Photodiode Theory of Operation
Understanding how photodiodes work helps them to be used more effectively..
Other diodes: Diode types
There is a number of different types of photodiode: all use the same basic quantum principle but the implementation of the theory of operation is slightly different for each type.
The differences in the operation of the different types of photodiode enables their individual characteristics to be utilised in different ways, and in this way their advantages can be maximised and the best circuit operation gained.To achieve this it is necessary to have a basic understanding of the actual way in which they work.
Basic photodiode theory of operation
Light energy can be considered in terms of photons or packets of light. When a photon of sufficient energy enters the depletion region of a semiconductor diode, it may strike an atom with sufficient energy to release the electron from the atomic structure. This creates a free electron and a hole (i.e. an atom with a space for an electron). The electron is negatively charged, while the hole is positively charged.
The electrons and holes may remain free, or other electrons may combine with holes to form complete atoms again in the crystal lattice. However it is possible that the electrons and holes may remain free and be pulled away from the depletion region by an external field. In this way the current through the diode will change and a photocurrent is produced.
PIN / PN photodiode operation
The photodiode is operated under a moderate reverse bias. This keeps the depletion layer free of any carriers and normally no current will flow. However when a light photon enters the intrinsic region it can strike an atom in the crystal lattice and dislodge an electron. In this way a hole-electron pair is generated. The hole and electron will then migrate in opposite directions under the action of the electric field across the intrinsic region and a small current can be seen to flow. It is found that the size of the current is proportional to the amount of light entering the intrinsic region. The more light, the greater the numbers of hole electron pairs that are generated and the greater the current flowing.
Operating diodes under reverse bias increases the sensitivity as it widens the depletion layer where the photo action occurs. In this way increasing the reverse bias has the effect of increasing the active area of the photodiode and strengthens what may be termed as the photocurrent.
It is also possible to operate photodiodes under zero bias conditions in what is termed as a photovoltaic mode. In zero bias, light falling on the diode causes a current across the device, leading to forward bias which in turn induces "dark current" in the opposite direction to the photocurrent. This is called the photovoltaic effect, and is the basis for solar cells. It is therefore possible to construct a solar cell using a large number of individual photodiodes. Also when photodiodes are used in a solar cell, the diodes are made larger so that there is a larger active area, and they are able to handle higher currents. For those used for data applications, speed is normally very important and the diode junctions are smaller to reduce the effects of capacitance.
When not exposed to light the photo diode follows a normal V-I characteristic expected of a diode. In the reverse direction virtually no current flows, but in the forward direction it steadily increases, especially after the knee or turn on voltage is reached. This is modified in the presence of light. When used as a photo-diode it can be seen that the greatest effect is seen in the reverse direction. Here the largest changes are noticed, and the normal forward current does not mask the effects due to the light.
Avalanche diode operation
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 PIN 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.
The different types of photodiode have slightly different modes of operation, but all rely on the same basic principle of operation. As the different types have different characteristics, the right type can be chosen to best fit the needs of the circuit in question.
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