Semiconductors

Semiconductor

is a material whose resistivity is between that of a good conductor and a good insulator.

Examples of materials which are semiconductors are Silicon and Germanium.

What happens when the temperature of a semiconductor increases?

The resistance of a semiconductor decreases as its temperature increases

Intrinsic Conduction

is the movement of charges through a pure semiconductor due to electrons moving from negative to positive and equal number of holes moving in the opposite direction

Thermistor

is an electrical component whose resistance decreases rapidly with increasing temperature

Insulator

is a medium that does not allow electric charge to flow through it easily

Cold Silicon

Has no free electron, cannot conduct electricity and therefore behaves like an insulator

A Light Dependant Resistor (LDR)

is an electrical component whose resistance decreases rapidly when light shines on it.

Doping

 is the addition of a small amount of atoms of another element to a pure semiconductor to increase its conductivity

Extrinsic Conduction

is the movement of charges through a doped semiconductor.

An n-type semiconductor

is a semiconductor in which electrons are the majority charge carriers.

In this case there is one ‘extra’ electron due to the addition of the Phosphoros atom.

A p-type semiconductor

is a semiconductor in which holes are the majority charge carriers.

• In this case there is one ‘missing’ electron due to the addition of the Boron atom. This is equivalent to a ‘Positive Hole’ which moves in the opposite direction to an electron.

The p-n junction

When a piece of p-type semiconductor is joined to a piece of n-type semiconductor, the junction between the two is known as a p-n junction. 

The Depletion Region

Forms where p-type and n-type semiconductors meet. Due to thermal motion, electrons from the n-type side diffuse into the p-type side and recombine with holes, while holes from the p-type side diffuse into the n-type side and recombine with electrons. This removes free charge carriers near the junction, leaving a region with no mobile charges. As a result, the depletion region acts as an insulator and is essential to how semiconductor devices function. Moving charge across the depletion region requires energy, creating a potential difference across the junction. This is called the junction voltage, typically about 0.1 V for germanium diodes and 0.6 V for silicon diodes.

Forward-Biased PN Junction

When the positive terminal is connected to the p-type and the negative terminal to the n-type, electrons and holes are pushed toward the junction. This reduces the width of the depletion region. If the applied voltage exceeds the junction voltage, the depletion region breaks down and current flows, allowing the junction to conduct.

Reverse-Biased PN Junction

When the positive terminal is connected to the n-type and the negative terminal to the p-type, electrons and holes are pulled away from the junction. This increases the width of the depletion region, preventing charge carriers from crossing it, so the junction acts as an insulator.

Applications of the PN Junction

The PN junction is the basis of the semiconductor diode, used in many electronic devices such as LEDs, computers, and integrated circuits. It is also used for rectification, converting AC (alternating current) into DC (direct current).

A (semiconductor) diode

will only allow current to flow in one direction.