Before commenting further on the elemental "semiconductors", it is worth
mentioning another group of technologically important solids which possess
semiconducting properties to varying degrees, namely the III-V compounds.
These are formed when equal numbers of group III and group V elements
combine with the same basic arrangement as the group IV elemental solids.
The difference lies in the fact that whereas the elemental solids contain
only one type of atom such that every atom in the (perfect) lattice is bonded
to four identical nearest neighbour atoms, in the III-V compounds a group III
atom is bonded to four group V nearest neighbours, and a group V atom is bonded
to four group III nearest neighbours. The two interpenetrating fcc sub-lattices
now contain either all group III atoms or all group V atoms. Figures 4 and 5
show the 3-D and 2-D representations, respectively, for these materials.
Figure 4 : Diagram to show the 3D unit cell of a III-V semiconductor compound
(eg. GaAs, gallium arsenide) with the zinc blende lattice.
Figure 5 : 2-D representation of a III-V semiconductor. Note the way in
which the group III and V alternate through the lattice on their individual fcc sub-lattices.
Although the Silicon Devices and Technology 3 course will concentrate on Si,
it is important to remember that the electronic band structure of Si (and Ge)
makes it unsuitable for certain applications; a prime example is light emitting
devices. Most semiconductor lasers and LEDs are made from the III-V materials;
it is NOT possible to get efficient light emission from Si. (There is now a
great deal of interest world-wide in the II-VI compounds because it has been
shown that blue and green LEDs and laser diodes can made from them, but
that's yet another story!).
