Semi conductor

Prepared By: Mohammed Qashlan

Energy Band In any material, there are 2 energy band:

1. Valence band : the outermost shell that determines the conductivity

2. Conduction band : the band outside the valence shell.

The 2 bands are separated by one energy gap called – forbidden gap. The valence band contains with electrons.

The electrons can move to the conduction band if it have enough energy ( eg: light or heat).

When the electron absorbs enough energy to jump from valence band to the conduction band, the electron is said to be in excited state.

The concept of energy bands is particularly important in classifying materials as conductors, semiconductors, and insulators.

• Semiconductor : has a smaller forbidden band and requires less energy to move an electron from the valence band to the conduction band.

• Therefore, for a certain amount of applied voltage, more current will flow in the semiconductor than in the insulator.

semiconducting elements:

– low electrical conductivity at room temperature

– Electrical conductivity increases with temp.

Gap between valence and conduction band is intermediate in size.

Semiconducting elements form the basis of solid state electronic devices.

Metalloids (such as silicon or germanium) are semiconducting elements whose electrical conductivity increases as temperature increases.

A striking property of these elements is that their conductivities increase markedly when they are doped with small quantities of other elements.

Made from materials that have four valence electrons in their outer orbitals.

Germanium and silicon are the most common.

Silicon is preferred due to its ability to withstand heat.

A pure semiconductor material such as silicon or germanium has no special properties and will make a

poor conductive material.

When silicon is doped with phosphorus, it becomes an n-type

semiconductor, in which electrical current is carried by negatively charged electrons.

When silicon is doped with boron, it becomes a p-type

semiconductor, in which an electrical current is carried by positively charged holes.

Joining a p-type semiconductor to an n-type semiconductor produces a p-n junction, which can function as a rectifier.

A rectifier is a device that allows current to flow in one direction, but not the other.

Types of Semiconductor: Semiconductors are mainly classified into two Semiconductors are mainly classified into two

categories: categories:

ii. . IntrinsicIntrinsic

ii. Extrinsicii. Extrinsic

i.i. IntrinsicIntrinsic : chemically very pure and possesses : chemically very pure and possesses poor conductivity. poor conductivity.- It has equal numbers of negative It has equal numbers of negative carriers (electrons) and positive carriers (electrons) and positive carriers (holes).carriers (holes). - Impurities do not affect its electrical Impurities do not affect its electrical behavior. behavior.

Intrinsic SemiconductorSilicon has 4 outer shell valence electrons

Forms into a lattice structure to share electrons

The pure semiconductor material without impurities atoms.example: Silicon and Germanium

Extrinsic semiconductor :

improved intrinsic semiconductor with a small amount of impurities added by a process, known as doping process, which alters the electrical properties of the semiconductor and improves its conductivity.

Introducing impurities into the semiconductor materials (doping process) can control their conductivity.

Adding impurities atom into intrinsic semiconductor = extrinsic semiconductor.

The process of adding specific types of atoms to a semiconductor to favorably alter electric characteristics – Doping

2 types of extrinsic (impure) semiconductor;

N-type P-type

When an impurity increases the number of free electrons, the doped semiconductor is negative or n-type.

An impurity that reduces the number of free electrons, causing more holes, creates a positive or p-type semiconductor.

Doping

Doping : Adding impurities to the silicon crystal lattice to increase the number of carriers.

Add a small number of atoms to increase either the number of electrons or holes.

Donors n-Type MaterialDonors

-Add atoms with 5 valence-band electrons

-ex. Phosphorous (P)

-“Donates” an extra e- that can freely travel around

-Leaves behind a positively charged nucleus (cannot move)

-Overall, the crystal is still electrically neutral

-Called “n-type” material (added negative carriers)

+

N– type materialN– type material

Antimony (Sb) impurity in n-type material

- Diffused impurities with 5 valence electrons are called donor atoms.

Acceptors Make p-Type Material

––

h+

Acceptors• Add atoms with only 3 valence-

band electrons

• ex. Boron (B)

• “Accepts” e– and provides extra h+ to freely travel around

• Leaves behind a negatively charged nucleus (cannot move)

• Overall, the crystal is still electrically neutral

• Called “p-type” silicon (added positive carriers)

P-type materialP-type material

Boron (B) impurity in p-type material

- The diffused impurities with 3 valence electrons are called acceptor

atoms.

PN Junction Formation A PN junction is fabricated from a single slice of

semiconductor. One side doped with acceptor impurity atoms – p region One side doped with donor impurity atoms – n region The interface separating the n and p regions is referred

as the metallurgical junction.

The PN junction

A p-n junction as a rectifier.

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Semiconductor Properties

For T > 0K

Electron shaken free and can cause current to flow

e–h+

-Generation – Creation of an electron (e-) and hole (h+) pair.-h+ is simply a missing electron, which leaves an excess positive charge (due to an extra proton).-Recombination – if an e- and an h+ come in contact, they annihilate each other-Electrons and holes are called “carriers”. because they are charged particles – when they move, they carry current.-Therefore, semiconductors can conduct electricity for T > 0K … but not much current (at room temperature (300K), pure silicon has only 1 free electron per 3 trillion atoms).