Chemistry Reference
In-Depth Information
1.2. ENERGETICS OF SEMICONDUCTOR/ELECTROLYTE INTERFACE
The energy spectrum of electrons in an ideal crystal consists of two different types
of energy bands: those with filled energy levels (allowed bands) and those with no
energy levels (band gaps). For a semiconductor the upper unfilled band is called the
conduction band and the lower almost filled band is called the valence band as shown
in Fig. 1.1. The energy levels in a semiconductor are characterized by the conduction
and valence band edge and by the Fermi level The Fermi level
describes the equilibrium distribution of carriers in the bands and is the chemical poten-
tial of electrons in the semiconductor. The width of the band gap, depends
on the strength of the chemical bonds. For silicon at 300K the band gap is 1.12eV. The
effective band gap is reduced by heavy doping, larger than For silicon, a band
gap reduction of more than 100mV is associated with a dopant concentration of
band edge
The electronic conductivity of semiconductors, as expected from the band struc-
ture, can be generated by electrons of atoms of the basic substance in the crystal (in-
trinsic conductivity) as well as by electrons of impurity atoms or by the presence of
defects (extrinsic conductivity). In intrinsic semiconductors at the generation
of current carriers occurs as a result of the thermal excitation of some electrons from
the valence band to the conduction band, with the corresponding thermal rupture of
some chemical bonds. Simultaneously, an equal number of positively charged holes are
created in the valence band. In an electric field, these holes behave like particles pos-
sessing a positive charge equal in absolute value to the charge of the electron. For
extrinsic semiconductors, impurities and defects (which have energy levels located in
the band gap) are classified as either donors or acceptors as shown in Fig. 1.1. Donors,
usually located at energy levels slightly below the conduction band, give up excess
electrons to the conduction band, thereby creating electron conductivity (
-type semi-
conductors). Acceptors, located at energy levels slightly above the valence band,
capture valence electrons from atoms of the basic substance, producing hole conduc-
tivity (
p
-type semiconductors).
Similar to
trons in electrolytes associated with ions are characterized by the redox potential,
The redox potential describes the tendency of the species to give up or accept electrons
and can be considered as the effective Fermi level of the solution.
n
for the energy levels in semiconductors, the energy levels of elec-
