Electron state density in redox electrode reactions

3 Electron state density in redox electrode reactions 

In contrast to metal electrodes in which the electrostatic potential is constant, in semiconductor electrodes a space charge layer exists that creates an electrostatic potential gradient. The band edge levels and in the interior of semiconductor electrodes, thereby, differ from the analogous and at the electrode interface hence, the difference between the band edge level and the Fermi level in the interior of semiconductor electrodes is not the same as that at the electrode interface as shown in Fig. 8-14 and expressed in Eqn. 8—47  

In the state of band edge level pinning, the electron level of redox particles with the state density of DredoxCe), relative to the electron level rf semiconductor with the state density of Dsc(e), remains unchanged at the electrode interface irrespective of electrode potential. On the other hand, in the state of Fermi level pinning, the electron level of redox particles relative to the electron level of semiconductor electrode depends on the electrode potential in the same way as occurs with metal electrodes (quasi-metallization of semiconductor electrodes). 

Since semiconductor electrodes are usually in the state of band edge level 

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