PHOTOCONDUCTIVITY AND PHOTOCONDUCTIVE MATERIALS: From Semiconductor Physics to Detector Design: Theory, Measurement, and Worked Examples

Everything you need to build, measure, and trust a photoconductive detector.

A photoconductor is a slab of semiconductor with two contacts. Bias it, shine light on it, and the current rises. That's the whole device — no junction, no depletion region, nothing to complicate the analysis.

So why does the same material give you a gain of ten thousand in one sample and a gain of one in the next? Why does a response that's beautifully linear over six decades go sublinear the moment you raise the power? Why does a lifetime measurement disagree with the datasheet by an order of magnitude?

Because the behavior is set by defects — states present at one part in ten million — and most treatments stop long before they explain how.

Photoconductivity and Photoconductive Materials is a complete working treatment of the subject, built for engineers and researchers who have to produce numbers rather than descriptions. It follows the four processes every photoconductive device executes — absorption, generation, transport, recombination — and develops each one to the point where you can calculate with it.

By the end you can:

Predict responsivity, gain, and bandwidth from material parameters

Compute a noise floor and compare detectors on specific detectivity

Diagnose an unexpected spectral roll-off or an injection artefact

Tell whether your measurement reflects the bulk material at all

The physics: optical absorption and the absorption edge · quasi-Fermi levels, drift, diffusion, and the continuity equations · the steady-state photoconductivity equation and the mobility–lifetime product · Shockley–Read–Hall recombination · traps, demarcation levels, and sensitization · transient response, time-of-flight, and dispersive transport · contacts and space-charge-limited current · spectroscopy · Johnson, generation–recombination, shot, and 1/f noise.

The materials: silicon and germanium, intrinsic and extrinsic · the cadmium chalcogenides · lead salts · mercury cadmium telluride · quantum well infrared photodetectors · amorphous selenium and hydrogenated amorphous silicon · organics, halide perovskites, colloidal quantum dots, and 2D photogating.

The practice: how to measure a photoconductor without generating artefacts, a catalogue of the artefacts themselves, uncertainty budgeting, and four end-to-end design case studies — a light-level sensor, a mammography panel, a terahertz emitter, and a cooled infrared focal-plane array.

Every chapter carries fully worked examples with unit tracking at every step, objective questions built around the misconceptions that actually trip people up, analytical problems, and a complete answer key that explains the reasoning rather than just marking the box.

Reference material at the back: symbol list, glossary, physical constants, material property tables, mathematical methods, unit conversions, and a full index.

Who it's for: upper-level undergraduates reading it straight through, graduate students entering optoelectronics research, and practicing engineers who need to open one chapter and follow every assumed result back to where it was derived. You need electrostatics, basic quantum mechanics, first-order differential equations, and circuit analysis. You don't need any prior exposure to recombination statistics, defect physics, or noise analysis — those are built from the ground up.

The field started in 1873 with a submarine telegraph cable, a selenium resistor, and an engineer who noticed that his readings changed when someone opened the shutters. It runs today through every thermal imager and X-ray panel in service.

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