Electronic Band Structure

Question 1

What do network solids have instead of molecular orbitals

Answer 1

Electronic bands

Question 2

Define the valence and

Answer 2

Band of occupied orbitals

Question 3

Define the conduction band

Answer 3

Band of unoccupied orbitals

Question 4

Define band gap

Answer 4

Energy gap between valence and and conduction band. The band gap is the minimum energy a network wiki must absorb to promote an electron from valence band to conduction band

Question 5

What appearance qualities does a band gap determine?

Answer 5

Colour
Small band gap (e.g. Silicon) absorbs all visible light - appears black

Big band gap (e.g. Diamond) absorbs no visible light. Appears transparent

Question 6

Relate conductivity to band gap

Answer 6

In order for an electron to conduct electricity it must have access to an unoccupied energy level.

The valence band of diamond is full so electrons in the valence band cannot move as there are no nearby empty orbitals

In order for diamond to conduct electricity it must absorb high enough energy to promote electrons from the valence band to the conduction band

Question 7

Account for the differences in allotropes, graphite and diamond

Answer 7

Graphite is soft solids with tiny band gap. Absorbs all light - black. Highly conducting.

Diamond - big band gap - absorbs no light - transparent - hard solid and insulator

Question 8

Describe the differences between insulators, intrinsic semiconductors and metal

Answer 8

Insulator: large band gap - electrons cannot be promoted to conduction band

Intrinsic semiconductor: elections can be promoted from valence to conduction upon heating

Metal: no band gap

Question 9

Identify the 2 different categories of semiconductors

Answer 9

Intrinsic semiconductor is pure and has small band gap - electrons can be promoted to conduction band leaving holes in the valence band

Extrinsic semiconductors are doped - atoms are substituted (with impurities) to increase conductivity.

Question 10

Define and draw n-type doping

Answer 10

Element with more electrons is added to semiconductive element with less electrons. Extra electrons reside in donor levels just below the conduction band. As the material is heated these electrons are promoted to the conduction band and vastly outnumber any holes in the valence band

Question 11

Describe and draw a pic of p-type doping

Answer 11

P type doping - there are fewer electrons and more positive charge carriers. This is achieved by substituting with an element tot the left on the periodic table which has fewer electrons.

The electron poor atoms generate acceptor levels just above the valence band. Valence band electrons are promoted to the acceptor levels leaving fm holes in the valence band. I’m a P type semiconductor the major conduction is due to holes

Question 12

Describe how solar cells work

Answer 12

Solar cells can be generated by combining an intrinsic semiconductor with both p and n type extrinsic semiconductors