# Homework 5 – Conductors, Semiconductors and Insulators ```Homework 5 – Conductors, Semiconductors and Insulators
1. Give examples of two conductors, two insulators and two semiconductors.
2. The behaviour of conductors, insulators and semiconductors can be explained
using band theory.
a) State the name of the band that contains no free electrons.
b) State the name of the band that contains electrons that are free to move.
c) How does the band gap of semiconductor material compare with the
band gap of an insulator?
3. The conductivity of a semiconductor material can be increased by ‘doping’.
a) Explain what is meant by the ‘conductivity’ of a material.
b) Explain, giving an example, what is meant by ‘doping’ a
semiconductor.
c) Explain why does ‘doping’ decreases the resistance of a semiconductor
material.
4. A sample of pure germanium (four electrons in the outer shell) is doped with
phosphorus (five electrons in the outer shell). What kind of semiconductor is
formed?
5. Why does a sample of n-type semiconductor still have a neutral overall charge?
6. Describe the movement of the majority charge carriers when a current flows in:
i.
ii.
an n-type semiconductor material
a p-type semiconductor material.
End of Exercise 5
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4. A p-n junction is used as a photodiode as shown.
(a) State the mode that the photodiode is operating in.
(b) Describe the effect of light on the p-n junction of the photodiode.
(c) The irradiance of light incident on the p-n junction is increased.
State what happens to the reading on the ammeter.
(d) The photodiode is placed at a distance of 1&middot;2 m from a small lamp. The reading
on the ammeter is 3&middot;0 mA.
The photodiode is now moved to a distance of 0&middot;80 m from the same lamp.
Calculate the new reading on the ammeter.
End of Exercise 6
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