Responsivity
Author: the photonics expert Dr. Rüdiger Paschotta (RP)
Definition: photocurrent per unit optical power incident on a photodetector
Alternative term: radiant sensitivity
Category:
DOI: 10.61835/xal Cite the article: BibTex plain textHTML Link to this page LinkedIn
The responsivity (or radiant sensitivity) of a photodiode or some other kind of photodetector is the ratio of generated photocurrent and incident (or sometimes absorbed) optical power (neglecting noise influences), determined in the linear region of response. In the case of photodiodes, the responsivity is typically highest in a wavelength region where the photon energy is somewhat above the band gap energy, and declining sharply in the region of the bandgap, where the absorption decreases. It can be calculated according to
$$R = \eta \frac{e}{{h\nu }}$$where <$h \nu$> is the photon energy, <$\eta$> is the quantum efficiency, and <$e$> the elementary charge. From this, one sees that the result units of <$R$> are C/J = A/W; the latter is most common.
For example, a silicon photodiode with 90% quantum efficiency at a wavelength of 800 nm, the responsivity would be ≈ 0.58 A/W. Values for other types of photodiode are basically always of that order of magnitude.
For avalanche photodiodes and photomultipliers, there is an additional factor for the internal current multiplication, so that values far above 1 A/W are possible. Note that the current multiplication is usually not subsumed in the quantum efficiency.
Note that the term responsivity cannot be directly applied to photoconductive detectors (photoresistors), where the response to incident light is not a photocurrent proportional to the incident intensity but rather an increase of conductivity – often with a quite nonlinear behavior.
The responsivity is usually defined for the steady state. The photodiode response typically falls off for signal frequencies above some detection bandwidth.
The term sensitivity is often used instead of responsivity, but that is not recommended, since the term can also have other meanings. It should be avoided particularly when a clear quantitative meaning is intended.
The responsivity is usually meant to be a wavelength-dependent quantity and thus considered for monochromatic light. However, one may consider an effective responsivity for non-monochromatic light with a certain spectral bandwidth.
A photodetector should ideally be operated in a spectral region where its responsivity is not far below the highest possible value because this leads to the lowest possible detection noise and thus to a high signal-to-noise ratio and high sensitivity.
If some detector has a voltage rather than a current output, one can define its responsivity as the ratio of output voltage and optical power. This leads to units of V/W (volts per watt). If a photodiode is combined with some detector electronics generating a voltage output, the output voltage is the photocurrent times the so-called transimpedance of the electronics. In the simplest case, one uses a shunt resistor, and the transimpedance is then its resistance.
Thermal detectors usually have a responsivity with a weak wavelength dependence in a broad spectral range – in contrast to photodetectors like photodiodes, where the responsivity typically drops sharply for photon energies around the band gap energy.
More to Learn
Encyclopedia articles:
Suppliers
The RP Photonics Buyer's Guide contains 118 suppliers for photodetectors. Among them:
CSRayzer Optical Technology
CSRayzer CR2000AH-1550-70M includes a 200 μm InGaAs avalanche photodiode and a hybrid preamplifier for the use in high speed, ultra-low light detection, in laser range finding, LIDAR and free space communications.
NLIR

Our D2250 2.2–5.0 µm single wavelength detector is super-fast (up to 10 GHz bandwidth) and ultra-sensitive (NEP of a few fW/√Hz).
Our D2250’s are based upon a new measuring paradigm where mid-infrared light is converted into near-visible light and then measured with conventional near-visible detector technology.
Gentec Electro-Optics

Gentec Electro-Optics offers a great range of power detectors based on silicon or germanium photodiodes for powers up to 750 mW.
Menlo Systems

Menlo Systems offers a series of photodetectors for lowest light level signals. From avalanche to PIN photodiodes, you can find the detector that is best for your specific application.
Analog Modules

The 710 Series are high sensitivity, low noise photodetector-amplifier modules that offer the flexibility of incorporating a variety of silicon or InGaAs, PIN or APD photodetectors. AMI also offers the capability of integrating other detectors on a custom basis.
Hamamatsu Photonics

Hamamatsu Photonics is a leading manufacturer of a very wide range of photodetectors, essential components in a vast array of modern scientific and commercial instruments and devices.
ALPHALAS

Ultrafast photodetectors from ALPHALAS for measurement of optical waveforms with rise times starting from 10 ps and total spectral coverage from 170 to 2600 nm (VUV to IR) have bandwidths from DC up to 30 GHz. Configurations include free-space, fiber receptacle or SM-fiber-pigtailed options and have compact metal housings for noise immunity. The UV-extended versions of the Si photodiodes are the only commercial products that cover the spectral range from 170 to 1100 nm with a rise time < 50 ps. For maximum flexibility, most models are not internally terminated. A 50 Ohm external termination supports the highest speed operation, while a high impedance load generates large amplitude signals. Applications include pulse form and duration measurement, mode beating monitoring and heterodyne measurements. Balanced photodiodes complement the large selection of more than 70 unique models.
FEMTO Messtechnik

FEMTO offers a wide range of very low-noise photodetectors. They offer bandwidths up to 2 GHz and gains up to approximately 1012 V/W, resulting in measurement ranges from fW to mW. The NEP reaches a minimum of 0.7 fW/√Hz. There is a choice of Si or InGaAs photodiodes covering a wavelength range from 190 nm to 1700 nm, with either a free space or fiber input (FC/PC, FC/APC). Many types and models are switchable in gain and bandwidth and can be controlled manually or remotely via PC. Customized models are available upon request.
2020-12-06
Consider, for example, the responsivity of a PIN photodiode at 1.3 μm and 1.55 μm, in both cases with a quantum efficiency of 80%. Why is the photodiode more responsive at 1.55 μm?
The author's answer:
This is because in light with the longer wavelength you have a larger number of photons per joule of optical energy.