Semi conductors and PN junctions

Question 1

Explain how doping a semiconductor with a group 5 element changes its resistance

Answer 1

• a group 5 element has 5 outer electrons in its outer shells , 4 of which fill up spaces in the valence band
• the extra electron enters the conduction band where it is free to conduct
• as a result the resistance of the semiconductor decreases

Question 2

Explain why doping a semi conductor with a group 3 element changes its resistance

Answer 2

• a group 3 element has only 3 outer electrons which fill up spaces in the valence band
• this creates a positive hole in the valence band which is free to conduct
• as a result the resistance of the semiconductor decreases

Question 3

An LED is made from a semiconductor doped with impurities to create a p-n junction. A voltage is applied across an LED so that it is forward biased and emits light. Explain why this occurs.

Answer 3

• the applied voltage causes the electrons to move from the conduction band of the n type material across to the conduction band of the p type material
• the electrons then drop into the valence band of the p type material to fill holes
• the electron energy is released in the form of a photon of light which is emitter

Question 4

Explain what happens when a semiconductor is heated

Answer 4

• a semiconductor has a small energy gap between its valence band and its conduction band
• an electron absorbs thermal energy which causes it to jump up from the valence band to the conduction band
• the electron can now flow freely and as a result the resistance of the semiconductor decreases

Question 5

Explain why a semi conductor does not conduct at absolute zero

Answer 5

• a semiconductor is effectively an insulator with a small energy gap between its valence band and its conduction band
• at absolute zero there is no thermal energy to excite the electrons up from the valence band to the conduction band
• as a result there are no electrons in the higher energy conduction band and the semiconductor does not conduct

Question 6

Explain the properties of a p-n junction diode

Answer 6

1. consists of an n type material with outer electrons in its conduction band
2. consists of a p type material which has holes in its valence band
3. they are separated by a depletion layer which acts as a narrow insulating layer
4. the p-n junction conducts if it is forward biased as electrons are given energy to move across from the conduction band of the n type layer to the p type layer where they then drop down to fill holes in the valence band
5. it does not conduct if it is in reversed biased as the depletion layer widens and so no electrons can move across the insulating layer