Semiconductors

Semi conductors

• A semiconductor is a material whose

resistance is between that of a

conductor and an insulator. Eg

Silicon

Intrinsic SemiconductorIntrinsic conduction is conduction in a pure

semiconductor.

Extrinsic Semiconductors

p-typen-type

Pb

Extrinsic conduction is conduction in a doped semiconductor.

Electrons are the majority charge carriers.

Holes are the majority charge carriers.

Doping• The addition of a small amount of impurity to a

semiconductor to improve its conductivity.

• Doping produces two types of semiconductor…n-type and

p-type

• Electrons are the majority charge carriers in n-type material

• Holes are the majority charge carriers in p-type material

Doping

Factors that affect conduction in semiconductors

• Temperature - Conductivity increases with temperature as the heat energy liberates electrons e.g. the thermistor

• Light - Conductivity increases with light intensity e.g. the LDR light dependent resistor

PN Diode

Structure

Symbol

Depletion layer

The Diode

• When p-type and n-type materials are placed in contact current will flow in one direction but not in the other

• Because of the charge movement at the junction, the p-type develops a negative charge and the n-type a positive charge A voltage is then created called the junction voltage.

• The depletion layer is the area between the p and n type material where there are no free charge carriers.

PN Diode

Structure

Symbol

Forward Bias

P Type N Type

Reverse Bias

P TypeN Type

A

B

Forward or reverse bias?

Half Wave Rectifier

Full Wave Rectifier

Light Emitting Diode

Used to indicate that power is on in electrical devices.

Uses less power than a bulb and does not heat up.

Light Dependant Resistor LDR

Used in automatic street lights

Answer the following questions

• Define the following:– Semiconductor– Doping– Extrinsic conduction– Intrinsic conduction– Diode– Depletion layer

• Draw and label the structure of a diode.• Draw a diagram to show the semi conductor

diode in reverse bias and forward bias.