traplevels

Trap levels are defect-related energy levels within the band gap of a semiconductor that can capture and release charge carriers. They arise from point defects (such as vacancies or interstitials), impurities, dislocations, or complex defect structures, and they introduce localized states that lie between the conduction and valence bands. The position of a trap, measured as an energy depth from the conduction or valence band, determines how easily it captures electrons or holes and how it thermally ionizes at a given temperature.

Traps are commonly categorized by their electrical behavior as donor-like or acceptor-like. Donor-like traps preferentially capture electrons and may be positively charged when occupied, while acceptor-like traps capture holes and can be negatively charged when occupied. Traps can be shallow, near the band edges, or deep, closer to the middle of the gap. The occupancy of traps depends on the Fermi level and temperature, which in turn affects carrier lifetimes and transport properties.

A central role of trap levels is in Shockley–Read–Hall (SRH) recombination, where trapped carriers participate in nonradiative recombination with carriers of the opposite type. This trap-assisted recombination reduces minority-carrier lifetimes and diffusion lengths, influencing device performance in solar cells, LEDs, and transistors. Traps also cause sub-bandgap absorption, leakage currents, hysteresis, and threshold shifts in various devices.

Modeling trap levels typically uses the SRH framework, parameterized by trap density, energy level, and capture cross sections for electrons and holes. Experimental identification and characterization employ techniques such as deep-level transient spectroscopy (DLTS), admittance spectroscopy, and photoluminescence. Understanding trap levels is essential for material quality control and device optimization across semiconductor technologies.