What Is Photoconductivity?

Photoconductivity is the increase in the electrical conductivity of a material caused by the absorption of electromagnetic radiation. In a photoconductive material, the absorption of photons generates additional free charge carriers, electrons and holes, that reduce the material's resistance while the illumination persists. Once the light source is removed, the excess carriers recombine and the conductivity returns toward its dark value at a rate governed by the carrier lifetime of the specific material. The phenomenon is central to the operation of photoresistors, photodetectors, and photoconductive switches used across many fields of electrical engineering and applied physics.

Photoconductivity was first observed in selenium in 1873 by Willoughby Smith, who found that the element's resistance decreased substantially under illumination. The effect was later understood through the quantum mechanical band theory of solids, which explains how the energy structure of a material determines whether an absorbed photon can produce free carriers or simply deposit energy as heat.

Physical Mechanism

The generation of excess carriers requires that the absorbed photon carry energy equal to or greater than the material's band gap in the case of an intrinsic photoconductor, or equal to or greater than the ionization energy of an impurity level in an extrinsic photoconductor. When this condition is met, the photon promotes an electron from a lower to a higher energy state, leaving a positively charged hole. Both the electron and the hole are mobile under an applied electric field and contribute to increased current flow. The magnitude of the photoconductivity depends on the absorption coefficient of the material at the illumination wavelength, the quantum efficiency of carrier generation, and the carrier lifetime before recombination eliminates the excess carriers. As described in technical references on photoconductive detectors and their performance characteristics, the ratio of carrier lifetime to transit time defines the photoconductive gain, which can exceed unity when carriers complete multiple circuits across the device before recombining.

Intrinsic and Extrinsic Photoconductivity

Intrinsic photoconductivity occurs through band-to-band transitions in which the photon elevates an electron across the fundamental band gap of the semiconductor. Silicon, germanium, gallium arsenide, and cadmium sulfide are among the most studied intrinsic photoconductors. Each material's response is spectrally bounded at the long-wavelength end by its band gap: silicon cuts off near 1.1 micrometers, while narrower-gap materials such as indium antimonide or mercury cadmium telluride respond into the mid- and long-wave infrared. Extrinsic photoconductivity exploits dopant-induced energy levels within the band gap, allowing detection of photons with energies below the intrinsic threshold. Germanium doped with copper or mercury, for instance, detects radiation at wavelengths exceeding 100 micrometers, relevant to far-infrared astronomy. As surveyed in an overview of photoconductors across material classes, extrinsic devices typically require cryogenic operation to suppress dark current from thermally excited carriers.

Transient and Steady-State Photoconductivity

Photoconductivity can be measured in two regimes. In steady-state photoconductivity, the illumination is continuous and constant, and the conductivity reaches a stable elevated value set by the balance between carrier generation and recombination. Transient photoconductivity is measured after a brief light pulse, with the decaying conductivity signal revealing carrier lifetime and mobility information. Time-resolved microwave conductivity and optical pump-probe techniques allow contactless measurement of carrier dynamics in thin films and nanostructured materials, including two-dimensional materials with unconventional photoconductive responses. These measurements are widely used to evaluate candidate photovoltaic and photodetector materials without requiring the fabrication of electrical contacts.

Applications

Photoconductivity has applications in a range of fields, including:

• Light-dependent resistors for ambient light sensing in consumer devices
• Infrared and thermal imaging detector arrays
• High-speed photoconductive switches driven by ultrashort laser pulses
• Terahertz emitters and receivers in time-domain spectroscopy systems
• Photoconductive sampling for ultrafast waveform measurement
• Characterization of photovoltaic and semiconductor thin-film materials