= Photoconductor
= Photoconductive detector
{synonym}
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.<How does a PbS photoconductive detector work? by Shimadzu Scientific Instruments>[operating-principle paragraphs, band excitation and external readout]{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,
$$
\sigma=e(n\mu_n+p\mu_p),\qquad G=\frac{A\sigma}{\ell},\qquad I=V_bG.
$$
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,
$$
\boxed{\Delta I=V_b\frac{Ae}{\ell}(\mu_n\Delta n+\mu_p\Delta p).}
$$
The assumptions exclude contact barriers, strongly nonuniform illumination and a bias changed by the readout; those effects require an expanded model.
For $\ell=1\,\mathrm{mm}$, $A=10^{-8}\,\mathrm{m^2}$, $\mu_n=0.10\,\mathrm{m^2/(V\,s)}$, $\mu_p=0.05\,\mathrm{m^2/(V\,s)}$ and $\Delta n=\Delta p=10^{18}\,\mathrm{m^{-3}}$, the conductivity increment is about $0.0240\,\mathrm{S/m}$. At fixed $V_b=5\,\mathrm V$, the predicted current increment is about $1.20\,\mathrm{\mu A}$. 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 is
$$
V_{\rm out}=V_s\frac{R_L}{R_d+R_L}.
$$
For $V_s=5\,\mathrm V$ 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 $\tau=100\,\mathrm{\mu s}$, 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.<How does a PbS photoconductive detector work? by Shimadzu Scientific Instruments>[temperature-dependence paragraph]{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.
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