Before discussing in more detail the peculiarities of semiconductor-electrolyte interfaces it is worth to discuss the so called Schottky interface. The Schottky barrier is not only of interest due to the fact that is one of the best investigated barriers in the solid state physics, but also because has many similarities with semiconductor/electrolyte interfaces.
Figure 2.7: a) A schematic representation of energy bands of isolated metal and isolated n-type semiconductor. b) The band diagram at the contact between the metal and the semiconductor. c is the electron affinity in the semiconductor; and are the work functions of the metal and semiconductor respectively.
A Schottky barrier is a junction between a metal and a semiconductor. Figure 2.7 shows the band diagrams before and after the Schottky contact between metal and n-type semiconductor was fabricated. At equilibrium, in the absence of externally applied voltages, the Fermi level must be constant throughout the sample, otherwise a current would flow. As it was already mentioned, the Fermi level in metals represents the top of the electron sea, while in semiconductors, far from the interface, the Fermi level is determined ('pinned') by the doping levels. Before equilibrium, in the case presented in Figure 2.7b, the Fermi level is higher in the semiconductor, i.e. the work function of the semiconductor, φs, is smaller than that of the metal, φm. Therefore, electrons will flow from the semiconductor into the metal. This generates an electric field and therefore a potential gradient (barrier) along the interface.
The potential gradient leads to band bending in the semiconductor in such a way that the electrons in the semiconductor will be forced to go away from the semiconductor-metal interface. As a result, a region of width W containing a surplus of positive donor ions is formed. This region is called 'space charge region' or 'depletion region'. The width of the depleted area is a function of the doping profile as follows:
where ND is the concentration of donor impurity in the semiconductor, VD is the height of potential barrier.
Depending on the metal and semiconductor, a Schottky contact may show a diode like or an ohmic behaviour. This depends on the relative position of the Fermi levels in the metal and semiconductor. However, the ohmic contact can be always "forced" by doping the semiconductor to very high levels, thus forcing it to degenerate. In such a case the Fermi level penetrates the conduction/valence band of the semiconductor and the width of the space charge region becomes negligible. This way the tunneling probability will increases considerably and the current flow will not depend on the sign of the applied potential anymore, i.e. the diode behaviour will be suppressed.