Responsivity
Author: the photonics expert Dr. Rüdiger Paschotta (RP)
Definition: photocurrent per unit optical power incident on a photodetector
Alternative term: radiant sensitivity
Category: 
Related: photodetectorsphotodiodesnoise-equivalent powersensitivitydetectivity
Units: A/W
Formula symbol: ($R$)
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DOI: 10.61835/xal Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
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What is a Responsivity?
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 band gap, 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. The 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.
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.
For photoconductive detectors, the responsivity depends strongly on the applied bias voltage, because both the quantum efficiency and the photoconductive gain depend on that voltage.
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.
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
What is the responsivity of a photodetector?
The responsivity of a photodetector, such as a photodiode, is the ratio of the generated photocurrent to the incident optical power. Its typical units are amperes per watt (A/W).
How is a photodiode's responsivity related to its quantum efficiency?
Responsivity ($R$) is proportional to the quantum efficiency ($\eta$) and inversely proportional to the photon energy ($h\nu$). The formula is ($R = \eta \frac{e}{{h\nu }}$), where ($e$) is the elementary charge.
Why does the responsivity of a photodiode depend on the light's wavelength?
Responsivity is wavelength-dependent because both the quantum efficiency and the photon energy vary with wavelength. For a photodiode, the responsivity drops sharply for photon energies below the band gap energy, where light is no longer absorbed.
Can a photodetector have a responsivity greater than 1 A/W?
Yes, detectors with an internal current multiplication mechanism, such as avalanche photodiodes and photomultipliers, can exhibit responsivity values far greater than 1 A/W.
What is the difference between responsivity and sensitivity?
Responsivity is a well-defined quantitative measure (e.g., in A/W or V/W). The term 'sensitivity' is often used as a synonym, but this is not recommended because 'sensitivity' can also refer to other metrics, such as the minimum detectable optical power.
Suppliers
Sponsored content: The RP Photonics Buyer's Guide contains 131 suppliers for photodetectors. Among them:
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.
FEMTO offers a wide range of very low-noise photodetectors with bandwidths up to 2 GHz and gains up to 1012 V/W. Intensities ranging from fW to mW can be measured with an NEP down to 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. Customized models are available upon request.
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.
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.
Qubitrium offers the InGaAs single-photon detector QDC-T4-00 with the following specs:
- Dark count: 5000 cps
- Timing jitter: 1000 ps
- Wavelength range: 1000–1650 nm
- Peak PDE: > 20% @ 1550 nm
- CMOS, TTL compatible
- Active area: 16 µm diameter
- Fiber: SMF/MMF
- Output: SMA
- Operation mode: CW/gated
The ULN-PDB module is a plug-and-play ultralow noise balanced photodetector in a compact and user-friendly package. It offers the best performances in terms of signal-to-noise ratio. ULN-PDB is proposed with InGaAs or Si photodiodes (with FC connectors) and offers a bandwidth of 100 MHz (adjustable on demand) with a high gain of 39 kV/A (adjustable on demand) in a DC- or AC-coupled configuration.
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.
AeroDIODE offers a choice of high-speed Si photodiodes and InGaAs photodiodes, available in free-space or fiber-optic coupled configurations, along with associated high-speed photodiode amplifiers that integrate the photodetector. The InGaAs IR detector combines high sensitivity, ultra-low dark current, and fast response up to a bandwidth of 10 GHz. The Silicon PIN photodiode is available in three models, including a free-space sensor with an extended infrared detection range and increased sensitivity.
Gentec Electro-Optics offers a great range of power detectors based on silicon or germanium photodiodes for powers up to 750 mW.
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.
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.