Other diodes: Diode types
The PN photodiode and the PIN photodiode are two of the most common formats for photdiodes.
Both the PIN photodiode and PN photodiodes are widely used for a variety of photo-detection applications because of their characteristics.
The basic PN photodiode is used in a number of instances. The photo detection occurs within the depletion area of the diode. As this is relatively small, the sensitivity is not as great as that for some other forms of photo diode.
PIN photodiode basics
The PIN photodiode provides additional sensitivity and performance over that of the basic PN junction photodiode.
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, and DVD drives, etc. 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 / PIN photodiode comparison
Both PN photodiodes and PIN photodiodes can be obtained from many suppliers. When designing photo detector circuit, it is necessary to choose the correct type of photo diode dependent upon the performance and characteristics needed:
- A PN photodiode does not require a reverse bias and as a result is more suitable for low light applications as a result of the improved noise performance.
- Reverse bias required by the PIN photodiode introduces a noise current which reduces signal to noise ratio
- The reveres bias offers better performance for high dynamic range applications
- The reverse bias required offers better performance for high bandwidth applications as the capacitance between the P and N regions as well as charge storage is small.
In this way it can be seen that the PN photodiode and PIN photodiode have different characteristics that enable them to be used in different applications.
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