Identify the material class: A photoconductor is best described as a(n) ________. (Choose the most accurate fundamental classification.)

Introduction / Context:
Photoconductors are materials whose electrical conductivity increases when illuminated. They are essential in light sensors, imaging arrays, and some photovoltaic components. Recognizing their fundamental classification reveals why light controls their conductivity and how devices using them operate.

Given Data / Assumptions:

• Illumination generates additional free carriers via photon absorption.
• Energy of incident photons is sufficient to excite electrons across the band gap or liberate carriers from traps.
• Steady temperature and bias conditions.

Concept / Approach:

In semiconductors, the band gap is small enough that photons of visible/near-IR energy can promote electrons from the valence band to the conduction band, creating electron–hole pairs and increasing conductivity. In pure conductors (metals), abundant carriers already exist and light does not significantly change conductivity. In insulators, very large band gaps prevent significant photogeneration at ordinary photon energies.

Step-by-Step Solution:

Verification / Alternative check:

Classic photoconductive materials include CdS, CdSe, PbS, and a-Si:H—each a semiconductor whose conductivity rises with illumination intensity.

Why Other Options Are Wrong:

• Conductor: already high carrier density; little photoconductive effect.
• Insulator: band gap too large for effective photogeneration.
• Superconductor: zero DC resistance below critical temperature; unrelated mechanism.
• Dielectric-only: ignores the semiconducting band structure required.

Common Pitfalls:

Confusing photoconductors with photodiodes or photovoltaic devices; while related, photoconductors rely primarily on increased conductivity rather than junction-based current generation.

Final Answer:

Semiconductor