Parasitic Light Sensitivity Limits in Stacked Backside Illuminated Global Shutter Image Sensors
Stacked BSI global shutter sensor parasitic light sensitivity depends on silicon depth, metal shield alignment, and active illumination wavelength.
Electron Hole Pair Generation
Semiconductor Activation
Excitation of a valence electron into the conduction band creates a free charge carrier and a corresponding vacancy within a crystalline lattice. This process of electron hole pair generation occurs when an external energy source provides sufficient force to overcome the bandgap of the material. Photons or thermal energy usually provide the necessary input for this transition.
Quantum Efficiency
Detection performance depends on the ratio of generated pairs to the number of incident photons. For silicon sensors, the bandgap of 1.1 electron volts determines the spectral response limits. Excess thermal energy increases the rate of electron hole pair generation without light, leading to undesirable dark current.
High precision instruments use thermoelectric cooling to suppress this background noise.
Recombination Limit
Lifetime of these carriers determines how far they travel before being lost to crystal defects or impurities. If the electron hole pair generation happens far from the depletion region, the carriers might recombine before they can be collected as signal. This diffusion length is a primary specification for photodiode design and solar cell efficiency.
Doping levels and crystal purity are the primary controls used by manufacturers to maximize this duration.
Noise Coefficient
Statistical fluctuations in the generation rate contribute to shot noise in the output signal. Random arrivals of photons produce a predictable noise floor that limits the minimum detectable signal.