Basic Theories of Semiconductor Electrochemistry - Electrochemistry of Silicon and Its Oxide - page 33

diffusion. The distance these nonequilibrium carriers travel during their lifetime is on
the order of the diffusion length. Thus, the holes generated deeper in the bulk,
mainly recombine before they reach the surface.
In the case of strong light absorption so that (Fig. 1.20a), the region for
photo hole generation is within the depletion layer. The photocurrent is independent of
the potential and represents the maximum attainable. On the other hand, when light
absorption is weak (Fig. 1.20b), the photocurrent is proportional to and thus
depends on the potential which determines the width of the space charge layer.
Photocurrent can generally be considered to consist of two parts, that due to gen-
eration in the depletion layer,

and that from generation in the bulk,

With the assumptions that (1) there is negligible recombination in the space charge layer
and at the surface and all the carriers generated in the space charge layer are driven by
the field to the surface, and (2) the electrode reactions are sufficiently fast, using Eq.
(1.83) the contribution due to photogeneration in the depletion layer can be described
by

The photocurrent due to generation in the bulk, can be described by