Germanium semiconductor/electrolyte junction

Photoelectrochemical effect involves production of a voltage and an electric current when light falls on a semiconductor electrode immersed in an electrolyte solution and connected to a counter electrode (Becquerel, 1839). Working with germanium electrodes, Brattain Garrett (1955) showed that the Becquerel effect was due to the formation of a semiconductor-electrolyte junction. The idea of using an illuminated... 

Although this model is a natural extension of that derived for metal/semi-conductor or p-n junctions, it has proved remarkably difficult to verify it for semiconductors in contact with those electrolytes normally employed by electrochemists. As an example, the electrochemistry of germanium initially proved very difficult to understand in aqueous solution [2] and it was only with DeWaid s studies of n-ZnO [3] that a paradigmatic example of the classical model was discovered. The data found by DeWaid in his study of the ZnO electrolyte interface confirmed quantitatively the behaviour of the a.c. response of the semiconductor/electrolyte as predicted by the classical model. In particular, DeWald confirmed that the series capacitance of the interface obeyed the Mott-Schottky relationship [1]... 

Brattain and Garrett took a thin wafer of germanium and by alloying one side of it with indium formed a p-n junction with the p-side very much more heavily doped than the n-side, so that any current flow across the junction would be due to holes (5). The n-side of the junction was brought into contact with the electrolyte and current was then passed across the electrolyte interface under various potentials. Under these conditions the voltage which develops between the two ohmic contacts to the semiconductor is a measure of the minority-carrier density on the less-heavily-doped side of the junction. 

A thin slice of the semiconductor serves as the electrode, one side of which is contacted with another material to form a p-n-junction. If the distance between the surface of the slice used as electrode and the p-n-junction on the other side is in the order of the mean free path of the minority carriers, then injection or extraction of minority carriers through the p-n-junction diode will increase or reduce the supply of minority carriers for the electrode surface. This can clearly be seen in the influence of the diode voltage on the electrolytic current as shown in Fig. 23 for the case of a Germanium-electrode. Such a tool is very useful for quantitative measurements because the number of minority carriers reaching the electrode surface can be calculated and compared with the change of the electrolysis current. 

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