Photodiode

- an overview of the photodiode detailing its operation and some of its applications.

The photo-diode is widely used within the electronics industry in a variety of areas from detectors in CD players to wide bandwidth optical telecommunications systems. The photodiode owes much of its success to its simple, low cost yet rugged structure. Despite this speeds of 30 GHz and more have been reported for the latest technology photo diodes, showing how much the technology can achieve.

The most widely used photo-diode is in the form of a p-i-n diode. It was developed in the late 1950s from the more conventional p-n diode, over which it has many advantages in this application.

Photodiode structure
Although an ordinary p-n junction can be used as the basis of a photodiode, the p-i-n junction is far more satisfactory. In the photo diode fabrication process a thick intrinsic layer is inserted between the p-type and n-type layers. The middle layer may be either completely instrinsic, or very lightly doped to make it and n- layer. In some instances it may be grown as an epitaxial layer onto the substrate, or alternatively it may be contained within the substrate itself.

One of the main requirements of the diode is to ensure that the maximum amount of light reaches the intrinsic layer. One of the most efficient ways of achieving this is to place the electrical contacts at the side of the device as shown. This enables the maximum amount of light to reach the active area. It is found that as the substrate is heavily doped, there is very little loss of light due to the fact that this is not the active area.

As light is mostly absorbed within a certain distance, the thickness of the intrinsic layer is normally made to match this. Any increase in thickness over this will tend to reduce the speed of operation - a vital factor in many applications, and it will not improve the efficiency greatly.

It is also possible to have the light enter the photo diode from the side of the junction. By operating the photo diode in this fashion the intrinsic layer can be made much less to increase the speed of operation, although the efficiency is reduced.

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.

Photodiode materials
The materials used within a photodiode determine many of its critical properties. The wavelength of light to which it responds and the level of noise are both critical parameters that are dependent upon the material used in the photodiode.

The wavelength sensitivity of the different materials occurs because only photons with sufficient energy to excite an electron across the bandgap of the material will produce significant energy to develop the current from the photodiode.

Material	Wavelength sensitivity (nm)
Germanium	800 - 1700
Indium gallium arsenide	800 - 2600
Lead sulphide	~1000 - 3500
Silicon	190 - 1100

Wavelength ranges for commonly used photodiode materials

While the wavelength sensitivity of the material is very important, another parameter that can have a major impact on the performance of the photodiode is the level of noise that is produced. Because of their greater bandgap, silicon photodiodes generate less noise than germanium photodiodes. However it is also necessary to consider the wavelengths for which the photodiode is required and germanium photodiodes must be used for wavelengths longer than approximately 1000 nm.

Applications
The p-i-n 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.

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.