Introduction: The modern interest for phenomena at the semiconductor–electrolyte interface dates back to experiments performed in the 1950s with germanium, and has extended to most semiconducting materials for reasons of fundamental knowledge or potential application, going from semiconductor processing technology to heterogeneous photocatalysis to sensors. The subject is highly interdisciplinary and involves ﬁelds like electrochemistry, solid-state physics, and surface science. | Semiconductor Electrodes S Cattarin F Decker 200 Introduction The modern interest for phenomena at the semiconductor-electrolyte interface dates back to experiments performed in the 1950s with germanium and has extended to most semiconducting materials for reasons of fundamental knowledge or potential application going from semiconductor processing technology to heterogeneous photocatalysis to sensors. The subject is highly interdisciplinary and involves fields like electrochemistry solid-state physics and surface science. The aim of this article is to provide a concise survey of the basic concepts involved in the formation and operation of a semiconductor-electrolyte junction both in the dark and under illumination. In most cases terminologies and symbols recommended by IUPAC are used. Some important equations are reported but not derived. po Nv exp pEF - Ev kT 2 where Nc and Nv are the effective density of states per unit volume at the bottom of the conduction band and at the top of the valence band respectively and are a function of temperature T and the effective masses of electrons or holes. Typical values of Nc and Nv are in the range 1018 1019 cm 3 to be compared with a density of atoms on the order of 1022 cm . At equilibrium electrons and holes have the same Fermi level nEF pEF EF and the product between the densities is constant at a given temperature analogous to the law of mass action in chemistry. ofo NcNv exp kT n 2 3 Energy Levels in Semiconductors and in Solution The electronic states in semiconductors are described by the quantum theory of crystalline solids developed taking advantage of the properties of periodicity of their structures. The peculiar characteristic of semiconductors is that for a given interval of energies there are no electronic states available an interval of prohibited energies Eg called the energy gap separates a band of filled energy states the valence band with upper edge Ev from a band of empty energy states the conduction .