Photodiode

A particle detector doesn't "see" a particle pass through it directly — it sees the tiny, fleeting flash of light that particle triggers in a scintillator, and turns that flash into a measurable electrical pulse. The device that does the converting, light in and electrical signal out, is the photodiode.

A light-sensitive semiconductor device

A photodiode is a semiconductor device that responds to light falling on it by generating or modulating an electrical signal. Photodiodes can be operated in more than one way, but this lesson focuses on photoconductive mode — the device connected in reverse bias — since this is the mode used when a photodiode is acting as an optical detector, the role that matters for the applications covered here.

The underlying physical idea is straightforward: incident photons, provided they carry enough energy, generate electron-hole pairs inside the semiconductor. Under the reverse-bias electric field, these charge carriers are swept apart and collected, producing a measurable current. The brighter the light falling on the device, the more electron-hole pairs are generated each second, and so the larger the resulting current — the device's electrical output tracks the light striking it.

The characteristic curve

In photoconductive (reverse-biased) mode, a photodiode's current-voltage characteristic looks different from an ordinary diode's. In darkness, only a small, roughly constant reverse (leakage) current flows. As light falling on the device becomes more intense, the reverse current increases significantly and roughly proportionally to that intensity. Plotting a family of characteristic curves for several different light intensities shows this clearly: each curve sits at a higher current level than the last as illumination increases, and — usefully — the current for a given intensity stays fairly constant across a wide working range of reverse voltage, rather than depending sensitively on the exact voltage applied.

This combination — output current set mainly by light intensity rather than by the exact operating voltage — is what makes a reverse-biased photodiode behave as a reliable, predictable light sensor rather than just another diode.

Spectral response

A photodiode isn't equally sensitive to every wavelength (colour) of light. Its spectral response curve plots the device's sensitivity against wavelength, and typically shows a peak response at some particular wavelength or narrow range of wavelengths, with sensitivity falling off on either side of that peak. In practice this means a given photodiode responds strongly to light of certain colours and weakly to others.

This matters directly when choosing or using a photodiode in a real application: a photodiode should be matched to the wavelength of light it is intended to detect — for instance, matched to the particular colour of light emitted by whatever light source or light-emitting process it's paired with — otherwise a substantial fraction of the light reaching it may fall outside its region of good sensitivity and produce a disappointingly weak signal.

Application: photodiode with a scintillator, for particle detection

One important application combines a photodiode with a scintillator — a material that emits a brief flash of light when struck by an ionising particle. On its own, a scintillator only produces a flash of light; it needs a way to convert that flash into an electrical signal that can be recorded, counted, or processed electronically. A photodiode, positioned so that light from the scintillator falls on it, does exactly that: each flash from the scintillator generates a corresponding pulse of current from the photodiode.

The result is an electronic detection system: an ionising particle strikes the scintillator, the scintillator emits a flash of light, and the photodiode converts that flash into an electrical pulse that can be counted or timed. Counting these pulses gives a way of counting individual particle interactions — the basis of many practical particle-detection instruments.

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