An increase in a material’s electrical conductivity caused by absorption of electromagnetic radiation that generates mobile charge carriers (for example, electron–hole pairs or free electrons), reducing resistivity while illumination persists and altering current under a given applied field.

Principle

When photons with sufficient energy are absorbed, they produce additional mobile carriers; the net photoconductive change depends on the photon absorption rate, the carriers' generation-to-recombination balance (lifetime), and their mobility, so conductivity change scales with the steady-state excess carrier density under the illumination and applied electric field.

Demonstration

Illustrative scenario — Situation: A photoconductive semiconductor strip is placed in a DC measurement circuit in the dark with low current. Recognition: A visible LED is switched on, illuminating the strip with photon energy above its bandgap. Action: Measured current through the strip is recorded before, during and after illumination. Consequence: Current rises while illuminated (resistance falls) and returns toward the dark value after the LED is off; the magnitude and temporal response reflect absorption, carrier lifetime and applied bias.

Misapplication

Confusing photoconductivity with the photoelectric effect or photovoltaic effect. The error is treating any light-induced current as emission of electrons from the material or as a generated open-circuit voltage, rather than as a bulk increase in carrier concentration that changes conductivity within the material.

Consequence

Designers must account for photoconductivity in sensors and circuits: it enables light detectors and imaging elements but can cause unwanted leakage or noise in optically exposed circuits. Temporal response and magnitude determine suitability for sensing, communications or stability-sensitive applications.

Reversal

If carrier recombination is extremely fast, traps dominate, or photons are absorbed only in an insulating surface layer, the observable photoconductive change can be negligible or non-linear; conversely, some materials exhibit persistent photoconductivity where elevated conductivity remains after illumination due to slow recombination or trapped charges.

Boundary

Clearly within: A bulk semiconductor whose conductivity measurably increases when illuminated above its bandgap. Boundary case: A weakly absorbing indirect-bandgap material with only surface photoconductivity. Clearly outside: Photoemission (photoelectric effect) where electrons leave the material, and photovoltaic junction effects that produce a built-in voltage rather than primarily changing bulk resistivity.

Semantic Tension

Photoconductivity ↔ Photovoltaic/Photoemission — all involve light-generated carriers, but photoconductivity describes a change in bulk/resistive properties under bias, whereas photovoltaic/photoemission describe generation of voltage or emitted electrons; design choices must trade detection mode, sensitivity and circuit conditions.

Synthesis

Photoconductive effect is the bulk electrical response to photon-generated carriers: it is operationally the way illumination alters a material’s conductive path under applied field, distinct in mechanism and application from surface emission or junction voltage generation.