Chemistry Reference
In-Depth Information
minority carriers and the width of the space charge layer. As illustrated in Fig. 1.20,
the depth of light penetration is on the order of
where
α
is the light absorption
coefficient defined as follows
962
:
where
I
is the light intensity, the light intensity entering the electrode, the frequency
of light, and
x
the distance from the surface into the bulk. The number of absorbed
photons per unit time and unit volume is and the rate of carrier generation for
one photon to one electron-hole pair is described by
The penetration depth of light may vary in a wide range since is a function of light
frequency as shown in Fig. 1.21.
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For silicon, the absorption of photons having ener-
gies greater than the band gap is almost entirely due to the generation of holes and elec-
trons. The optical penetration depth, is less than 10 nm when silicon is illuminated
with high-energy ultraviolet light whereas it is
with light having a wavelength
of
Figure 1.20 shows the two extreme cases of light absorption,
and
in relation to the diffusion length and the width of the space charge layer. In
the holes generated in the depletion layer, are transported
by the electric field to the surface where they are consumed in the electrode reactions.
Outside the depletion layer at
the region
the photogenerated holes are transported by
