Environmental Engineering Reference
In-Depth Information
Table 4.2
Band Gap for Various Semiconductors at 300 K
IV semiconductors
III-V semiconductors
II-VI semiconductors
Material
E
g
Material
E
g
Material
E
g
Si
1.107 eV
GaAs
1.35 eV
CdTe
1.44 eV
Ge
0.67 eV
InSb
0.165 eV
ZnSe
2.58 eV
Sn
0.08 eV
InP
1.27 eV
ZnTe
2.26 eV
GaP
2.24 eV
HgSe
0.30 eV
Source:
data from Lechner, 1992
semiconductor is cadmium telluride (CdTe). Table 4.2 shows the different band
gaps of various semiconductors.
Silicon is the material most commonly used in photovoltaics. Silicon is the
second most abundant element in the earth's crust after oxygen, but it cannot
be found in a chemically pure form. Silicon is an elementary semiconductor of
the group IV of the periodic table of elements, i.e. silicon has four valence
electrons in the outer shell. In order to obtain the most stable electron
configuration, two electrons of neighbouring atoms in the silicon crystal lattice
form an electron pair binding. In other words, two atoms jointly use these
electrons. Electron pair bindings (covalent bonds) with four neighbours give
silicon a stable electron configuration similar to that of the noble gas argon
(Ar). In the energy band model, the valence band is now fully occupied and
the conduction band is empty. Supplying sufficient energy by incident light or
heat can elevate an electron from the valence band into the conduction band.
This electron now can move freely through the crystal lattice. A so-called
defect electron, or hole, remains in the valence band. Figure 4.5 illustrates this
process. The formation of defect electrons is responsible for the so-called
intrinsic conduction
of semiconductors.
Electrons and holes always arise in pairs, i.e. there are exactly as many
electrons as holes. This is described by the following equation using the
electron density n
and
hole density p
:
Figure 4.5
Crystal Structure of Silicon (left), Intrinsic Conduction due to
Defect Electron in the Crystal Lattice (right)
