Basic Theories of Semiconductor Electrochemistry - Electrochemistry of Silicon and Its Oxide - page 30

At the limiting current in the dark under a reverse bias, all the change in poten-
tial occurs across the space charge layer. But at current below the dark limiting current
the applied potential can be distributed over both the Helmholtz layer and the space
charge layer. 717 The anodic charge transfer at a reverse bias involves two processes that
are electrically in series:
1. Electrochemical charge transfer through the Helmholtz layer according to
where is the exchange current, the overpotential across the Helmholtz
layer, and the charge transfer coefficient.
2. Electron transfer from the surface to the bulk of the semiconductor through
the space charge region with limiting current of
The total overpotential is
This equation indicates that potential change occurs mainly in the Helmholtz layer at
a current significantly less than the limiting current whereas it occurs mainly in the
space charge layer at a current near or equal to the limiting current and the Helmholtz
potential is almost fixed.