Band structure of III-V and group IV semiconductors
Group IV semiconductors (Si,Ge)
The band structure of these semiconductors is very similar because:
- They do crystallize in the same crystallographic structure (diamond)
- They have similar electronic outer orbitals
The structure of silicon is purely covalent. The last orbital of atomic silicon has the electronic configuration 3s2p2. There are therefore 4 electrons (2s et 2p) sharing an orbital that could contain 8 (2 for the s orbital, 6 for the p orbital). Silicon has therefore 4 valence bands. The band structure of silicon and germanium, two most important semiconductors formed using the column IV of the periodic table, is shown in Fig. 2.4.
Figure 2.4: Germanium (left), Silicon (center) and Gallium Arsenide (right) band structures.
The valence band maximum is at k = 0 and is degenerate with the heavy and light hole bands. A third important valence band is the “spin-split” called this way because it is splitby the spin-orbit interaction. Finally, most important in the band structure of Silicon and Germanium is the fact that the minimum of the conduction band does not coincide with the maximum of the valence band. The semiconductor is called “indirect”.
The conduction band minimum in silicon is in the direction  and, as a result, also in the directions , , , ,  for a total of six minima.
Figure 2.5: Minima of the conduction band of Si, Ge and GaAs
In Germanium, in contrast, the conduction band minimum is in the directions corresponding to the cube’s diagonal, and we have therefore 8 conduction band minima.
III-V Semiconductors (GaAs, InP, ..)
The band structure of III-V semiconductors is similar since the tetrahedral bonds have the same structure as the ones in Silicon or Germanium. In fact, the missing electron of the group III with the electron configuration 4s24p (for example Gallium) is provided by the column V element (for example Arsenic) of configuration 4s24p3 and these bonds have a low ionicity.
For a large number of III-V semiconductors, the bandgap is direct.
Figure 2.6: Structure de bande du GaAs
As an example, the computed band structure of GaAs is shown in Fig. 2.6.