15293_conductors,insulators & semiconductors

8/3/2019 15293_Conductors,Insulators & Semiconductors

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CONDUCTORS,

INSULATORS&

SEMICONDUCTORS


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CONDUCTORS: A substance or object that allowselectricity to flow through it with low resistance.

For example: Silver, Copper ,Gold, Iron etc.

INSULATORS: A substance or object that does not

conduct electricity.For example: Glass, Rubber, Oil, Ceramic etc.

SEMICONDUCTORS: A substance or object with

conductive properties between those of a conductorand an insulator.

For example: Germanium, Silicon, Gallium

arsenideetc.


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Classes of Materials

Good electrical conductors contain a high densityof free charge carriers.

The density of free charge carriers in an insulator isnearly zero.

Semiconductors are materials with a chargedensity between those of insulators and conductors.


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Metals

To be a good conductor, the charge carriers in amaterial must be free to move in response to anelectric field.

The motion of electrons in response to an electricfield represents an increase in the energy of thesystem.

When an electric field is applied to a conductor, theelectrons move up to an available higher energystate.


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Metals Energy Band


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Insulators

Although an insulator has many vacant states in theconduction band, these states are separated from

the filled band by a large energy gap.

Only a few electrons can occupy the higher states,so the overall electrical conductivity is very small.


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Insulators Energy Band


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Semiconductors

The band structure of a semiconductor is like that of

an insulator but with a smaller energy gap.

Charge carriers in a semiconductor can benegative, positive or both.


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Semiconductors - Energy Band


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Movement of Charges


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Intrinsic Semiconductors

A pure semiconductor material containing only oneelement is called an intrinsic semiconductor.

It will have equal numbers of conduction electronsand holes.

Such combinations of charges are called electron-

hole pairs.


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Doping in Semiconductors

The process of adding impurities to a semiconductoris called doping.

Doping results in:

The modification of the band structure of asemiconductor.

The modification of its resistivity.

Doping is very important in controlling theconductivity of semiconductors.


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n-typeSemiconductors

The energy level ofthe extra electron is

just below theconduction band.

The electron of thedonor atom can moveinto the conductionband as a result of asmall amount ofenergy.


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p-typeSemiconductors

The energy level ofthe hole is just abovethe valence band.

An electron from thevalence band can fillthe hole with an

addition of a smallamount of energy.


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Extrinsic Semiconductors

When conduction in a semiconductor is the resultof acceptor or donor impurities, the material is

called an extrinsic semiconductor.

For these semiconductors, doping densities rangefrom 1013 to1019 cm-3.

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15293_conductors,insulators & semiconductors

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