Other diodes: Diode types
Photo diode technology has many applications within the electronics industry. Photo diodes are used as light sensing photo detectors in many applications ranging from CD / DVD drives to optical communications and burglar alarms to general light sensing.
Photodiode technology is straightforward and yet it can provide very high levels of performance. Speeds of 30GHz and more are attainable using the right photodiode technology and this enables it to be used for very high speed optical data transmission.
Photodiode technology developments came out of the basic developments of the PN junction diode that started in the 1940s in earnest. Applications for the use of the PN junction diode were found outside the basic use of rectifying signals. It was found that they could be used for many photonic applications - photodiodes, solar cells and light emission.
Photodiode technology was refined in the 1950s and in the latter part of that decade the PIN photodiode was developed. Light absorption in the wide depletion area of the PIN structure was first investigated in a paper published in 1959 by Gartner. Although silicon has been the favoured material for photodiodes, germanium can also be used, and its use was first demonstrated in 1962 by Riesz.
While PIN photodiode technology has been the most widely used format for diodes, other types including the avalanche diode were also demonstrated. The first step along the road was undertaken in 1953 by McAfee and McKay who first addressed the concept of avalanche multiplication and later work appeared on avalanche photodiodes in 1963 and the following years.
Another form of photodiode, named the Schottky photodiode has also been addressed. Some of the first research on point contact photodetectors appears to have been undertaken around 1962, and later diodes using evaporated metals films were also studied.
The photodiode symbol utilises the basic diode symbol but with the addition of two arrows shown pointing towards the device. This indicates the direction of the light, i.e. striking the device. The photo diode symbol is similar to that of the light emitting diode, but with the arrows in the opposite direction for obvious reasons.
Types of photodiode
Although the term photodiode is widely used, there are actually a number of different types of photodiode technology that can be used. As they offer different properties, the different photodiode technologies are used in different areas.
- PN photodiode: The PN photodiode was the first form of photodiode to be developed. Its performance is not as advanced as some of the other types and therefore its use is less now than it used to be. . . . . . Read more about the PN photodiode.
- PIN photodiode: The PIN photodiode is one of the most widely used forms of photodiode today. The PIN photodiode collects the light photons more efficiently than the more standard PN photodiode because of the wide intrinsic area between the P and N regions allow for more light to be collected, and in addition to this it also offers a lower capacitance. . . . . . Read more about the PIN photodiode.
- Avalanche photodiode: Avalanche photodiode technology is used in areas of low light because of its high levels of gain. Against this it produces high levels of noise. Accordingly this photodiode technology is not suitable for all applications. . . . . . Read more about the Avalanche photodiode.
- Schottky photodiode: The Schottky photodiode tis based upon the Schottky diode. The small diode junction means that there is very little junction capacitance and this means that it can operate at high speeds. As a result, this form of photo diode is often used in high bandwidth optical communication systems, e.g. fibre optic links. . . . . . Read more about the Schottky photodiode.
Each type of photodiode technology has its own advantages and disadvantages and this means that the actual type of photo diode can be chosen for a given application. Parameters including noise, reverse bias constraints, gain, wavelength, can all be considered when choosing the type of photo diode technology that is most applicable for a given situation. With PIN, PN, avalanche and Schottky photodiodes all available, an informed choice can be made to ensure the optimum photodiode technology is used.
Although the different types of photodiode work in slightly different ways, the basis of operation of photodiode technology remains the same despite the different types of diode used.
. . . . . Read more about the Photodiode theory of operation.
To explain how photo diode technology works, it is necessary to consider light in terms of photons or packets of light.
When a photon of sufficient energy enters the depletion region of a semiconductor diode, it may strike an atom with sufficient energy to release the electron from the atomic structure. This creates a free electron and a hole (i.e. an atom with a space for an electron). The electron is negatively charged, while the hole is positively charged.
The electrons and holes may remain free, or other electrons may combine with holes to form complete atoms again in the crystal lattice. However it is possible that the electrons and holes may remain free and be pulled away from the depletion region by an external field. In this way the current through the diode will change and a photocurrent is produced within the photo diode.
Photo diode technology is well established and widely used. Many very high performance diodes are available and the exact photo diode technology can be chosen for any given situation. PN junction, Avalanche, PIN, and Schottky types of photo diode are all available.
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