Carrier Mechanism
Semiconductor depletion regions generate electron-hole pairs via localized energy states located within the forbidden bandgap. Solid-state photodetectors, image sensors and junction diodes experience thermal carrier generation where crystal defects act as stepping stones across the bandgap. This mechanism, formally designated as shockley read hall generation, governs dark current generation in reverse-biased p-n junctions and active pixel areas.
The process is bounded by the intrinsic carrier concentration of the semiconductor and diminishes toward zero as absolute temperature approaches zero Kelvin.
Thermal Generation
Mid-gap trap states introduced by metallic impurities, crystal dislocations or unpassivated interface bonds facilitate thermal transitions between conduction and valence bands. Within a reverse-biased space charge region, the electric field sweeps generated electrons and holes in opposite directions before recombination can occur. This continuous carrier separation produces a steady leakage current under complete darkness.
The rate of shockley read hall generation depends exponentially on temperature and aligns closely with the energetic depth of the trap state relative to the intrinsic Fermi level. Mid-gap states exhibit the highest generation rates, maximizing dark current in uncooled sensors. Chemical purity and crystal growth perfection dictate the total density of effective generation centres across the active sensing volume.
Dark Current
High operating temperatures exponentially multiply the thermal generation rate, doubling dark current with every few degrees of thermal rise. Elevated dark current consumes available dynamic range, adds fundamental shot noise and creates fixed-pattern noise across multi-element sensor arrays. Heavy ion irradiation or manufacturing plasma etch processes introduce lattice displacement damage, creating deep-level traps that permanently degrade sensor noise performance.
High reverse bias voltages expand the depletion volume, capturing more defect centres and increasing the total generated leakage current.
Sensor Threshold
Testing protocols measure reverse dark current across temperature sweeps to construct Arrhenius plots, allowing extraction of effective activation energies and trap state densities. Deep-level transient spectroscopy identifies specific impurity species by monitoring thermal emission rates from localized traps under pulsed bias conditions. Sensor qualification limits specify maximum permissible dark current density at room and elevated operating temperatures.
Mitigating shockley read hall generation through high-purity silicon processing and effective gettering is necessary to maximize signal-to-noise ratios in scientific image sensors.
