Photodiode operation in photoconductive mode

A photodiode is a semiconductor diode that produces a current when light generates mobile charge carriers within it. The carriers are electrons and holes: holes are places where electrons are missing and can behave as mobile positive charges.

In photoconductive mode, the photodiode is connected with a reverse bias. The resulting electric field separates the light-generated electrons and holes, driving them in opposite directions. Their movement produces a photocurrent in the external circuit.

Greater incident light intensity generates more mobile carriers, increasing the magnitude of the reverse current. The current can therefore indicate changes in light intensity in an optical system. A small reverse current can also flow without illumination; this is called the dark current.

Photodiode current–voltage characteristics

A current–voltage characteristic shows how the current through the photodiode depends on the voltage across it. To see the effect of light, a family of curves can show the characteristic at several illumination levels.

Compare the curves at the same reverse-bias voltage: the magnitude of the reverse current is greater at higher illumination. The curve for no illumination shows the dark current. Whether reverse voltage and reverse current appear as positive or negative values depends on the graph’s sign convention, so the direction of any shift on the graph must be read from its labelled axes.

Photodiode spectral response

A photodiode produces an electric current when it detects light. Its spectral response describes how sensitive it is to light of different wavelengths. If the same optical power falls on the photodiode at two different wavelengths, it may produce a larger photocurrent at one wavelength than at the other.

A spectral response curve plots wavelength on the horizontal axis and sensitivity or responsivity on the vertical axis. The curve shows the wavelength range over which the device responds. Its peak identifies the wavelength at which the plotted response is greatest.

Responsivity and relative sensitivity

Responsivity compares the photocurrent produced with the incident optical power at a particular wavelength:

Here, $$R$$ is responsivity, $$I_{\text{ph}}$$ is photocurrent and $$P_{\text{incident}}$$ is incident optical power. Responsivity is measured in amperes per watt ($$\mathrm{A/W}$$). A greater responsivity means more photocurrent is produced for the same incident power.

Some curves show relative sensitivity instead of responsivity. They express the response at each wavelength as a fraction or percentage of the maximum response. Such a curve shows how the response changes with wavelength, but it does not by itself give the absolute photocurrent per watt.

Why the response varies with wavelength

Light transfers energy in photons. For a photon to generate charge carriers in the photodiode, it must have sufficient energy. Photon energy decreases as wavelength increases, so beyond a sufficiently long wavelength the photons do not have enough energy and the response falls to zero.

Having enough photon energy does not guarantee an equally strong response at every shorter wavelength. The response also depends on how effectively the photodiode absorbs light and collects the charge carriers produced. These factors help determine the shape and peak of the spectral response curve.

Scintillation detection with a photodiode

A particle can be detected by converting some of the energy it deposits into light, then converting that light into an electrical signal. The two components that perform these steps are a scintillator and a photodiode.

When an incident particle interacts with the scintillator, it deposits energy in the material. The scintillator converts some of this energy into a brief flash of light, called a scintillation. The photodiode detects some of the light and converts it into an electrical signal.

The signal indicates that a particle has interacted with the scintillator. Its size can also provide information about the energy deposited: a larger deposited energy generally produces more light and a larger signal. The detector must be suitably calibrated, so that signal sizes can be related to deposited energies. The energy deposited is not necessarily the particle’s total incident energy.