Other diodes: Diode types
The photodiode structure has a large bearing on the way any device works as a photodetector.
The photodiode structure and materials govern the way in which the photodiode works and factors like the size of the junction area including an intrinsic area increase the size of the area or volume over which light photos can be collected.
As a result the structure, materials and the mechanism used for the photodiode are all of great importance.
A variety of different photodiode structures are used and these vary according to the type of photodiode in question. Avalanche photodiode structures are different to those used for PIN or PN photodiodes. The Schottky photodiode structure is again different. However all the photodiode structures are designed to optimise the light collection and conversion
PN & PIN photodiode structures
The standard PN junction diode can provide the functions of a photodiode. However one of the key requirements for a photodiode is a suitable area for collection of the light. Within a standard PN junction this is relatively small, but the area can be increased by using a PIN diode. As the intrinsic area is included in the active junction for light collection, there is a much larger area for light collection, making the PIN photodiode more effective.
In the photodiode fabrication process a thick intrinsic layer is inserted between the P type and N type layers. This middle intrinsic 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 photodiode 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.
In some instances a heterojunction may be used. This form of structure has the additional flexibility that light can be received from the substrate and this has a larger energy gap which makes it transparent to light.
The heterojunction format for a PIN photodiode uses less standard technology often using materials such as the InGaAs and InP depicted in the diagram. Being a less standard process, it is more expensive to implement and as a result tends to be used for more specialist products.
PN / PIN photodiode materials
The materials for the photodiodes determine many of its characteristics. One of the key properties or characteristics is the wavelength of light to which the diode responds. Another is the level of noise. Both of these are governed to a large extent by the material used in the photodiode.
The varying response to the wavelength caused by the use 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.
|Wavelength ranges for commonly used photodiode materials|
|Germanium||800 - 1700|
|Indium gallium arsenide||800 - 2600|
|Lead sulphide||~1000 - 3500|
|Silicon||190 - 1100|
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.
Avalanche photodiode structure
The avalanche photodiode structure is relatively similar to that of the more commonly used PN photodiode structure or the structure of the PIN photodiode. However as the avalanche photodiode is operated under a high level of reverse bias a guard ring is placed around the perimeter of the diode junction. This prevents surface breakdown mechanisms.
Avalanche photodiode materials
Like the standard PN or PIN photodiodes, the materials used have a major effect on determining the characteristics of the avalanche diode.
|Commonly used avalanche photodiode materials|
|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.|
For optimum noise performance the large difference in the ionisation coefficients for electrons and holes is needed. Silicon provides a good noise performance with a ratio between the different coefficients of 50. Germanium and many group III-V compounds only have ratios of less than 2. While the noise performance of these materials is much inferior, they need to be used for longer wavelengths that require the smaller energy gap offered.
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