There is a variety of types of photodiode that are available. The p-n and p-i-n photodiodes are the most widely used, but avalanche photodiodes are also available.
Avalanche photodiodes have advantages in some applications although their use may be more specialised.
Avalanche photodiode basics
The avalanche photodiode possesses a similar structure to that of the PIN or PN photodiode. A structure similar to that of a Schottky photodiode can also be used but this is less common. However the structure is optimised for avalanche operation.
The main difference of the avalanche photodiode operates under a slightly different scenario to that of the more standard photodiodes. It operates under a high reverse bias condition to enable avalanche multiplication of the holes and electrons created by the initial hole electron pairs created by the photon / light impact.
The avalanche action enables the gain of the diode to be increased many times, providing a much greater level of sensitivity.
Avalanche photodiode advantages and disadvantages
The avalanche photodiode has a number of different characteristics to the normal p-n or p-i-n photodiodes, making them more suitable for use in some applications. In view of this it is worth summarising their advantages and disadvantages..
The main advantages of the avalanche photodiode include:
- Greater level of sensitivity
The disadvantages of the avalanche photodiode include:
- Much higher operating voltage may be required.
- Avalanche photodiode produces a much higher level of noise than a p-n photodiode
- Avalanche process means that the output is not linear
Circuit conditions
Avalanche photodiodes require a high reverse bias for their operation. For silicon, a diode will typically require between 100 and 200 volts, and with this voltage they will provide a current gain effect of around 100 resulting from the avalanche effect. Some diodes that utilise specialised manufacturing processes enable much higher bias voltages of up to 1500 volts to be applied. As it is found that the gain levels increase when higher voltages are applied, the gain of these avalanche diodes can rise to the order of 1000. This can provide a distinct advantage where sensitivity is of paramount importance.
The avalanche photodiodes are not as widely used as their p-i-n counterparts. They are used primarily where the level of gain is of paramount importance, because the high voltages required, combined with a lower reliability means that they are often less convenient to use.
Schottky photodiode
The Schottky photodiode is based around the Schottky barrier diode and it may also be called the metal-semiconductor diode as a result of its structure.
The Schottky photodiode provides additional capabilities over other forms of photodiode in terms of speed and long wavelength detection capability. As a result the Schottky photodiode has a unique niche in amongst the other forms of photodiode that are available.
Schottky photodiode basics
As the name indicates, the Schottky photodiode uses the Schottky diode (or Schottky barrier diode as it is sometimes called) as its basis of the photodetector.
The Schottky photodiode is unique as a photodetector as it is able to operate in two photo-detection modes:
- Electron pair generation: This occurs from band to band or energy gap excitation in the semiconductor.
- Emission of carriers: The emission of carriers occurs from the metal to the semiconductor over the Schottky barrier. This often referred to as internal photoemission.
The metal-semiconductor junction provides a similar action to that of the intrinsic layer of the PIN photodiode. Accordingly is provides a larger areas for capture of the photon energy.
Schottky photodiode applications
The Schottky photodiode is particularly compatible with mature silicon and silicide technology. As a result these photodiodes have been widely used in CCD - charge coupled device - as the image sensing photodetector.
The Schottky photodiode can be integrated into a single chip along with CCD transfer gate - these are also compatible with silicon technology. The CCD itself forms a shift register to allow the data from the array of Schottky photodiode detectors to transferred out of the overall chip in a managed way - providing method of extracting the huge data files from the large array of photodetectors with a sensible number of leads on the integrated circuit.
As a result of these advantages the Schottky photodiode detector has been the most widely used technology for focal plane arrays.
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