Direct Energy, 2018a

116 6.3 Semiconductors and Energy Level Diagrams
Material Gap in
eV
Material Gap in
eV
Material Gap in
eV
AlP 2.45 ZnS 3.6 GaP 2.26
GaP 2.26 ZnSe 2.7 GaAs 1.43
InP 1.35 ZnTe 2.25 GaSb 0.70
Table 6.1: Energy gap of various semiconductors.
Why are solar cells and optical photodetectors made from semiconductors
instead of conductors? When light shines on a solar cell or photodetector,
photons of light are absorbed by the material. If the photon absorbed
has energy greater than the energy gap of the material, the electron quickly
decays to the top of the conduction band. With some more time, it decays
back to the lowest energy state. In a solar cell or photodetector, a pn
junction is used to cause the electrons to ow before decaying back to the
ground state. The amount of energy converted to electricity per excited
electron depends on the energy gap of the material, not the energy of the
incoming photon. Only energy E g per photon absorbed is converted to
electricity regardless of the original energy of the photon. Thus, the energy
gap of the material used to make a solar cell or photodetector should be
large so that as much energy per excited electron is converted to electricity
as possible. The material should not have an energy gap that is too small
otherwise very little of the energy will be converted to electricity. The electron
and hole will release the excess energy, hf − E g , quickly in the form
of heat or lattice vibrations called phonons.
Each semiconductor has a dierent energy gap E g . Many solar cells and
photodetectors are made from silicon, which is a semiconductor with E g =
1.1 eV. Predicting the energy gap of a material is quite dicult. However,
all else equal, if an element of a semiconductor is replaced with one below
it in the periodic table, the energy gap tends to get smaller. This trend is
illustrated in Table 6.1. Data for the table comes from [9]. This trend is
also illustrated in Fig. 6.6, which plots the energy gap and lattice constant
for various semiconductors. Figure 6.6 is taken from reference [71]. The
horizontal axis represents the interatomic spacing in units of angstroms,
where one angstrom equals 10 10 meters. The vertical axis represents the
energy gap in eV. This gure illustrates energy gaps and lattice constants
for materials of a wide range of compositions. For example, the energy
gap for aluminum phosphide can be found from the point labeled AlP, and
the energy gap of aluminum arsenide can be found from the point labeled
AlAs. Energy gap for semiconductors of composition AlAs x P 1−x can be
found from the line between these points.