Avalanche photodiode

- an overview or tutorial describing the basics of the of the avalanche photodiode and detailing how it differs from the basic p-n and p-i-n photodiodes.

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 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

Avalanche diode structure and operation
The structure is somewhat more complicated than that of the ordinary p-i-n device. It consists of four layers. There are n+, p, un-doped, and p+ regions. Light absorption takes place in the un-doped region and as before this may be relatively thick. The avalanche region occurs between the n+ and p regions.

Light enters the un-doped region of the avalanche photodiode and causes the generation of hole-electron pairs. Under the action of the electric field the electrons migrate towards the avalanche region. Here the electric field causes their velocity to increase to the extent that collisions with the crystal lattice create further hole electron pairs. In turn these electrons may collide with the crystal lattice to create even more hole electron pairs. In this way a single electron created by light in the un-doped region may result in many more being created.

The avalanche photodiode has a number of differences when compared to the ordinary p-i-n diode. The avalanche process means that a single electron produced by light in the un-doped region is multiplied several times by the avalanche process. As a result the avalanche photo diode is far more sensitive. However it is found that it is not nearly as linear, and additionally the avalanche process means that the resultant signal is far noisier than one from a p-i-n diode. The structure of the avalanche diode is also more complicated. An n-type guard ring is required around the p-n junction to minimise the electric field around the edge of the junction. It is also found that the current gain is dependent not only on the bias applied, but also thermal fluctuations. As a result it is necessary to ensure the devices are placed on an adequate heat sink.

Materials used
Like the standard p-n or p-i-n photodiodes, the materials used have a major effect on determining the characteristics of the avalanche diode.

Material	Properties
Germanium	Can be used for wavelengths in the region 800 - 1700 nm. Has a high level of multiplication noise.
Silicon	Can be used for wavelengths in the region between 190 - 1100 nm. Diodes exhibit a comparatively low level of multiplication noise when compared to those using other materials, and in particular germanium.
Indium gallium arsenide	Can be used for wavelengths to 1600 nm and has a lower level of multiplication noise than germanium.

Summary of materials commonly used for avalanche photodiodes and their properties

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.

Photodiode parameters and characteristics
There are a number of parameters that are important in the specification of an avalanche photodiode. These parameters include:

1. Photodiode material



2. Diode size



3. Bandwidth



4. Responsivity and gain



5. Dark and noise current



6. Excess noise factor

These parameters represent some of the more important items to be specified, but it does not include a comprehensive list for all applications. The parameters will be addressed individually:

1. Photodiode material:   The affect of the different materials on the photodiode performance has already been discussed. The application for the avalanche diode will often dictate the material used, especially in terms of the wavelength.



2. Size of the avalanche photodiode:   The area over which light is to be collected may determine the actual size of the photodiode itself. However the larger the diode, the greater the cost. As a result, it is often more beneficial to utilise optical methods of focussing the light from a given area onto a smaller avalanche photodiode.



3. Bandwidth:   It is important to specify the bandwidth required for the avalanche photodiode. It is necessary to ensure that the diode can respond to the changes as rapidly as needed so that data at the required speed can be received. While there is a temptation to over specify, the required bandwidth should be carefully analysed as there is a penalty in the signal to noise ratio for choosing a wider bandwidth than is required.
   
   
4. Responsivity and gain:   The responsivity of a photodiode is measured in amps per watt and is an indication of the current generated for a given excitation in watts. This must be given for a particular bias voltage as the responsivity varies with the level of bias.
   
   
5. Dark and noise current:   The darm current is the current that flows in the device when it is not exposed to any light. Dark current is dominated by surface current, and since the dark and spectral noise current are a strong function of the gain of the avalanche photodiode, these should be specified at a stated responsivity level.
   
   
6. Excess noise factor:   All avalanche photodiodes generate excess noise due to the statistical nature of the avalanche process. In data on avalanche photodiodes, this factor is generally denoted by the letter F. In essence it can be viewed as the factor by which the statistical noise on the diode current exceeds that which would be expected from a noiseless multiplier on the basis of statistics (shot noise) alone. Accordingly this factor gives an indication of the amount of noise a diode introduces above that which would be expected on the basis of shot noise alone.
   

Summary
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.