Photodiode

A photodiode is a semiconductor built with a PN junction, sensitive to the incidence of visible or infrared light. In order for it to function correctly, it is reverse polarized, with which a certain flow of current will be produced when it is excited by light. Due to their construction, photodiodes behave like photovoltaic cells, that is, when illuminated in the absence of an external power source, they generate a very small current with the positive at the anode and the negative at the cathode.

The casing of a photodiode allows light (or infrared or ultraviolet radiation, or X-rays) to reach the sensitive part of the device. The package may include optical lenses or filters. Devices specially designed for use as a photodiode use a PIN junction instead of a p-n junction, to increase the speed of response. Photodiodes typically have a slower response time as their surface area increases. A photodiode is designed to operate in reverse bias. A solar cell used to generate electrical solar power is a large area photodiode.

Photodiodes are used in scientific and industrial instruments to measure the intensity of light, either by itself or as a measure of some other property (smoke density, for example). A photodiode can be used as a receiver of encoded data in an infrared beam, such as in home remote controls. Photodiodes can be used to form an optocoupler, which allows signals to be transmitted between circuits without a direct metallic connection between them, allowing them to be isolated from high voltage differences.

Principle of operation

A photodiode is a PN junction or P-I-N structure. When a beam of light of sufficient energy falls on the diode, it excites an electron by giving it motion and creating a positively charged hole. If absorption occurs in the depletion zone of the junction, or at diffusion distance from it, these carriers are removed from the junction by the depletion zone field, producing a photocurrent.

A photodiode will be sensitive to only one wavelength of incident light. What that wavelength is will depend on what is known as the energy gap of the device. Since the energy is equal to Planck's constant times the frequency of the incident photon (E=h·f), it is easy to see the relationship. Only photons with the right energy to move an electron from the conduction band to the valence band (thus generating current) will be up to the task.

In order to be used according to this purpose, the photodiode must be reverse biased (higher voltage at the cathode than at the anode). An increase in current flow will occur when the diode is excited by light incident on it. In the absence of light the current present is very small and is called the current of darkness. In this way, the light intensity that falls on the photodiode can be calculated knowing the value of the reverse current that circulates through it. It will suffice to subtract the heat of the darkness current from said current and divide by the sensitivity of the device to do so.

Avalanche photodiodes They have a similar structure, but work with higher reverse voltages. This allows the photogenerated charge carriers to be multiplied in the avalanche zone of the diode, resulting in an internal gain, which increases the response of the device. All these features can be found in the manufacturer's manual.

Composition

The material used in the composition of a photodiode is a critical factor in defining its properties. They are usually composed of silicon, sensitive to visible light (wavelength up to 1µm); germanium for infrared light (wavelength up to approx. 1.8 µm); or any other semiconductor material.

The sensitivity of these materials can be expressed with a parameter called "responsibility", which expresses the amount of electric current generated with respect to the power of the light that falls on the material (Io=R.Po). This value varies with respect to the wavelength of the incident light and based on measurements and experiments, it has been known at which wavelengths the materials are most efficient.

It is also possible to fabricate photodiodes for use in the mid-infrared field (wavelength between 5 and 20 µm), but these require liquid nitrogen cooling.

In the past, exposure meters were manufactured with a selenium photodiode with a large surface area.

Use

P-n photodiodes are used in applications similar to other photodetectors, such as photoconductors, charge-coupled devices (CCDs), and photomultiplier tubes. They can be used to generate an output that depends on lighting (analog for metering), or to change the state of circuitry (digital, either for control and switching or for digital signal processing). They are used in fiber optic systems,

Photodiodes are used in consumer electronics devices such as compact disc players (recovering the information recorded in the CD groove by transforming the light from the laser beam reflected in it into electrical impulses to be processed by the system and obtain as result the recorded data.), smoke detectors, medical devices, and the receivers for infrared remote control devices used to control equipment from televisions to air conditioners. For many applications photodiodes or photoconductors can be used. Either type of photosensor can be used to measure light, as in camera light meters, or to respond to light levels, as in turning on street lights after dark.

Photosensors of all types can be used to respond to incident light or to a light source that is part of the same circuit or system. A photodiode is often combined in a single component with a light emitter, typically a light emitting diode (LED), either to detect the presence of a mechanical obstruction to the beam (slot optical switch) or to couple two circuits digital or analog 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 to accurately measure 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 for analyzing samples (immunoassay), and pulse oximeters.

PIN diodes are much faster and more sensitive than p-n junction diodes, so they are often used for optical communication and lighting regulation.

P-n photodiodes are not used to measure extremely low light intensities. Instead, if high sensitivity is needed, avalanche photodiodes, boosted charge coupling devices, or photomultiplier tubes are used for applications such as astronomy, spectroscopy, night vision equipment, and laser rangefinders.

Comparison with photomultipliers

Advantages compared to photomultipliers:

1. Excellent linearity of the output current according to incident light
2. Spectral response of 190 nm to 1100 nm (silt), longer wavelengths with other semiconductor materials
3. Low noise
4. Robust to mechanical tension
5. Low cost
6. Compact and lightweight
7. Long service life
8. High quantum efficiency, typically 60–80%
9. No high voltage required
10. Unlike the LDR the photodiode responds to the changes of darkness to lighting and vice versa with much more speed, and can be used in circuits with smaller response time.

Disadvantages compared to photomultipliers:

1. Small area
2. No internal gain (except photodium of avalanche, but its profit is usually 10²-10³ compared to 10⁵- 10⁸ for photomultiplier)
3. Much lower overall sensitivity
4. Photon count is only possible with specially designed photods, usually cooled, with special electronic circuits
5. The response time for many designs is slower
6. effect latent

Anchored Photodiode

The tethered photodiode (PPD) is shallowly implanted (P+ or N+) in an N-type or P-type diffusion layer, respectively, on a P-type or N-type (respectively) substrate layer, so that the intermediate diffusion layer can be completely depleted of majority carry, like the base region of a bipolar junction transistor. The PPD (usually PNP) is used in the CMOS active pixel sensor; In 1975, Sony invented a precursor NPN variant with a synchronized upper N-layer for use in CCD image sensors.

Early charge-coupled image sensor devices suffered from shutter lag. This was largely resolved with the invention of the fixed photodiode. It was invented by Nobukazu Teranishi, Hiromitsu Shiraki, and Yasuo Ishihara at NEC in 1980. They recognized that the delay can be eliminated if the signal carriers can be transferred from the photodiode to the CCD. This led to his invention of the tethered photodiode, a photodetector structure with low delay, low noise, high quantum efficiency, and low dark current. It was first reported publicly by Teranishi and Ishihara with A. Kohono, E. Oda, and K. Arai in 1982, with the addition of an anti-bloom structure. The new photodetector structure invented at NEC was named the "pinned photodiode" (PPD) by B.C. Burkey at Kodak in 1984. In 1987, PPD began to be incorporated into most CCD sensors, becoming an accessory in consumer electronics camcorders and later digital still cameras.

In 1994, Eric Fossum, while working at NASA's Jet Propulsion Laboratory (JPL), proposed an improvement to the CMOS sensor: pinned photodiode integration. A CMOS sensor with PPD technology was first manufactured 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. Read. Since then, PPD has been used in almost all CMOS sensors. The CMOS sensor with PPD technology was perfected by R.M. Guidash in 1997, K. Yonemoto and H. Sumi in 2000, and I. Inoue in 2003. This led CMOS sensors to achieve image performance comparable to, and later superior to, CCD sensors..

Research

Worldwide research in this field focuses (around 2005) especially on the development of cheap solar cells, miniaturization and improvement of CCD and CMOS sensors, as well as faster and more sensitive photodiodes for use in fiber optic telecommunications.

Since 2005 there are also organic semiconductors. The company NANOIDENT Technologies was the first in the world to develop an organic photodetector, based on organic photodiodes.

Spectral response of silicon photodiodes

Silicon photodetectors with a spectral response defined by design are described. To do this, modern micromachining technologies in general are used, as well as two properties of the integrated silicon photodetector in particular. First, the wavelength dependence of the absorption coefficient is exploited. Second, it takes advantage of the fact that the multilayer interference filter at the pn junction is developed by processing a silicon wafer. The refractive index of the silicon complex, n * = n - jk, is wavelength dependent in the perceivable part of the spectrum due to an indirect band gap at 1.12 eV and the possibility of a direct transition at 3, 4 eV, which makes the material highly absorb ultraviolet radiation and also act practically as a transparent material for wavelengths greater than 800 nm. This mechanism allows the design of color sensors and also photodiodes with discernible response in the IR or UV array. The transmission of light from the event with a shallow stack of thin films to bulk silicon is wavelength dependent. The necessary compatibility with conventional microelectronic processes in silicon limits the range of ideal materials to the silicon-compatible materials traditionally used for integrated circuit fabrication. Accurate data is provided on: crystalline Si, thermally grown SiO2, LPCVD polysilicon, silicon nitride (low-loss and stoichiometric) and also oxides (LTO, PSG, BSG, BPSG), PECVD oxynitrides, as well as thin-film metals for increased the predictive quality of the simulation. In the case of a complete microspectrometer, micromachining actions are often used to fabricate the diffusion component. Devices operating in the visible or infrared spectrum based on a Fabry-Perot grating or an etalon are presented.

Related Devices

An avalanche photodiode has a structure optimized to operate with high reverse bias, approaching the reverse breakdown voltage. This allows each photo-generated carrier to be multiplied by avalanche breakdown, giving rise to an internal gain within the photodiode, which increases the effective responsiveness of the device.

Electronic symbol of a phototransistor.

A phototransistor is a light-sensitive transistor. A common type of phototransistor, the bipolar phototransistor, is essentially a bipolar transistor enclosed in a transparent box so that light can reach the base-collector p-n junction. It was invented by Dr. John N. Shive (most famous for his wave machine) at Bell Laboratories in 1948 but it was not announced until 1950. The electrons that generate the photons at the base-collector junction are injected into the base, and this photodiode current is amplified by the current gain β of the transistor (or h_fe). If the base and collector leads are used and the emitter is left unconnected, the phototransistor becomes a photodiode. Although phototransistors have a higher responsiveness to 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 a 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 gateless phototransistor. A compact class of two-terminal phototransistors or solaristors has been demonstrated in 2018 by ICN2 researchers. The novel concept is a two-in-one power supply plus transistor device that runs on solar energy by taking advantage of a memresistive effect in the photogenerated carrier flux.