What is it?
This describes a class of materials that are more or less insulators (they do not pass electrical current),
but can under certain circumstances be conductors (they readily pass electrical current). In a material such as rubber - an insulator - an electric field (like
from a battery) does not induce a free flow of electrons, and so the material is said to be an insulator (no current flow). However a conductor (all metals for example) readily allow electrons to
migrate through it under the effect of an electric field. This is because the outer shells of electrons around the atoms need only a very low energy to knock them
out of their attachment to their nucleus. In effect, within the metal there is a veritable sea of electrons that can migrate through the conductor when an electric field is applied.
Now a semiconductor is a crystal with tightly bound electrons that do not readily give up their attachment
to their nucleus - just like an insulator. However, if the pure semiconductor is ‘doped’ with a tiny amount of an element that has a ‘spare’ electron in its
outermost valence shell, then this electron can move through the crystal, temporarily latching onto each atom on its migration path. Such a doped semiconductor is called n-type, as it has the
capability of passing electric current through the mechanism of ‘spare’ electrons (which have a negative charge (n).
If the material is doped with an element that has an electron short in its outermost valence shell then this
‘hole’ can also migrate across the crystal structure acting like a positively charged electron. In reality of course, it is the electrons that move, but the ‘hole’
that is left behind acts like a positive charge, and the semiconductor is then called p-type.
This behaviour is from the science of quantum theory. Practical devices that use semiconductors influence the conductivity of the material by combining different semiconductor types and
possible applying a control current or voltage to influence the behaviour of the semiconductor. A semiconductor diode allows current to flow only in
one direction. A transistor can amplify a current by having its conductivity controlled by a ‘gate’ or a ‘base’ that has a small current or voltage applied to it.
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