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.
Photodiode basics
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.
Photodiode symbol
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.
The two most common forms of photodiode are the PIN or p-i-n photodiode and the PN photodiode.
Both the PIN photodiode and PN photodiodes are widely used for a variety of photo-detection applications. Both the PIN photodiode and the PN photodiode have their advantages and disadvantages.
PIN photodiode basics
One of the key requirements for any photodetector is a sufficiently large area in which the light photons can be collected and converted. This is achieved by creating a large depletion region - the region where the light conversion takes place - by adding an intrinsic area into the PN junction to create a PIN junction.
One of the key parameters within the design of the PIN photodiode is to enable the light to enter the intrinsic region. The physical design of the photodiode needs to take account of this so that the light collection is optimised.
Photodiodes in general and in this case the PIN photodiode will respond differently to different light wavelengths. It is generally the thickness of the top p type region or layer that is one of the key parameters in determining the response sensitivity.
PIN photodiode applications
The PIN photo-diode does not have any gain, and for some applications this may be a disadvantage. Despite this it is still the most widely used form of diode, finding applications in audio CD players, DVD players as well as computer CD drives. In addition to this they are used in optical communication systems.
PIN photodiode are also used as nuclear radiation detectors. There are several types of nuclear radiation. The radiation may be in the form of high energy charged or uncharged particles, or it may also be electromagnetic radiation. The diode can detect all these forms of radiation. The electromagnetic radiation, of which light is a form, generates the hole-electron pairs as already mentioned. The particles have exactly the same effect. However as only a small amount of energy is required to generate a hole-electron pair a single high-energy particle may generate several hole-electron pairs.
PN photodiode
While the PIN photodiode is the most widely used, the PN photodiode is also used in some circumstances. It is essentially the same as the PN photodiode, except that it does not have an intrinsic layer within the depletion region.
PN / PIN photodiode comparison
Both PN photodiodes and PIN photodiodes are available on the market. When designing circuit it is necessary to choose the correct type. Both PN photodiodes and PIN photodiodes have their advantages and disadvantages:
- A PIN photodiode needs reverse bias for its operation as a result of the presence of the intrinsic region. This reverse bias has several consequences:
- Reverse bias introduces a noise current which reduces signal to noise ratio
- Reverse bias offers better performance for high bandwidth applications
- Reveres bias offers better performance for high dynamic range applications
- A PN photodiode does not require a reverse bias and as a result is more suitable for low light applications.
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