Semiconductors & P-N Junctions

Do energy levels in different atoms have different energies?

Yes.

Many more slightly different energy levels come into existence to create…

A band of permitted energy levels.

What are the 2 highest energy bands, and which is the higher energy band?

Valence Band & the Conduction Band, the Conduction band being higher.

What happens to electrons in a solid?

They are held tightly in the valence band, but move freely in conduction band, meaning the solid conducts.

Using band theory, explain how the LED emits light?

* Voltage applied causes electrons to move towards the conduction band of a p-type towards the junction.


* Electrons drop from conduction to valence band.
* Photons emitted when electrons drops.

What is a conductor?

A material which allows the flow of electricity.

What is an insulator?

A material which doesn’t conduct electricity easily.

What is a semiconductor?

Materials which are insulators when pure, but will conduct an when impurity is added and/or in response to light, heat, voltage etc.

Give 3 examples of conductors, and their resistance?

Metals/Semi-Metals like Graphite, Antimony, & Arsenic - Low Resistance

Give 3 examples of insulators, and their resistance?

Plastic, Wood, Glass - High Resistance

Explain the band gap in a conductor?

There is no band gap between the valence & conduction bands.

Explain the band gap in a insulator?

There is a large energy gap between the bands. It’s so large that electrons almost never cross the gap and the solid never conducts.

In some metals the conduction and valence band partially overlap? What does this mean?

The electrons can move freely between the valence band & conduction band.

Explain the valence band in an insulator?

It is full, but their are no electrons in the conduction band.

What happens if you supply enough energy in an insulator?

The solid with conduct but often, large amounts of energy needed ends up destroying the solid

Give 3 examples of semiconductors, and their resistance?

Silicon, Germanium, Selenium