PIN / PN Photodiode

- an overview of the PIN photodiode and the PN photodiode detailing its differences and features when compared to other forms of photodiode.

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.

By Ian Poole

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