Types of Photodetectors: Photodiodes, APDs, Single-Photon and Thermal Detectors Compared

Scope

This article compares the photodetectors used in photonics and gives the questions that decide between them: the wavelength, how much light there is, how fast the signal changes, and whether single photons must be counted. The underlying quantities are defined in the responsivity and quantum efficiency entries, and the measurement of a detector's responsivity, noise and bandwidth in Photodetector Characterization and How to Measure Photodetector Bandwidth.

The main families

Photodiodes: material sets the wavelength

A photodiode absorbs only photons with more energy than its bandgap, so the material fixes the long-wavelength limit: about 1100 nm for silicon, about 1700 nm for standard InGaAs lattice-matched to InP, and about 1800 nm for germanium. Within its range, a photodiode's responsivity grows in proportion to wavelength at constant quantum efficiency, reaching $\lambda/1.24$ A/W (λ in μm) for a detector that converts every photon; that ceiling is 1.25 A/W at 1550 nm, and good InGaAs detectors reach about 0.9 A/W. Silicon peaks near 900 nm and falls steeply toward its bandgap, which is why silicon is weak at 1064 nm and blind at 1310 nm.

The structure then sets speed and noise. A PIN photodiode adds an undoped absorption layer that is fully depleted under bias; a thin layer and small area give high bandwidth at the cost of responsivity and collection area. The UTC photodiode lets only electrons cross the depletion region, which raises both speed and the photocurrent it can handle before space-charge effects slow it. Germanium discrete detectors have far higher dark current than InGaAs, which limits them at low light.

Avalanche photodiodes: gain with a noise cost

An APD biased near breakdown multiplies each photogenerated carrier by impact ionization, with mean gain $M$. The gain lifts the signal above the noise of the following amplifier, which improves sensitivity when amplifier noise dominates. The multiplication is random, and its excess noise grows with gain; the growth is much slower in silicon, where electrons ionize far more readily than holes, than in the InP multiplication layers of InGaAs APDs. Each APD therefore has an optimum gain, above which the excess noise outweighs the benefit, and the gain depends strongly on temperature, so bias is usually temperature-compensated.

Single-photon detectors

SPADs are APDs biased above breakdown, so a single carrier triggers a self-sustaining avalanche that is then quenched. Each detection is followed by a dead time during which the detector is blind, which limits the count rate, and a trapped carrier can release later and trigger an afterpulse. Silicon SPADs are efficient in the visible; InGaAs SPADs cover the telecom bands with lower efficiency and higher dark counts.

SNSPDs are thin superconducting wires biased just below their critical current; an absorbed photon breaks the superconductivity locally and produces a voltage pulse. They reach system detection efficiencies above 90% at 1550 nm with very low dark count rates and timing jitter of tens of picoseconds or less, but need cooling to a few kelvin.

Photomultipliers convert photons to electrons at a photocathode and multiply them through a chain of dynodes. They combine single-photon sensitivity with large areas and nanosecond response, and their photocathodes cover the UV and visible well; their quantum efficiency in the near infrared is low.

Thermal detectors

Thermal detectors respond to the heat deposited by absorbed light rather than to individual photons, so their response is nearly independent of wavelength across a wide range. A thermopile measures average power, from milliwatts to kilowatts, and responds in about a second. A pyroelectric detector responds only to changes in temperature, so it measures the energy of individual pulses or the power of a chopped beam. Bolometers, including the microbolometer arrays of thermal cameras, measure the change in a resistance. Thermal heads are the usual choice for high-power lasers and for wavelengths outside the range of any photodiode; the optical power meter entry compares them with photodiode heads.

Choosing a detector

1. Wavelength. It excludes most options at once: silicon up to about 1000 nm, InGaAs from 1000 to 1700 nm, thermal detectors wherever the wavelength is unusual or the band is broad.
2. Light level. Milliwatts and above call for a photodiode or a thermal head; nanowatts for a photodiode with a low-noise transimpedance amplifier or an APD; single photons for a SPAD, SNSPD or photomultiplier.
3. Speed. The required bandwidth sets the detector's size and structure. A large-area detector collects light easily but is slow; a detector fast enough for tens of gigahertz is a few tens of micrometres across or waveguide-coupled.
4. Area and coupling. Free-space beams need an active area larger than the beam; fiber-coupled detectors trade area for speed and capture the fiber's light fully.
5. Timing. For time-of-flight and time-correlated counting, the jitter of the detector and its electronics matters as much as its efficiency.

References: S. M. Sze and K. K. Ng, Physics of Semiconductor Devices (3rd ed., Wiley, 2007), chapter on photodetectors; B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics (3rd ed., Wiley, 2019), chapter on photodetectors; R. H. Hadfield, "Single-photon detectors for optical quantum information applications," Nature Photonics 3, 696 (2009).