P- and n-type silicon

Unlike in genuinely conductive materials (metals), in semiconductors only a small part of the electrons is available for transporting electricity. Exactly how much depends on among other things the material composition and the temperature. In a pure silicon crystal, the atoms are in a neat network. Each silicon atom is combined with 4 other silicon atoms (figure 8).

: Figure 8: p- (right) and n-type (left) silicon. In the n-type silicon, many free electrons are available; in the p-type, many free holes.

The 4 ‘valence electrons’ of each silicon atom are shared with the neighbouring atoms. It is these shared electrons that at higher temperature, or under the influence of sunlight, are able to get away and then move freely through the material. If for example we replace a silicon atom in the lattice by an atom with 5 valence electrons, that atom does not fit well in the network (illustration on the left in figure 5). Phosphor is such a 5-value atom. It only has 4 neighbouring atoms of silicon with which it can share 4 valence electrons, so there is 1 electron left over. That 5^th electron easily detaches (so that a positively charged phosphor atom remains: the ‘hole’) and creates an ‘electron pressure’ in the material. Put properly the silicon is ‘doped’ with phosphor, and this material is called an n-type semiconductor. It is obvious that you could also do the reverse: introduce a 3-value atom (usually borium) so that in fact locally there is one bonding electron too few and ‘electron attraction’ occurs (illustration on the right). Silicon doped with borium is called p-type silicon.