- an overview of photodiode technology detailing the basics of photodiode technology as well as the different types of photodiode along with their applications.
The photo-diode is widely used within the electronics industry in a variety of areas from detectors in CD players to wide bandwidth optical telecommunications systems.
Photodiode technology owes much of its success to its simple, low cost yet rugged structure. Despite this speeds of 30 GHz and more have been reported for the latest photodiode technologies, showing how much the technology can achieve.
Photodiode history and development
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.
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.
- PIN photodiode: This type of photodiode is one of the most widely used forms of photodiode today. Although the PIN or p-i-n photodiode was not the first type of photodiode to be used, it collects the light photons more efficiently than the more standard PN photodiode, and also offers a lower capacitance.
- PN photodiode: The PN photodiode was the first form of photodiode to be developed and used. Nowadays, it is not as widely used as other types which are able to offer better performance parameters. Nevertheless it is still used in some instances.
- Avalanche photodiode: Avalanche photodiode technology is used in areas of low light. The avalanche photodiode offers very high levels of gain, but against this it has high levels of noise. Accordingly this photodiode technology is not suitable for all applications and it tends to be used
- Schottky photodiode: As the name indicates, Schottky photodiode technology is based upon the Schottky diode. In view of the small diode capacitance it offers a very high speed capability and is used in high bandwidth communication systems.
Although the different types of photodiode work in slightly different ways, the basis of operation of all photodiodes remains the same.
Light energy can be considered 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.
The photodiode symbol shows the basic format for a diode. However the photodiode symbol also shows the light in the form of arrows striking the diode junction - the arrows are in the opposite direct to that of a light emitting diode where they emanate from the device.
Photodiode symbol used for circuit schematics
There are several forms of photodiode that are available. Each type of photodiode has its own advantages and disadvantages, thereby allowing a choice of photodiode technology to be made to gain the best results. Factors including noise, reverse bias constraints, gain, wavelength, and more all play a part. With PIN, PN, avalanche and Schottky photodiodes all available, an informed choice can be made to ensure the optimum photodiode technology is used.
By Ian Poole
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