Photodiode
A photodiode is a
semiconductor diode that functions as a photodetector. Photodiodes are packaged with either a window or
optical fibre connection, in order to let in the light to the sensitive part of the device. They may also be used without a window to detect
vacuum UV or
X-rays.
A phototransistor is in essence nothing more than a
bipolar transistor that is encased in a transparent case so that
light can reach the
base-collector junction. The phototransistor works like a photodiode, but with a much higher sensitivity for light, because the electrons that are generated by photons in the base-collector junction are injected into the base, and this current is then amplified by the transistor operation.
Encyclopedia
A
photodiode is a
semiconductor diode that functions as a photodetector. Photodiodes are packaged with either a window or
optical fibre connection, in order to let in the light to the sensitive part of the device. They may also be used without a window to detect
vacuum UV or
X-rays.
A
phototransistor is in essence nothing more than a
bipolar transistor that is encased in a transparent case so that
light can reach the
base-collector junction. The phototransistor works like a photodiode, but with a much higher sensitivity for light, because the electrons that are generated by photons in the base-collector junction are injected into the base, and this current is then amplified by the transistor operation. However, a phototransistor has a slower response time than a photodiode.
Principle of operation
A photodiode is a
p-n junction or
p-i-n structure. When light with sufficient
photon energy strikes a semiconductor, photons can be absorbed, resulting in
generation of a mobile
electron and electron hole. If the absorption occurs in the junction's depletion region, these carriers are swept from the junction by the
built-in field of the depletion region, producing a
photocurrent.
Photodiodes can be used under either zero bias leading to a current in the forward bias direction. This is called the
photovoltaic effect, and is the basis for
solar cells — in fact, a solar cell is just a large number of big, cheap photodiodes.
Diodes usually have extremely high
resistance when reverse-biased. This resistance is reduced when light of an appropriate frequency shines on the junction. Hence, a reverse-biased diode can be used as a detector by monitoring the current running through it. Circuits based on this effect are more sensitive to light than ones based on the photovoltaic effect.
Avalanche photodiodes have a similar structure, but they are operated with much higher reverse bias. 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.
Materials
The material used to make a photodiode is critical to defining its properties, because only
photons with sufficient energy to excite an
electron across the material's
bandgap will produce significant photocurrents.
Materials commonly used to produce photodiodes:
Because of their greater bandgap, silicon-based photodiodes generate less noise than germanium-based photodiodes, but germanium photodiodes must be used for wavelengths longer than approximately 1 µm.
Features
Critical performance parameters of a photodiode include:
;responsivity: The ratio of generated photocurrent to incident light power, typically expressed in
A/W when used in photoconductive mode. The responsivity may also be expressed as a
quantum efficiency, or the ratio of the number of photogenerated carriers to incident photons and thus a unitless quantity.
;dark current: The current through the photodiode in the absence of any input optical signal, 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 calibration if a photodiode is used to make an accurate optical power measurement, and it is also a source of noise when a photodiode is used in an optical communication system.
;noise-equivalent power: The minimum input optical power to generate photocurrent, equal to the rms noise current in a 1 hertz bandwidth. The related characteristic
detectivity is the inverse of NEP, 1/NEP; and the
specific detectivity is the detectivity normalized to the area of the photodetector, . The NEP is roughly the minimum detectable input power of a photodiode.
When a photodiode is used in an optical communication system, 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 ratio.
Applications
P-N photodiodes are used in similar applications to other photodetectors, such as
photoconductors,
charge-coupled devices, and
photomultiplier tubes.
Photodiodes are used in consumer electronics devices such as
compact disc players,
smoke detectors, and the receivers for remote controls in
VCRs and
televisions.
In other consumer items such as
camera light meters, clock radios and street lights,
photoconductors are often used rather than photodiodes, although in principle either could be used.
Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a better, more linear response than photoconductors.
They are also widely used in various medical applications, such as detectors for
Computed tomography or instruments to analyze samples . They are also used in
Blood gas monitors.
PIN diodes are much faster and more sensitive than ordinary 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 range finding.
Comparison with photomultipliers
Advantages compared to
photomultipliers:
- Excellent linearity of output current as a function of incident light
- Spectral response from 190 nm to 1100 nm , longer wavelengths with other semiconductor materials
- Low noise
- Ruggedized to mechanical stress
- Low cost
- Compact and light weight
- Long lifetime
- High quantum efficiency, typically 80%
- No high voltage required
Disadvantages compared to
photomultipliers:
- Small area
- No internal gain
- 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
See also
References
- Portions of this article are adapted from Federal Standard 1037C and from the FAA Glossary of Optical Communications Terms
- Gowar, John, Optical Communication Systems, 2 ed., Prentice-Hall, Hempstead UK, 1993
External links
- Principles and techniques of integrating photodiodes into regulation lighting systems
