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:
- Photodiode material
- Diode size
- Bandwidth
- Responsivity and gain
- Dark and noise current
- 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:
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
|
