What Is Photoconductivity? (Physics Guide)
Photoconductivity is a change in a material’s electrical conductivity caused by light. When a material absorbs photons, it may create mobile electrons and holes, which carry current. The size and speed of the response depend on the material, wavelength, illumination, temperature, and electric field. Measuring it well means separating light-driven conductivity from heating and contact effects.
Light can make some materials conduct more easily, but a brighter current reading alone does not prove photoconductivity. The same reading may result from heat, a light-generated voltage, or a change at an electrical contact. Knowing the difference helps you interpret lab data and understand how light sensors work.
I use a simple sequence to analyze the effect: identify what light the material absorbs, trace the resulting charge carriers, then compare its electrical response in the dark and under controlled illumination. The equations below connect those steps.
Photoconductivity: Definition, Mechanism, and Measurement
Photoconductivity is a light-induced change in a material’s electrical conductivity. In many semiconductors, absorbed light creates mobile electrons and holes, so conductivity rises. The effect is measured by comparing electrical behavior in darkness and light under stated conditions, including wavelength, applied bias, temperature, and sample geometry.
How light changes conductivity
A photon is a packet of light energy. When a material absorbs a photon with enough energy, it can move an electron into a state where it can move through the material. This leaves behind a positively charged vacancy called a hole. Both electrons and holes can carry current.
The basic sequence is:
- The material absorbs light.
- Absorption creates charge carriers, or frees carriers trapped in the material.
- An electric field moves those carriers through the sample.
- Carriers recombine or become trapped over time.
- The number and mobility of carriers determine the conductivity change.
Conductivity, written as (\sigma), describes how readily charge flows through a material. Its SI unit is siemens per metre, or S·m⁻¹. For electrons and holes, a common model is:
[ \sigma=q(n\mu_n+p\mu_p) ]
Here, (q) is the elementary charge, (n) and (p) are electron and hole concentrations, and (\mu_n) and (\mu_p) are their mobilities. Mobility describes how readily a carrier moves in response to an electric field.
The light-induced change is:
[ \Delta\sigma=q(\Delta n\mu_n+\Delta p\mu_p) ]
In an intrinsic or weakly doped material under simple conditions, light may create electron–hole pairs in roughly equal numbers, so (\Delta n\approx\Delta p). That approximation does not apply to every material. Doping, defects, trapping, and complex transitions can make the carrier changes unequal.
How researchers measure the effect
A basic measurement compares current before, during, and after illumination. Keep the applied voltage fixed, record the dark current, illuminate the sample, and record the new current. A reversible change that tracks the light is evidence of a light response, but more checks are needed to identify its cause.
For a uniform sample shaped like a bar, conductivity can be estimated from resistance:
[ \sigma=\frac{L}{RA} ]
Here, (L) is the distance between voltage contacts, (A) is the cross-sectional area, and (R) is the measured resistance. This estimate assumes the sample is uniform and the contacts do not dominate the measurement.
Report the sample dimensions and contact layout. Also record wavelength, optical power at the sample, bias voltage, temperature, and response time. Without these conditions, two reported values may not be meaningfully comparable.
Carrier Physics, Key Equations, and Material Requirements
Photoconductive response depends on how light interacts with a material and what happens to the carriers afterward. Photon energy must match an allowed absorption process, while carrier mobility and lifetime shape the current. These factors explain why two materials can respond very differently to the same lamp.
Photon energy and the absorption edge
Photon energy depends on wavelength:
[ E_\gamma=\frac{hc}{\lambda} ]
Here, (h) is Planck’s constant, (c) is the speed of light, and (\lambda) is wavelength. Shorter wavelengths carry more energy per photon. For band-to-band absorption, a photon generally needs at least the material’s bandgap energy, (E_g), to create an electron–hole pair.
A useful estimate of the corresponding wavelength cutoff is:
[ \lambda_g\approx\frac{1.24}{E_g}\ \mathrm{\mu m} ]
Use (E_g) in electronvolts. For example, if a material’s bandgap is (2.0) eV, the estimated cutoff is (0.62) µm. This is a guide, not a promise of a sharp cutoff. Defects, excitons, and indirect transitions can alter the measured response.
Lifetime, transit time, and gain
Carrier lifetime, (\tau_{\text{life}}), is the typical time a carrier remains mobile before recombination or trapping. Transit time, (\tau_{\text{transit}}), is the time a carrier takes to cross the active region. A common idealized estimate of photoconductive gain is:
[ G\approx\frac{\tau_{\text{life}}}{\tau_{\text{transit}}} ]
Both times must use the same units. If carriers persist much longer than they take to cross the sample, one absorbed photon can support more than one circuit charge passing through the device. Gain above one can boost signal, but a long lifetime often slows the response and may increase noise or persistent after-effects.
Response metrics and what they mean
Responsivity is photocurrent divided by incident optical power:
[ R=\frac{I_{\text{ph}}}{P_{\text{opt}}} ]
It is measured in A·W⁻¹. Here, (I_{\text{ph}}) is the light-related current after the dark-current contribution is removed, and (P_{\text{opt}}) is the optical power reaching the active area.
External quantum efficiency (EQE) estimates the number of collected charge carriers per incident photon. For a simple single-carrier photodiode:
[ \mathrm{EQE}=\frac{Rhc}{q\lambda} ]
Use consistent units. EQE is often reported as a percentage. Responsivity and EQE are not interchangeable: responsivity depends on wavelength, while EQE expresses a carrier-per-photon ratio. For a photoconductor with gain, EQE can exceed 100% because one absorbed photon may support multiple charges through the circuit.
| Quantity | Meaning | Unit or relationship | What to report |
|---|---|---|---|
| Conductivity, (\sigma) | Ease of charge flow | S·m⁻¹ | Geometry and temperature |
| Photoconductivity, (\Delta\sigma) | Light-related conductivity change | S·m⁻¹ | Dark and illuminated conditions |
| Responsivity, (R) | Photocurrent per optical power | A·W⁻¹ | Wavelength and optical power |
| EQE | Collected carriers per incident photon | Ratio or percent | Wavelength and device conditions |
| Gain, (G) | Idealized carrier multiplication in the circuit | (\tau_{\text{life}}/\tau_{\text{transit}}) | Bias and response speed |
Experimental Methods, Learning Milestones, and Common Pitfalls
A sound photoconductivity test changes one condition at a time and records enough information to repeat the result. The main challenge is separating carrier generation from heating, photovoltaic voltage, and contact effects. A step-by-step method makes the conclusion more reliable than a single current reading.
A controlled measurement, step by step
- Record the sample. Note material, dimensions, contact spacing, electrode layout, and temperature. These details affect resistance and current.
- Choose the electrical condition. Apply a known voltage, or use a known current, and keep it steady. Do not compare measurements taken at different bias without accounting for the change.
- Measure the dark state. Shield the sample from light and record current after it settles. This is the dark current.
- Set the illumination. Record wavelength and optical power at the sample, not just the lamp’s rated output. Keep the beam position and illuminated area consistent.
- Track the response over time. Record current during illumination and after the light is removed. A slow rise or decay can reveal trapping or long carrier lifetimes.
- Check for other causes. Monitor sample temperature, test more than one wavelength if possible, and check whether contact behavior changes with light.
- Repeat the cycle. Repeated dark-light-dark readings can show whether the response is stable and reversible.
For a simple fixed-voltage test, the light-related current is often estimated as (I_{\text{ph}}=I_{\text{light}}-I_{\text{dark}}). This is an operational measure, not proof of the mechanism. If illumination heats the sample, conductivity may change even without direct carrier generation.
Illustrative measurement cases
These examples show how to reason about results; they are not reports of specific experiments.
| Observation | Possible interpretation | Useful next check |
|---|---|---|
| Current rises under light and returns toward baseline in darkness | A reversible light response is present | Check wavelength dependence and sample temperature |
| Current rises slowly and remains high after the light is removed | Trapping or long-lived carriers may be involved | Measure rise and decay times over repeated cycles |
| Current changes as the sample warms | Heating may explain some or all of the response | Stabilize temperature or compare at matched temperatures |
| Current changes when the light reaches a contact | A contact barrier may affect the reading | Alter contact geometry or illuminate the active region separately |
| A voltage appears with no external bias | A photovoltaic contribution may be present | Measure open-circuit voltage and short-circuit current separately |
A photovoltaic effect and photoconductivity are related but distinct. Photoconductivity means light changes conductivity. The photovoltaic effect means light produces an electrical potential or current in a device. A device can show more than one effect, so the measurement setup matters.
Common interpretation errors
- Treating any current increase as proof: Heating and contact changes can also raise current.
- Comparing responsivity without wavelength: Photon energy and absorption vary with wavelength.
- Ignoring geometry: Resistance depends on sample dimensions and contact placement.
- Assuming more gain is always better: Gain can trade off with speed, noise, or recovery time.
- Reporting lamp settings as sample illumination: Optical power reaching the active region may differ from the source rating.
Applications, Device Performance, and Career Connections
Photoconductivity is used when a material’s electrical response to light can serve as a signal. Common examples include light sensors and photodetectors. Device performance depends on more than sensitivity: wavelength range, speed, noise, dark current, and operating bias all matter.
A photoconductive detector converts a change in illumination into a change in current or resistance. One material may be useful for detecting a particular wavelength, while another may respond faster or work better under a different operating condition. The best choice depends on the task, not on one headline number.
When evaluating a device, ask:
- What wavelength range does it detect?
- What optical power and illuminated area were used?
- What bias voltage was applied?
- What are the dark current and noise level?
- How quickly does the signal rise and return to baseline?
- Were temperature and device geometry reported?
These questions also guide coursework and lab work in semiconductor physics, materials science, and electrical engineering. A strong analysis links the measured signal to carrier generation and transport, then states the limits of the evidence. The most useful result is not simply “the current increased,” but a reproducible description of how much it changed, under what conditions, and which mechanisms could explain it.
Conclusion and FAQs
Photoconductivity connects light absorption to charge transport. To interpret it, follow the carriers, use conductivity and response equations with correct units, and compare dark and illuminated behavior under controlled conditions. Report wavelength, power, bias, geometry, temperature, and timing so others can judge the result.
What is photoconductivity in simple terms?
It is a change in a material’s ability to conduct electricity when light shines on it.
What causes photoconductivity?
Absorbed light can create or release mobile charge carriers, such as electrons and holes. Those carriers can increase current under an applied electric field.
Does photoconductivity always increase current?
No. The response depends on the material and conditions. Light may also cause heating or change contact behavior, which can alter the measured current.
How is photoconductivity measured?
Measure the sample’s electrical response in darkness and under controlled illumination, while keeping bias and other conditions known. Record wavelength, optical power, temperature, geometry, and response time.
What is the difference between photoconductivity and the photovoltaic effect?
Photoconductivity is a light-induced change in conductivity. The photovoltaic effect is the creation of electrical voltage or current by light in a device.
What does the bandgap tell me?
The bandgap helps estimate the photon energy needed for band-to-band absorption. The approximate wavelength cutoff is (\lambda_g\approx1.24/E_g) µm when (E_g) is in eV.
What is photoconductive gain?
It is an estimate of how much circuit charge can flow per absorbed photon. A simple model uses carrier lifetime divided by transit time, but higher gain can reduce speed or increase noise.
What is responsivity?
Responsivity is photocurrent divided by incident optical power. It is measured in A·W⁻¹ and should be reported with the wavelength and test conditions.
Why must wavelength be reported?
A material’s absorption changes with photon energy. Responsivity and other response measures can therefore differ at different wavelengths.
Why can two studies report different values?
They may use different materials, wavelengths, illumination levels, bias voltages, temperatures, geometries, or measurement methods. Comparing values requires matching or accounting for those conditions.
(This article was written by one of our staff writers, Nathan Sterling. Visit our Meet the Team page.)
