The negative electrons, orbiting the positively charged nucleus of an atom, all try to get
as close to the nucleus as they can. This is because opposite charges tend to attract each other. However, though no-one
knows why, Quantum mechanics forces them to sit at fixed, or quantised, distances from the nucleus and
only allow a limited number in each orbit level. Because of the link between the orbit and the energy, we tend to talk
about the electrons sitting at various energy levels in the atom.
The energy level furthest from the atom which still holds a fair number of electrons is called the valence level.
This is because these are the electrons which provide most of the forces which 'stick together' solids and liquids, and
control most chemical reactions. The level just above the valence level is called the conduction level. This is
because electrons here help a material to be able to conduct electricity.
When we remove an electron from the valence level (or one of the lower levels - nearer to the nucleus) we leave a
hole. The atom now has an overall positive charge, so it looks a bit like we've added a positively charged particle
to the atom. When we try to understand semiconductors, holes are very important. Engineers and solid-state physicists
tend to talk about "holes moving from place to place" and "the velocity of the holes", etc.
The holes don't really exist, but materials behave just as if they did. To make sense of this, when confused, just remember
that a hole is the absence of an electron. It's a bit like a bubble being the absence of some beer! When a bubble rises up a
glass we're actually watching some beer falling as gravity pulls it down. The fallen beer fills up the gap and opens up a new
gap above it.
When a hole moves from atom A to atom B what actually happens is an electron moves from B to A. It looks just as though
something positively charged went from A to B.
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Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)