Electrochemical processes at semiconductor-electrolyte interfaces play a critical role in applications ranging from photoelectrochemical water splitting to corrosion science. The behavior of these systems is governed by the interplay between the semiconductor’s electronic properties and the electrolyte’s chemical environment. Key phenomena include the formation of space charge layers, photoelectrochemical reactions, and corrosion mechanisms. This article examines these processes with reference to specific systems such as silicon in potassium hydroxide (KOH) and gallium nitride (GaN) for water splitting.

When a semiconductor is immersed in an electrolyte, a space charge region forms at the interface due to the difference in electrochemical potential between the two phases. This region is characterized by band bending, which arises from charge redistribution as the semiconductor and electrolyte reach equilibrium. For an n-type semiconductor, electrons transfer to the electrolyte, creating a depletion layer where the majority carrier concentration is reduced. Conversely, a p-type semiconductor may accumulate holes at the interface. The width of the space charge layer depends on the doping density and the applied potential. In silicon immersed in KOH, for example, the space charge layer extends over tens to hundreds of nanometers under typical doping conditions. The Helmholtz layer, a thin region of solvated ions, forms adjacent to the semiconductor surface, further influencing charge transfer kinetics.

Photoelectrochemical processes leverage the interaction of light with the semiconductor-electrolyte interface to drive redox reactions. When photons with energy exceeding the semiconductor’s bandgap are absorbed, electron-hole pairs are generated. These carriers are separated by the electric field in the space charge region, with electrons and holes migrating toward the bulk and surface, respectively. In GaN, a wide-bandgap semiconductor, ultraviolet illumination generates holes that oxidize water at the surface, while electrons reduce protons at the counter electrode, enabling water splitting. The efficiency of this process depends on factors such as carrier mobility, surface recombination, and the alignment of the semiconductor’s band edges with the redox potentials of the electrolyte. For GaN in aqueous solutions, the conduction band edge lies above the hydrogen evolution potential, making it suitable for photocathodic hydrogen production.

Corrosion mechanisms at semiconductor-electrolyte interfaces are influenced by both electrochemical and chemical dissolution. In silicon, the interaction with KOH proceeds through a two-step process. First, hydroxide ions attack the silicon surface, breaking Si-Si bonds and forming Si-H and Si-OH intermediates. Second, these intermediates react further, leading to the dissolution of silicon as silicate ions. The rate of this process is highly dependent on the potential applied to the semiconductor. At anodic potentials, hole accumulation accelerates oxidation, while cathodic potentials may suppress dissolution by driving hydrogen evolution instead. GaN, while more chemically stable than silicon in alkaline solutions, can still undergo corrosion under prolonged anodic polarization, particularly at defect sites or dislocations where the local electric field enhances ion migration.

The stability of semiconductor-electrolyte interfaces is also affected by surface states and adsorbates. Surface states can act as recombination centers, reducing photoelectrochemical efficiency, or as catalytic sites, enhancing reaction rates. In silicon, native oxide layers can passivate the surface, slowing corrosion, but may also introduce interface states that trap carriers. GaN surfaces, when properly treated, can achieve low surface state densities, improving charge transfer efficiency for water splitting. Adsorbates such as hydrogen or hydroxyl groups further modify the interfacial energetics, shifting band edges and altering reaction pathways.

Practical applications of these principles are exemplified by silicon etching in KOH and GaN-based photoelectrodes. Silicon micromachining relies on the anisotropic etching of silicon in KOH, where the (100) plane ethes faster than the (111) plane due to differences in bond density and stability of surface intermediates. This process is widely used in microfabrication for creating trenches and membranes. GaN, on the other hand, is employed in photoelectrochemical cells for solar hydrogen generation. Its resistance to photocorrosion in neutral and alkaline solutions, combined with its favorable band alignment, makes it a promising material for sustainable energy applications.

The study of electrochemical processes at semiconductor-electrolyte interfaces continues to advance with the development of new materials and characterization techniques. In situ methods such as electrochemical impedance spectroscopy and scanning probe microscopy provide insights into dynamic changes at the interface. Computational modeling further aids in predicting behavior under varying conditions. Understanding these processes is essential for optimizing performance in applications ranging from energy conversion to electronic device fabrication.

In summary, the electrochemical behavior of semiconductor-electrolyte interfaces is governed by space charge effects, photoelectrochemical reactions, and corrosion dynamics. Systems such as silicon in KOH and GaN for water splitting illustrate the complex interplay between material properties and electrochemical environment. Advances in this field hold promise for improving the efficiency and durability of semiconductor-based electrochemical systems.