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A photoconductor is a detector whose illuminated material changes electrical conductivity, allowing an external circuit to measure the change under applied bias. It uses photoconductivity: absorbed photons can produce mobile electrons and electron holes, changing the conductive path between contacts. A simple photoresistor reads this as a light-dependent resistance. Shimadzu’s primary description explains band-gap excitation and the resistance decrease in a PbS detector.^[ref]

An original uniform-bar model makes the signal mechanism quantitative. Let the contact separation be $\ell$, the conducting area $A$, electron and hole number densities $n,p$, and their mobilities ${\mu }_n,{\mu }_p$. In the low-field ohmic model,Here $e$ is the positive elementary charge and $V_b$ the actual detector electric potential difference. If illumination changes carrier densities while mobilities stay fixed,The assumptions exclude contact barriers, strongly nonuniform illumination and a bias changed by the readout; those effects require an expanded model.

For $\ell =1\mathrm{mm}$, $A=10^{-8}{\mathrm{m}}^2$, ${\mu }_n=0.10{\mathrm{m}}^2/(\mathrm{V}\mathrm{s})$, ${\mu }_p=0.05{\mathrm{m}}^2/(\mathrm{V}\mathrm{s})$ and $\Delta n=\Delta p=10^{18}{\mathrm{m}}^{-3}$, the conductivity increment is about $0.0240S/m$. At fixed $V_b=5\mathrm{V}$, the predicted current increment is about $1.20\mu \mathrm{A}$. These stipulated parameters illustrate the conversion of extra carriers into a signal, rather than specifications of a named detector.

Readout loading can change that result. In a divider powered at $V_s$, with detector resistance $R_d$ in series with load $R_L$, the output across the load isFor $V_s=5\mathrm{V}$ and $R_L=10\mathrm{k\Omega }$, changing $R_d$ from $100$ to $10\mathrm{k\Omega }$ raises the output from about $0.455$ to $2.50\mathrm{V}$. The detector’s own bias falls from about $4.55$ to $2.50\mathrm{V}$. A light-dependent conductance and a measured divider voltage therefore need not have the same linearity. Record the actual bias and illuminated area when measuring spectral responsivity.

Photoconductor response time also depends on carrier storage and loss. A simple spatially uniform excess-carrier model is $d\Delta n/dt=g(t)-\Delta n/\tau$. After a step to constant generation $g_0$, initially zero excess density becomes $g_0\tau (1-e^{-t/\tau })$. Small sinusoidal modulation has response proportional to $(1+i2\pi f\tau {)}^{-1}$. With stipulated $\tau =100\mu \mathrm{s}$, the single-pole cutoff is about $1.59\mathrm{kHz}$. Real traps, contacts and electronic filters need not yield one such time constant.

A proposed characterization biases the detector through a known readout, measures dark and illuminated currents at controlled wavelength and temperature, then chops or modulates the light and measures amplitude and phase versus modulation frequency. Repeat at several powers and biases to separate detector behavior from circuit loading. Temperature control matters: the manufacturer notes that PbS spectral sensitivity, response speed and dark resistance all change with temperature.^[ref]

The name photoconductive mode for a reverse-biased photodiode describes an operating choice of a junction detector; it does not make every such device the bulk-resistance detector modeled above. Likewise, a detector’s signal increase is not by itself a claim of unlimited useful gain: background response, dark current, noise, saturation and response time remain distinct characterization questions.

Photoconductor response time describes how quickly the electrical signal follows a change in illumination under stated bias, temperature, background and readout conditions. A rise time, fall time and modulation bandwidth summarize different features and require definitions. The manufacturer’s PbS description notes that sensitivity, response speed and dark resistance vary with temperature, so a timing number is not independent of operating conditions.^[ref]

An original one-lifetime model takes excess carrier density $n_e$ to obeyWith constant mobility and fixed detector bias, suppose the excess current is proportional to $n_e$. After a step from zero to generation $g_0$, the normalized signal is $1-e^{-t/\tau }$. The times to reach ten and ninety percent of its final value are $-\tau \ln 0.9$ and $-\tau \ln 0.1$, givingThe time from zero to ninety percent is instead $\tau \ln 10$. A quoted rise time without its thresholds is therefore incomplete.

For small sinusoidal generation changes, the normalized frequency response isThe half-power frequency is $f_c=1/(2\pi \tau )$, so this single-pole model gives $t_{10\text{–}90}{f}_c=\ln 9/(2\pi )\approx 0.3497$. This relationship is a consequence of the stated model, not a universal identity for arbitrary detectors.

For $\tau =100\mu \mathrm{s}$, the rise time is about $219.7\mu \mathrm{s}$ and the cutoff about $1.592\mathrm{kHz}$. At $1\mathrm{kHz}$, the signal amplitude is about $0.8467$ of its low-frequency value and its phase lags by $32.14^{\circ }$. The signal can therefore have a substantial phase delay before reaching the half-power point.

A readout adds its own dynamics. If the detector response is cascaded with a separate first-order electronic response of the same time constant, the overall magnitude is $1/[1+(2\pi f\tau {)}^2]$. Its half-power frequency is $\sqrt{\sqrt{2}-1}/(2\pi \tau )$, about $0.6436$ times the single-pole cutoff. Assigning that combined bandwidth entirely to the material would give an incorrect carrier-lifetime estimate.

Photoconductor rise and fall asymmetry tests another limitation of the simple model: linear one-lifetime dynamics predict equal normalized rise and fall behavior, but trap occupancy, changing background or a nonlinear circuit can produce different transients. A long relaxation tail should be measured rather than compressed into an unsupported single time constant.

A proposed experiment switches or modulates a monitored optical source, records detector current and actual bias, and measures ten-to-ninety and ninety-to-ten crossings after settling the initial illumination state. Repeat the modulation sweep with a characterized faster readout or independently measured amplifier response. Vary temperature and optical background while holding the wavelength and spot fixed. Agreement between step and frequency-response fits tests the one-pole approximation; disagreement identifies missing dynamics without by itself proving which physical mechanism caused them.

A photoresistor, also called a light-dependent resistor or LDR, is a two-terminal component whose resistance changes with incident light. A common photoconductive implementation uses an illuminated semiconductor between contacts; an external circuit converts its resistance change into a voltage or current signal. A manufacturer’s CdS datasheet explicitly identifies these names and describes resistance decreasing with visible illumination.^[ref]

The photoconductor mechanism is a change in conductivity through generated mobile carriers. A photoresistor’s resistance is consequently not a calibrated unit of optical power by itself. Material, wavelength distribution, temperature, illumination history and the measuring circuit help determine the observed response.

For an original illustrative model over a limited range, let $R(P)=R_0(P/P_0{)}^{-\gamma }$ with $\gamma >0$, fixed spectrum and geometry. Choose $R_0=20\mathrm{k\Omega }$ and $\gamma =0.7$. Four times the reference optical input gives $R\approx 7.579\mathrm{k\Omega }$, rather than one quarter of the original resistance. This stipulated power law illustrates a nonlinear response, not a fitted specification for the cited part.

In a divider with supply $V_s$ and fixed resistor $R_L$, measuring across $R_L$ givesAt $V_s=3.3\mathrm{V}$ and $R_L=10\mathrm{k\Omega }$, the model output rises from $1.10\mathrm{V}$ at $P_0$ to about $1.877\mathrm{V}$ at $4P_0$. The logarithmic sensitivity is greatest when $R(P)=R_L$, where it is $\gamma V_s/4$. Choosing a resistor therefore selects an operating region; it does not remove the material’s response law.

The electrical input must also be characterized. The divider seen at its output has Thevenin resistance $R(P)\parallel R_L$. A finite-input-resistance voltmeter or an analog-to-digital converter with a switched input can change the reading. Self-heating depends on the detector’s actual voltage and resistance, while photoconductor response time can make a rapidly sampled value differ from its settled value.

Photoresistor spectral calibration is essential when interpreting an illumination reading. Lux is photometrically weighted, so equal illuminance from different spectra need not produce equal absorption or equal resistance. The cited manufacturer specifies source conditions for light-resistance measurements, a waiting time for dark resistance and light-adaptation preconditioning before its tests.^[ref] Those conditions describe what was tested; they should not be silently omitted from a comparison.

A proposed characterization uses a stable monitored light source, fixed detector position and a controlled spectrum, stepping illumination in both directions and recording the settled resistance and transient. Measure actual bias and temperature, then test the intended readout separately. A component can work well as a light/dark threshold sensor while requiring substantially more characterization to estimate optical power quantitatively.