Semiconductors and Band Theory

Question 1

What makes a solid a conductor?

Answer 1

The material must have free electron and empty states

Question 2

Band (energy) gap in a conductor

Answer 2

Conduction band and the valence band overlap

Question 3

Energy gap in an insulator

Answer 3

Massive

Question 4

In a metal, both conduction and valance bands are

Answer 4

Partially filled

Question 5

In an insulator what state is the valance band in

Answer 5

Full

Question 6

For an insulator at room temperature the gap between the valance band and conduction band is large so electrons can’t

Answer 6

Move into the conduction band

Question 7

What is the energy gap like in a semiconductor compared to an insulator

Answer 7

Smaller

Question 8

What type of energy can move electrons from the valence band into the conduction band in a semiconductor

Answer 8

Heat energy

Question 9

What is doping

Answer 9

Adding an impurity to a semiconductor to increase the conductivity

Question 10

What is an n type semiconductor

Answer 10

A semiconductor doped with an atom that has extra electrons. These electrons end up free to move and so can more easily get into the conduction band

Question 11

What is a p-type semiconductor

Answer 11

A semiconductor that has been doped with an atom with too few electrons. This creates a hole in the valence band (empty state)

Question 12

What is a p-n junction semiconductor

Answer 12

It is just a layer of p type and n type semiconducting material

Question 13

State in terms of band theory how an LED works

Answer 13

The forward bias potential difference across the junction causes electrons to move from the conduction band of the n-type semiconductor towards the conduction band of the ptype semiconductor. Photons are emitted when electrons ‘fall’ from the conduction band into the valence band either side of the junction.

Question 14

Forward bias

Answer 14

N type to negative side of cell therefore electric field opposing conduction is reduced

Question 15

Reverse bias

Answer 15

N type to positive side of supply, therefore electric field opposing conduction is increased

Question 16

State in terms of band theory how a photovoltaic cell works (solar panel)

Answer 16

solar cells are p-n junctions designed so that a potential difference is produced when photons are absorbed. (This is known as the photovoltaic effect.) The
absorption of photons provides energy to ‘raise’ electrons from the valence band of the semiconductor to the conduction band. The p-n junction causes the electrons in the conduction band to move towards the n-type semiconductor and a potential difference
is produced across the solar cell

Question 17

What is a semiconductor?

Answer 17

is like an insulator with a much smaller band gap. The gap is small enough that
room temperature is enough to give some electrons the energy to jump up from the valence to
conduction band and increase the conductivity of the solid. Conductivity increases (ie resistance
decreases) with increasing temperature.

Question 18

What can you do to semi conductors?

Answer 18

Semiconductors can be doped by adding impurity atoms to reduce their resistance