Light-dependent resistance behavior of a photoconductor How does the electrical resistance of a photoconductor change as incident light intensity increases?

Introduction / Context:
Photoconductors (light-dependent resistors, LDRs) are semiconductor devices whose conductivity changes under illumination. They are used in light sensors, automatic lighting, optical encoders, and camera exposure controls. Understanding the resistance–illumination relationship is vital to designing biasing circuits and setting thresholds.

Given Data / Assumptions:

• Intrinsic or lightly doped photoconductive material (e.g., CdS, CdSe).
• Illumination increases the density of free carriers (electrons/holes).
• Temperature held approximately constant to isolate optical effects.

Concept / Approach:

Electrical conductivity σ = n q μ, where n is carrier density, q charge, and μ mobility. Absorbed photons generate electron–hole pairs, increasing n. Although mobility may change slightly, the dominant effect is increased carrier concentration; thus σ increases and resistance R = 1/σ decreases as light intensity rises.

Step-by-Step Solution:

Verification / Alternative check:

Typical LDR datasheets show R dropping by orders of magnitude between dark (megaohms) and bright light (kiloohms), confirming the inverse relationship.

Why Other Options Are Wrong:

(a) contradicts photogeneration; (c) is not correct for standard photoconductors—trend is monotonic; (d) ignores optical sensitivity; (e) oscillations require modulated light and specific circuitry, not a material property.

Common Pitfalls:

Confusing photoconductors with photovoltaic cells (which generate voltage); neglecting response time and spectral sensitivity that affect dynamic behavior.

Final Answer:

Decreases with increasing light intensity