# Diodes and Semiconductors 2 questions!

1. Mar 23, 2009

### Wahoowa

A semiconductor diode is simple and elegant, but understanding how it works takes some thought. Let's walking through the formation and use of a diode to see exactly why that diode acts as a one-way device for electric current. The first step in building a diode is to obtain the basic material from which it will be made. In most cases, that material is the element silicon. When you order silicon from a chemical supply company, you can purchase silicon that is 98.4% pure for $40/kg or 99.9999% pure for$800/kg. Why should you spend the extra money for the good stuff?

a) Because of its impurity atoms, the 98.4% pure silicon will have a much higher electrical resistance than the 99.9999% pure silicon.

b) Because of its impurity atoms, the 98.4% pure silicon will not conduct electric current well enough to become a diode.

c) Because of its impurity atoms, the 98.4% pure silicon will have no electrons in its conduction band and no vacancies in its valence band. It will therefore not be a true semiconductor.

d) Because of its impurity atoms, the 98.4% pure silicon will have too many electrons in its conduction band or too many vacancies in its valence band to be a true semiconductor. Producing both p-type and n-type semiconductors from it will be almost impossible.
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Suppose you start with the 99.9999% pure silicon and add a small fraction of aluminum atoms. The new material has the same band structure as before but each aluminum atom has one fewer electron than the silicon atom it replaces. The aluminum atom also has one fewer positive charge in its nucleus, so the material is still electrically neutral. Because of its aluminum content, this new material

A) has some empty levels in its valence band and is an n-type semiconductor.

B) has some filled levels in its conduction band and is an n-type semiconductor.

c) has some empty levels in its valence band and is a p-type semiconductor.

d) has some filled levels in its conduction band and is a p-type semiconductor.

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