction band except under break down conditions.

In part (b) semiconductors have a much narrower energy gap that permits some electrons to jump in to the conduction band and become free electrons.

In part (c) the energy band in conductors overlap.Therefore in conductive materials there is always a large number of free electrons.

CONDUCTION IN SEMICONDUCTORS

CONDUCTION ELECTRONS AND HOLES : An intrinsic (pure) silicon crystal at room temp has sufficient energy for some valence

electrons to jump the gap from the valence band in to the conduction band, becoming free electrons.

Free electrons are also called Conduction Electrons.

When an electron jumps to the conduction band a vacancy is left in the valence band within the crystal.

This vacancy is called a Hole.

When an electron is raised to the conduction band there is one hole left in the valence band creating what is known as Electron Hole pair

The number of holes are equal to the number of electrons which jump in to the conduction band.

EXTRINSIC MATERIAL

• The characteristic of semiconductor can be altered by adding impurity through doping process (extrinsic material)

• Two type:– N-type– P-type

N-TYPE• N-type is created by introducing

impurity elements that have five valence electrons (pentavalent) – antimony, arsenic, phosphorus

• Adding arsenic (doping) will allow four of the arsenic valence electrons to bond with the neighboring silicon atoms.

• The one electron left over for each arsenic atom becomes available to conduct current flow.

• Diffused impurities with five valence electrons are called donor atoms

Figure: Arsenic impurity in n-type material

P-TYPE• P-type is created by doping

with impurity atoms having three valence electrons – boron gallium, indium

• The 3 electrons in the outer orbit do form covalent bonds with its neighboring semiconductor atoms.

• This hole is ready to accept a free electron

• The diffused impurities with three valence electrons are called acceptor atoms.

Figure 1.11 Boron impurity in p-type material

Electron versus Hole Flow

Semiconductor Diode• Diode is formed by bringing these two material together p- and

n-type• Electrons and holes at joined region will combine, resulting in a

lack of carriers in the region near the junction (depletion region)

Current Flow in N-type Semiconductors

• The DC voltage source has a positive terminal that attracts the free electrons in the semiconductor and pulls them away from their atoms leaving the atoms charged positively.

• Electrons from the negative terminal of the supply enter the semiconductor material and are attracted by the positive charge of the atoms missing one of their electrons.

Current Flow in P-type Semiconductors

• Electrons from the negative supply terminal are attracted to the positive holes and fill them.

• The positive terminal of the supply pulls the electrons from the holes leaving the holes to attract more electrons.

Figure 1-16 Reverse-biased p-n junction

Figure 1.18 Forward-biased p-n junction

Figure 1.19 Silicon semiconductor diode characteristics