They typically have narrow bandgaps of < 2 eV
Semiconductors are generally covalently bonded substances.
The elements Si and Ge are intrinsic semiconductors.
Si has a band gap of 1.1 eV while that for Ge is 0.55 eV.
Modern synthetic semiconductors include the III-V compounds such as GaAs, GaP, InSb, InAs and InP. These have a range of different band gaps
Carbon, silicon, germanium and tin are atoms in ascending order of atomic number from column IV A of the period table. Each is characterised by having four valence electrons in its outermost shell of electrons, and requires a further four to make up the full complement of the shell. All can solidify to form elemental, covalently bonded crystals where the four valence electrons of one atom are shared between its four nearest neighbours so that every atom effectively gains eight electrons in its valence shell. A group IV atom and its four nearest neighbours from a tetrahedron as shown in Figure 1.
Figure 1: Schematic diagram to show the orientation of covalently bonded group 4 atoms. A tetrahedron is formed by the nearest neighbours, with the principal atom located at its centre.
Taking a larger scale perspective of the arrangement of the atoms, or crystal lattice , it is found that they organise themselves into two interpenetrating face centred cubic (fcc) sub-lattices, one displaced from the other by 1/4(a 0 , a 0 , a 0 ) along a diagonal of the unit cell . a 0 is called the lattice constant or lattice parameter and is a measure of the size of the unit cell, often expressed in Angstrom (A) units (1A=0.1nm or 1x10 -10 m). It is determined by techniques such as X-ray diffractometry. Figure 2 shows a complete unit cell for a group 4 crystal covalently bonded with the diamond structure.
This structure is of course difficult to visualise and draw, hence it is usually represented by an equivalent 2-D, "square" arrangement shown in figure 3.
In the example shown, it is assumed that the solid, Si say, is both crystallographically perfect and pure. At 0 K, all the covalent bonds are complete and there are no free charge carriers moving around randomly through the lattice; the crystal is an insulator.
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.