Oxide semiconductors, such as indium gallium zinc oxide (IGZO) and zinc oxide (ZnO), have gained significant attention for their applications in transparent electronics, thin-film transistors, and optoelectronic devices. However, their stability and degradation mechanisms under various environmental and operational conditions remain critical challenges. Understanding these mechanisms and developing effective mitigation strategies are essential for improving the performance and longevity of oxide semiconductor-based devices.

Environmental factors, including humidity and oxygen exposure, play a significant role in the degradation of oxide semiconductors. Moisture absorption is a primary concern, particularly for materials like ZnO, which exhibit hygroscopic behavior. Water molecules can adsorb onto the surface and diffuse into the bulk, leading to the formation of hydroxyl groups and oxygen vacancies. These defects act as electron traps, increasing off-state current and reducing carrier mobility. In IGZO, humidity-induced degradation manifests as a negative threshold voltage shift in thin-film transistors due to the interaction of water molecules with oxygen vacancies and metal cations. Oxygen exposure, on the other hand, can passivate oxygen vacancies but may also lead to excessive oxidation, altering the stoichiometry and electronic properties of the material.

Bias stress effects are another major contributor to instability in oxide semiconductors. Under prolonged positive or negative gate bias, charge trapping occurs at the semiconductor-dielectric interface or within the bulk of the material. Positive bias stress typically results in a positive threshold voltage shift due to electron trapping at interface states or oxygen vacancies. Negative bias stress, often exacerbated by illumination, leads to a negative threshold voltage shift attributed to hole trapping or ion migration. In IGZO transistors, the instability under bias stress is influenced by the composition and stoichiometry of the material, with gallium-rich films exhibiting better stability due to reduced oxygen vacancy concentration. ZnO-based devices are particularly susceptible to bias stress degradation due to their high intrinsic defect density.

Defect generation and migration are intrinsic to the degradation mechanisms of oxide semiconductors. Oxygen vacancies, the most common defects in these materials, act as shallow donors and contribute to n-type conductivity. However, excessive oxygen vacancies can lead to uncontrolled carrier concentration and Fermi level pinning. Interstitial metal ions, such as zinc interstitials in ZnO, also contribute to conductivity but can migrate under electric fields, causing device instability. Hydrogen impurities, often incorporated during deposition or from ambient moisture, can passivate oxygen vacancies but may also introduce additional electronic states. The interplay between these defects determines the overall stability of the material.

Mitigation strategies focus on minimizing environmental and operational degradation through material engineering and device design. Encapsulation is a widely used approach to protect oxide semiconductors from moisture and oxygen. Thin-film barriers made of inorganic materials like aluminum oxide or silicon nitride are deposited using atomic layer deposition or chemical vapor deposition to prevent gas permeation. Organic-inorganic hybrid encapsulation layers offer flexibility and improved adhesion, making them suitable for flexible electronics. Passivation layers, such as silicon oxide or hafnium oxide, can be applied to the semiconductor surface to reduce interface trap density and suppress charge trapping. These layers also act as diffusion barriers against ion migration.

Doping and composition control are effective in enhancing the stability of oxide semiconductors. In IGZO, increasing the gallium content reduces oxygen vacancy concentration and improves bias stress stability. Magnesium doping in ZnO has been shown to suppress oxygen vacancy formation and enhance thermal stability. Nitrogen doping can passivate defects and reduce carrier concentration, leading to more stable electronic properties. Tailoring the stoichiometry and crystallinity of the material during deposition also plays a crucial role in minimizing defect-related degradation.

Operational strategies include optimizing the device architecture and driving conditions. Dual-gate structures in thin-film transistors can mitigate bias stress effects by distributing the electric field and reducing charge trapping. Pulse-driven operation, as opposed to continuous bias, minimizes prolonged exposure to high electric fields and reduces degradation. Light shielding layers are employed in optoelectronic applications to prevent photo-induced instability, particularly in ZnO-based devices.

The long-term stability of oxide semiconductors depends on the synergy between material properties, device design, and environmental protection. Advances in defect engineering and encapsulation technologies continue to improve the reliability of these materials for practical applications. Future research directions include the development of self-healing materials and advanced barrier coatings to further enhance stability under extreme conditions. Understanding the fundamental degradation mechanisms remains key to unlocking the full potential of oxide semiconductors in next-generation electronics.