Video: Physics Past Exam • 2017/2018 • Pack 1 • Question 38A

Physics Past Exam • 2017/2018 • Pack 1 • Question 38A

01:50

Video Transcript

Compare P-type semiconductor with N-type semiconductor, point of comparison: valence impurity.

We can begin this comparison starting with a base material many semiconductors are made of. Silicon is a material often used this way, and we know that silicon has four electrons in its valence shell. This means that when silicon atoms are combined together in lattice formation, the interior atoms have a full valence electron shell with eight electrons.

This bulk semiconductor material has no net positive or negative charge. We create P for positive-type or N for negative-type semiconductors by adding an impurity in to this lattice formation. When we dope a semiconductor with an impurity that has three electrons in its valence shell, such as boron, for example, now when we surround this boron atom with silicon, there is a hole where an electron used to be, where the boron atom silicon.

This electron hole repeated many times over the lattice formation gives the semiconductor its overall positive charge. We can say that any atom with three electrons in its valence shell, that is, a trivalent atom, can help us create a P-type semiconductor.

On the other hand, an N-type semiconductor is created using an impurity that has five electrons in its valence shell. An example of an atom like this is antimony. Surrounded by silicon atoms, antimony has a full valence shell plus one electron which is free to move. It’s this excess electron that overall gives the semiconductor a negative-type charge. And it was created by doping the semiconductor with an impurity that has five electrons in its valence shell, that is, a pentavalent impurity. These then are the differences in the valence impurities between P- and N-type semiconductors.

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