|Working principle||Converts light into current|
|Pin configuration||anode and cathode|
A photodiode is a semiconductor p–n junction device that converts light into an electrical current. The current is generated when photons are absorbed in the photodiode. Photodiodes may contain optical filters, built-in lenses, and may have large or small surface areas. Photodiodes usually have a slower response time as their surface area increases. The common, traditional solar cell used to generate electric solar power is a large area photodiode.
Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fiber connection to allow light to reach the sensitive part of the device. Many diodes designed for use specially as a photodiode use a PIN junction rather than a p–n junction, to increase the speed of response. A photodiode is designed to operate in reverse bias.
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 generated in the absence of light) and the photocurrent, so the dark current must be minimized to maximize the sensitivity of the device.
To first order, for a given spectral distribution, the photocurrent is linearly proportional to the irradiance.
In photovoltaic mode (zero bias), photocurrent flows out of the anode through a short circuit to the cathode. If the circuit is opened or has a load impedance, restricting the photocurrent out of the device, a voltage builds up in the direction that forward biases the diode, that is, anode positive with respect to cathode. If the circuit is shorted or the impedance is low, a forward current will consume all or some of the photocurrent. This mode exploits the photovoltaic effect, which is the basis for solar cells – a traditional solar cell is just a large area photodiode. For optimum power output, the photovoltaic cell will be operated at a voltage that causes only a small forward current compared to the photocurrent.
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. 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.
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.
A phototransistor is a light-sensitive transistor. A common type of phototransistor, the bipolar phototransistor, is in essence a bipolar transistor encased in a transparent case so that light can reach the base–collector junction. It was invented by Dr. John N. Shive (more famous for his wave machine) at Bell Labs in 1948: 205 but it was not announced until 1950. The electrons that are generated by photons in the base–collector junction are injected into the base, and this photodiode current is amplified by the transistor's current gain β (or hfe). If the base and collector leads are used and the emitter is left unconnected, the phototransistor becomes a photodiode. While phototransistors have a higher responsivity for light they are not able to detect low levels of light any better than photodiodes. Phototransistors also have significantly longer response times. Another type of phototransistor, the field-effect phototransistor (also known as photoFET), is a light-sensitive field-effect transistor. Unlike photobipolar transistors, photoFETs control drain-source current by creating a gate voltage.
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.
The material used to make a photodiode is critical to defining its properties, because only photons with sufficient energy to excite electrons across the material's bandgap will produce significant photocurrents.
Materials commonly used to produce photodiodes are listed in the table below.
wavelength range (nm)
|Indium gallium arsenide||800–2600|
|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.
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 electromagnetic radiation (light) of wavelength suitable to produce a photocurrent. 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 due to induced photo-currents. Background radiation from the packaging is also significant. Radiation hardening mitigates these effects.
In some cases, the effect is actually wanted, for example to use LEDs as light-sensitive devices (see LED as light sensor) or even for energy harvesting, then sometimes called light-emitting and light-absorbing diodes (LEADs).
Critical performance parameters of a photodiode include spectral responsivity, dark current, response time and noise-equivalent power.
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.
P–n photodiodes are used in similar applications to other photodetectors, such as photoconductors, charge-coupled devices (CCD), and photomultiplier tubes. They may be used to generate an output which is dependent upon the illumination (analog for measurement), or to change the state of circuitry (digital, either for control and switching or for digital signal processing).
Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, medical devices 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 light-emitting diode (LED), either to detect the presence of a mechanical obstruction to the beam (slotted optical switch) or to couple two digital or analog circuits while maintaining extremely high electrical isolation between them, often for safety (optocoupler). The combination of LED and photodiode is also used in many sensor systems to characterize different types of products based on their optical absorbance.
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 computed tomography (coupled with scintillators), instruments to analyze samples (immunoassay), and pulse oximeters.
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, avalanche photodiodes, intensified charge-coupled devices or photomultiplier tubes are used for applications such as astronomy, spectroscopy, night vision equipment and laser rangefinding.
Advantages compared to photomultipliers:
Disadvantages compared to photomultipliers:
The First Pinned Photodiode was invented on March 5, 1975 by Yoshiaki Hagiwara. The Evidence was given in his application form to the IP department of Sony Corporation. Hagiwara filed three Japanese patents JPA1975-127646, JPA1975-127647 and JPA1975-134985 in 1975. Unfortunate the documentations were all written only in Japanese and never have been disclosed in the English speaking community till recently. See http:/www.aiplab.com/ in details. There is a big difference between the buried photodiode and the pinned photodiode. There are two kinds of buried photodiode, one is a buried photodiode with a floating P+ surface region and the other is a buried photodiode with the pinned P+ surface region, which is also called as Sony Hole Accumulation Diode (HAD). The floating surface is due to the RC delay caused by the undesired finite resistance. IEEE IEDM1982 NEC paper by Teranishi which did not show the adjacent P+ channel stops to make the zero resistance needed to pin the P+ heavily doped P+ surface hole accumulation region. Any small finite resistance value causes the RC delay time which invites the serious image lag problem. Hagiwara 1975 three Japanese patent figures show the Pinned connection to the surface region and also the empty potential well profile for the first time in the world which is the evidence of the no-image-lag feature.
The pinned photodiode (PPD) has a shallow P+ implant in N type diffusion layer over a P-type epitaxial substrate layer with no RC delay to the external controlled pinning voltage. It is not to be confused with the PIN photodiode. The PPD is used in CMOS active-pixel sensors. which explained that Hagiwara at Sony invented in 1975 and developed First Pinned Photodiode in 1978.
Early image sensor used the CCD type charge transfer device with the N+P single junction photodiode with the floating N+ surface which has the serious image lag problem and suffered from shutter lag . This was largely resolved with the invention of the pinned photodiode (PPD) by Hagiwara in 1975 , evidenced by three Japanese patents by Hagiwara JPA1975-134985, JPA1975-127646 and JPA1975-127646 which can be used in the interline transfer CCD type charge transfer devices and CMOS type low power digital signal type charge transfer device ( CMOS imager ). Hagiwara team at Sony developed the first Pinned Photodiode in SSDM1978 paper. Fossum in his 2014 paper did not quote properly and ignored the 1975 Hagiwara three basic patents on double and triple junction type photodiodes with the pinned surface and with the complete charge transfer capability with no image lag feature, and also with the anti-blooming control feature with the vertical overflow drain (VOD) function, which realize the electrical shutter function replacing completely the film and machanical shutter parts. Hagiwara in his JPA 1975-127646 and JPA 1975-127647 patents also invented the Global Shutter function with the MOS capacitor type buffer memory to avoid the rotary shutter effect. These works by Hagiwara and Sony were ignored and not quoted properly in Fossum 2014.  Fossum claimed incorrectly that Nobukazu Teranishi, Hiromitsu Shiraki and Yasuo Ishihara at NEC in 1980. They developed and reported the results years later after Hagiwara 1975 invention and 1978 first development efforts. Hagiwara 1975 patents showed that the lag can be eliminated since the signal carriers can be transferred from the double and triple junction type photodiode to the CC type charge transfer devid. Hagiwara 1975 reports, led to their invention of the pinned photodiode, a photodetector structure with low lag, low noise, high quantum efficiency and low dark current. Forsum 2014 was misleading and did not explained these facts clearly.  It was first invented by Hagiwara at Sony in the 1975 patents and Hagiwara team at Sony developed in 1978 and reported at SSDM1978, in Tokyo Japan for the first time in the world in public. The second runner is the NEC Teranish team in 1982. Teranishi and Ishihara with A. Kohono, E. Oda and K. Arai in 1982, with the addition of an anti-blooming structure. The new photodetector structure invented at Sony and later called in 1987 also as Hole Accmulation diode (HAD) was originally given the name "pinned photodiode" (PPD) by B.C. Burkey at Kodak in 1984. In 1987, the PPD began to be incorporated into most CCD sensors, becoming a fixture in consumer electronic video cameras Peter Noble invented the in-pixel Active pixel image sensor with the three-transistor type source follower current amplifier circuits in 1969. But we had to wait the CMOS scaling advancement till 2000. Meanwhile the CCD type charge transfer device was widely used with the double and triple junction type photodiode originally invented by Hagiwara in 1975.
In 1994, Eric Fossum, while working at NASA's Jet Propulsion Laboratory (JPL), proposed an improvement to the CMOS sensor: the integration of the pinned photodiode. A CMOS sensor with PPD technology was first fabricated in 1995 by a joint JPL and Kodak team that included Fossum along with P.P.K. Lee, R.C. Gee, R.M. Guidash and T.H. Lee. Since then, the PPD has been used in nearly all CMOS sensors. 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. Recently Sony developed the backlight CMOS image sensor which used the Hagiwara 1975 pinned buried photodiode originally developed by Hagiwara at Sony. Sony now has announced that Hagiwara was the true inventor of Pinned Photodiode. See https://www.sony.com/en/SonyInfo/News/notice/20200626/
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.
In recent years, one advantage of modern photodiode arrays (PDAs) is that they may allow for high speed parallel readout since the driving electronics may not be built in like a charge-coupled device (CCD) or CMOS sensor.
The passive-pixel sensor (PPS) is a type of photodiode array. It was the precursor to the active-pixel sensor (APS). A passive-pixel sensor consists of passive pixels which are read out without amplification, with each pixel consisting of a photodiode and a MOSFET switch. In a photodiode array, pixels contain a p–n junction, integrated capacitor, and MOSFETs as selection transistors. A photodiode array was proposed by G. Weckler in 1968, predating the CCD. This was the basis for the PPS.
Early photodiode arrays were complex and impractical, requiring selection transistors to be fabricated within each pixel, along with on-chip multiplexer circuits. The noise of photodiode arrays was also a limitation to performance, as the photodiode readout bus capacitance resulted in increased noise level. Correlated double sampling (CDS) could also not be used with a photodiode array without external memory. 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.
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