A brief review and analysis of the new results (published in the 21st century) concerning the probability of underpotential deposition (UPD) of adatom layers or submonolayers on the single-crystal semiconductor electrodes is presented. These results confirm the previous conclusions that the probability of these processes is very low on ordinary (commonly used) semiconductor single-crystal electrodes. This fact is explained by means of comparison of the characters of the bonds forming between adatoms and substrates of various types. The relatively high probability of underpotential deposition processes on metal electrodes is caused by formation of a strong metal-metal type bond between each adatom and metal substrate. On the contrary, comparatively low intensity of interphase electron transfers across the interface between a single-crystal semiconductor electrode and electrolyte prevents the formation of such strong bond between each adatom and substrate. However, some sorts of pretreatment or modification of the surfaces of these electrodes may promote the occurrence of UPD of adatom layers on them.
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