Photodiode
![]() One Ge (top) and three Si (bottom) photodiodes | |
Type | Passive, diode |
---|---|
Working principle | Converts light into current |
Pin configuration | anode and cathode |
Electronic symbol | |
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A photodiode is a semiconductor
Principle of operation
A photodiode is a PIN structure or p–n junction. When a photon of sufficient energy strikes the diode, it creates an electron–hole pair. This mechanism is also known as the inner photoelectric effect. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in electric field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. The total current through the photodiode is the sum of the dark current (current that is passed in the absence of light) and the photocurrent, so the dark current must be minimized to maximize the sensitivity of the device.[2]
To first order, for a given spectral distribution, the photocurrent is linearly proportional to the irradiance.[3]
Photovoltaic mode
![](http://upload.wikimedia.org/wikipedia/commons/thumb/0/06/Photodiode_operation.png/350px-Photodiode_operation.png)
In photovoltaic mode (zero
Photoconductive mode
In photoconductive mode the diode is reverse biased, that is, with the cathode driven positive with respect to the anode. This reduces the response time because the additional reverse bias increases the width of the depletion layer, which decreases the junction's capacitance and increases the region with an electric field that will cause electrons to be quickly collected. The reverse bias also creates dark current without much change in the photocurrent.
Although this mode is faster, the photoconductive mode can exhibit more electronic noise due to dark current or avalanche effects.[4] The leakage current of a good PIN diode is so low (<1 nA) that the Johnson–Nyquist noise of the load resistance in a typical circuit often dominates.
Related devices
Avalanche photodiodes are photodiodes with structure optimized for operating with high reverse bias, approaching the reverse breakdown voltage. This allows each photo-generated carrier to be multiplied by avalanche breakdown, resulting in internal gain within the photodiode, which increases the effective responsivity of the device.[5]
![](http://upload.wikimedia.org/wikipedia/commons/thumb/4/4a/IEEE_315-1975_%281993%29_8.6.16.svg/100px-IEEE_315-1975_%281993%29_8.6.16.svg.png)
A phototransistor is a light-sensitive transistor. A common type of phototransistor, the bipolar phototransistor, is in essence a
A solaristor is a two-terminal gate-less phototransistor. A compact class of two-terminal phototransistors or solaristors have been demonstrated in 2018 by ICN2 researchers. The novel concept is a two-in-one power source plus transistor device that runs on solar energy by exploiting a memresistive effect in the flow of photogenerated carriers.[8]
Materials
The material used to make a photodiode is critical to defining its properties, because only
Materials commonly used to produce photodiodes are listed in the table below.[9]
Material | Electromagnetic spectrum wavelength range (nm) |
---|---|
Silicon | 190–1100 |
Germanium | 400–1700 |
Indium gallium arsenide | 800–2600 |
Lead(II) sulfide | <1000–3500 |
Mercury cadmium telluride | 400–14000 |
Because of their greater bandgap, silicon-based photodiodes generate less noise than germanium-based photodiodes.
Binary materials, such as MoS2, and graphene emerged as new materials for the production of photodiodes.[10]
Unwanted and wanted photodiode effects
Any p–n junction, if illuminated, is potentially a photodiode. Semiconductor devices such as diodes, transistors and ICs contain p–n junctions, and will not function correctly if they are illuminated by unwanted light.[11][12] This is avoided by encapsulating devices in opaque housings. If these housings are not completely opaque to high-energy radiation (ultraviolet, X-rays, gamma rays), diodes, transistors and ICs can malfunction[13] due to induced photo-currents. Background radiation from the packaging is also significant.[14] Radiation hardening mitigates these effects.
In some cases, the effect is actually wanted, for example to use
Features
![](http://upload.wikimedia.org/wikipedia/commons/thumb/4/41/Response_silicon_photodiode.svg/260px-Response_silicon_photodiode.svg.png)
Critical performance parameters of a photodiode include spectral responsivity, dark current, response time and noise-equivalent power.
- Spectral responsivity
- The spectral responsivity is a ratio of the generated photocurrent to incident light power, expressed in A/W when used in photoconductive mode. The wavelength-dependence may also be expressed as a quantum efficiency or the ratio of the number of photogenerated carriers to incident photons which is a unitless quantity.
- Dark current
- The dark current is the current through the photodiode in the absence of light, when it is operated in photoconductive mode. The dark current includes photocurrent generated by background radiation and the saturation current of the semiconductor junction. Dark current must be accounted for by noisewhen a photodiode is used in an optical communication system.
- Response time
- The response time is the time required for the detector to respond to an optical input. A photon absorbed by the semiconducting material will generate an electron–hole pair which will in turn start moving in the material under the effect of the electric field and thus generate a current. The finite duration of this current is known as the transit-time spread and can be evaluated by using Ramo's theorem. One can also show with this theorem that the total charge generated in the external circuit is e and not 2e as one might expect by the presence of the two carriers. Indeed, the integral of the current due to both electron and hole over time must be equal to e. The resistance and capacitance of the photodiode and the external circuitry give rise to another response time known as RC time constant (). This combination of R and C integrates the photoresponse over time and thus lengthens the impulse response of the photodiode. When used in an optical communication system, the response time determines the bandwidth available for signal modulation and thus data transmission.
- Noise-equivalent power
- Noise-equivalent power (NEP) is the minimum input optical power to generate photocurrent, equal to the rms noise current in a 1 hertz bandwidth. NEP is essentially the minimum detectable power. The related characteristic detectivity () is the inverse of NEP (1/NEP) and the specific detectivity () is the detectivity multiplied by the square root of the area () of the photodetector () for a 1 Hz bandwidth. The specific detectivity allows different systems to be compared independent of sensor area and system bandwidth; a higher detectivity value indicates a low-noise device or system.[16] Although it is traditional to give () in many catalogues as a measure of the diode's quality, in practice, it is hardly ever the key parameter.
When a photodiode is used in an optical communication system, all these parameters contribute to the sensitivity of the optical receiver which is the minimum input power required for the receiver to achieve a specified bit error rate.
Applications
P–n photodiodes are used in similar applications to other
Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, medical devices[17] and the receivers for infrared remote control devices used to control equipment from televisions to air conditioners. For many applications either photodiodes or photoconductors may be used. Either type of photosensor may be used for light measurement, as in camera light meters, or to respond to light levels, as in switching on street lighting after dark.
Photosensors of all types may be used to respond to incident light or to a source of light which is part of the same circuit or system. A photodiode is often combined into a single component with an emitter of light, usually a
Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a more linear response than photoconductors.
They are also widely used in various medical applications, such as detectors for
PIN diodes are much faster and more sensitive than p–n junction diodes, and hence are often used for optical communications and in lighting regulation.
P–n photodiodes are not used to measure extremely low light intensities. Instead, if high sensitivity is needed,
Comparison with photomultipliers
Advantages compared to photomultipliers:[18]
- Excellent linearity of output current as a function of incident light
- Spectral response from 190 nm to 1100 nm (semiconductor materials
- Low noise
- Ruggedized to mechanical stress
- Low cost
- Compact and light weight
- Long lifetime
- High quantum efficiency, typically 60–80%[19]
- No high voltage required
Disadvantages compared to photomultipliers:
- Small area
- No internal gain (except avalanche photodiodes, but their gain is typically 102–103 compared to 105-108 for the photomultiplier)
- Much lower overall sensitivity
- Photon counting only possible with specially designed, usually cooled photodiodes, with special electronic circuits
- Response time for many designs is slower
- Latent effect
Pinned photodiode
The pinned photodiode (PPD) has a shallow implant (P+ or N+) in N-type or P-type diffusion layer, respectively, over a P-type or N-type (respectively) substrate layer, such that the intermediate diffusion layer can be fully depleted of majority carriers, like the base region of a bipolar junction transistor. The PPD (usually PNP) is used in CMOS active-pixel sensors; a precursor NPNP triple junction variant with the MOS buffer capacitor and the back-light illumination scheme with complete charge transfer and no image lag was invented by Sony in 1975. This scheme was widely used in many applications of charge transfer devices.
Early charge-coupled device
A CMOS image sensor with a low-voltage-PPD technology was first fabricated in 1995 by a joint JPL and Kodak team. The CMOS sensor with PPD technology was further advanced and refined by R.M. Guidash in 1997, K. Yonemoto and H. Sumi in 2000, and I. Inoue in 2003. This led to CMOS sensors achieve imaging performance on par with CCD sensors, and later exceeding CCD sensors.
Photodiode array
![](http://upload.wikimedia.org/wikipedia/commons/thumb/c/ce/Photodiode_array_chip.jpg/150px-Photodiode_array_chip.jpg)
![](http://upload.wikimedia.org/wikipedia/commons/thumb/d/d4/Optical_mouse_chip_detail.png/150px-Optical_mouse_chip_detail.png)
A one-dimensional array of hundreds or thousands of photodiodes can be used as a position sensor, for example as part of an angle sensor.[23] A two-dimensional array is used in image sensors and optical mice.
In some applications, photodiode arrays allow for high-speed parallel readout, as opposed to integrating scanning electronics as in a
Passive-pixel image sensor
The
The noise of photodiode arrays is sometimes a limitation to performance. It was not possible to fabricate active pixel sensors with a practical pixel size in the 1970s, due to limited microlithography technology at the time.[25]
See also
- Electronics
- Band gap
- Infrared
- Optoelectronics
- Optical interconnect
- Light Peak
- Interconnect bottleneck
- Optical fiber cable
- Optical communication
- Parallel optical interface
- Opto-isolator
- Semiconductor device
- Solar cell
- Avalanche photodiode
- Transducer
- LEDs as photodiode light sensors
- Light meter
- Image sensor
- Transimpedance amplifier
- Photoelectric sensor
References
This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22.
- ISBN 978-0-07-162935-5. Archived from the originalon 2021-08-17. Retrieved 2021-02-25.
- ISBN 1-4419-9924-8. Chapter 3 From Light to Electric Current – The Photodiode
- ^ ISBN 9781119978381. Retrieved 19 April 2019.
- ^ "Photodiode Application Notes – Excelitas – see note 4" (PDF). Archived from the original (PDF) on 2014-11-13. Retrieved 2014-11-13.
- .
- ISBN 9780393318517.
- ^ "The phototransistor". Bell Laboratories Record. May 1950. Archived from the original on 2015-07-04. Retrieved 2012-04-09.
- S2CID 102819292.
- ISBN 1-4200-7662-0
- S2CID 27038582.
- ^ Shanfield, Z. et al (1988) Investigation of radiation effects on semiconductor devices and integrated circuits[dead link], DNA-TR-88-221
- ISBN 978-1-4398-2694-2
- .
- .
alpha particles emitted from the natural radioactive decay of uranium, thorium, and daughter isotopes present as impurities in packaging materials were found to be the dominant cause of [soft error rate] in [dynamic random-access memories].
- ^ Erzberger, Arno (2016-06-21). "Halbleitertechnik Der LED fehlt der Doppelpfeil". Elektronik (in German). Archived from the original on 2017-02-14. Retrieved 2017-02-14.
- ISBN 9781891121746
- ^ E. Aguilar Pelaez et al., "LED power reduction trade-offs for ambulatory pulse oximetry," 2007 29th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, Lyon, 2007, pp. 2296–2299. doi: 10.1109/IEMBS.2007.4352784, URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4352784&isnumber=4352185
- ^ Photodiode Technical Guide Archived 2007-01-04 at the Wayback Machine on Hamamatsu website
- ISBN 978-0-470-13148-0
- ^ .
- ^ U.S. Patent 4,484,210, which was a floating-surface type buried photodioe with the similar structure of the 1975 Philips invention. Solid-state imaging device having a reduced image lag
- S2CID 44669969.
- ^
Gao, Wei (2010). Precision Nanometrology: Sensors and Measuring Systems for Nanomanufacturing. Springer. pp. 15–16. ISBN 978-1-84996-253-7.
- S2CID 123351913.
- ^ S2CID 10556755.
External links
![](http://upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/30px-Commons-logo.svg.png)
- Photodiode I–V characteristics Archived 2022-02-26 at the Wayback Machine
- Using the Photodiode to convert the PC to a Light Intensity Logger
- Design Fundamentals for Phototransistor Circuits (archived on February 5, 2005)
- Working principles of photodiodes Archived 2009-02-12 at the Wayback Machine
- Excelitas Application Notes on Pacer Website (archived on March 4, 2016)