Chemistry Reference
In-Depth Information
1.4.6. Current Multiplication
Current multiplication is a phenomenon associated with the limiting current when
a reaction of several charge transfer steps occurs and only a part of these steps involves
the minority carriers. For example, for an anodic oxidation of a reducing species A to
its oxidized form
on an
n
-type material, current multiplication occurs as follows:
where A is first oxidized by a hole to become a radical Because of the closer posi-
tion of the intermediates to the edge of the conduction band, further oxidation to
is accomplished by injecting (
n-m-
1) electrons into the conduction band. This results
in a current that is
n/
(
m
+ 1) times larger than the limiting current by the minority
carriers. It is commonly called current doubling when
n/
(
m +
1) is 2. Current multi-
plication with a multiplication factor of 2 to 4 has been observed during the anodic
dissolution of
n
-type silicon in fluoride-containing solutions.
908,909
1.5.
PHOTOEFFECTS
When a semiconductor/electrolyte interface is irradiated with a light of energy
photons are absorbed, breaking the covalent bonds holding
the electrons at atomic sites in the lattice, and electron-hole pairs are generated. Some
of these electrons and holes, especially those formed in the bulk semiconductor beyond
the space charge region, recombine with the generation of heat or emission of light.
However, the field in the space charge layer causes the separation of electrons and holes.
Thus, in the case of Fig. 1.19 the holes arrive at the surface at an effective potential
equivalent to the valence band edge and cause the oxidation of the redox couple R/O
from R to O while the electrons move into the external circuit through the semicon-
ductor electrode lead. The flow of holes and electrons in opposite directions can be
measured as photocurrent.
greater than the band gap
1.5.1. Photocurrent
The attainable photocurrent under a given illumination intensity depends on the
relative depth of light penetration in the semiconductor, the diffusion length of the
