The Main Sensor Types and What Each One Measures
Selecting sensors requires matching physical transduction principles to measurands while accounting for thermal drift, noise floor, and fab availability.
Optical Photodiodes
Spectral Responsivity
Semiconductor transducers convert incident radiation into measurable electrical current through internal photoelectric generation. Optical photodiodes operate within this domain as fundamental solid-state detectors that translate photon flux into linear output signals across specific wavelength bands. Photons absorbed within the active depletion region generate electron-hole pairs, which drift under the influence of an internal or external electric field to produce a terminal current.
Dark current originating from thermal carrier generation establishes the lower detection limit of the component. Manufacturers specify responsivity in amperes per watt at reference operating temperatures and calibrated bias voltages to define device sensitivity.
Quantum Efficiency
Photons striking the active surface do not all contribute to usable signal current due to reflection losses and incomplete absorption in thin semiconductor layers. External quantum efficiency measures the ratio of generated electron-hole pairs collected at the contacts to incident photons arriving at the window. Anti-reflective coatings applied to the silicon or indium gallium arsenide surface minimize surface scatter and maximize penetration into the junction region.
Wavelength boundaries restrict usable operation, because transparent transmission occurs below the bandgap energy, while excessive photon energy generates thermal phonons rather than free carriers.
Response Bandwidth
Junction capacitance and carrier transit time dictate the maximum frequency response of the sensor under modulated light input. Depletion width variations controlled by reverse bias voltage alter the transit time of generated carriers and simultaneously adjust the capacitance of the device. Load resistance connected in series with the photodiode forms an RC time constant that limits signal rise and fall times during high-speed pulsed measurements.
Parasitic inductance from packaging leads introduces ringing and overshoot in fast optical communication circuits, requiring impedance matching networks to preserve signal fidelity.
Calibration Drift
Environmental exposure and aging mechanisms alter the conversion accuracy of the sensor throughout its operational lifetime. Temperature fluctuations shift the fundamental bandgap energy of the semiconductor material, modifying responsivity and dark current characteristics over extended deployment periods. High-energy radiation degrades crystal lattice integrity, introducing trapping states that reduce carrier lifetime and suppress measured signal output.
Periodic verification against primary optical standards quantifies accumulated responsivity loss and establishes correction factors for subsequent measurement chains.