A photodiode is referred to that semiconductor equipment that transforms light into an electric current. The electric current is formed when photons get absorbed in photodiodes. This electrical equipment may consist of built-in lenses, optical filters, with small and large surface areas. One of the commonly and traditionally used solar cells known as electric solar power is an example of a large area photodiode.

Photodiodes are much similar to regularly used semiconductor diodes but they might be either uncovered or covered with an optical fiber connection or window to enable light to come and hit some of the sensitive device’s parts. To uplift the response of speed, several diodes intended for use as a photodiode utilizes a PIN junction in place of a P-N junction. A photodiode is designed basically to work in reverse bias.

Principle of Operation

As said earlier, a photodiode is a PIN structure or a P-N junction. Whenever a photon containing enough energy hits the diode, it forms an electron-hole pair. This process is called the photoelectric effect. If the assimilation happens in the depletion region of the junction, or one dispersion length away from it, these electron and holes are cleared from the junction by the electric field generated inside the depletion region. Thus the electrons go towards the cathode while the holes move toward the anode, producing a photocurrent. The net amount of current passing through the photodiode is equal to the sum of the dark current (current formed in the absence of light) and the photocurrent. Thus to get a maximum value of the device’s sensitivity, the dark current should be minimized.

Materials

The materials that can be utilized to create a photodiode is the basic to characterize its properties because only photons that are having enough energy to excite electrons across the bandgap of the material will generate notable photocurrents.

Materials that are used popularly to make photodiodes are:-

• Silicon – Range of electromagnetic spectrum wavelength is from 190nm to 1100nm.
• Germanium - Range of electromagnetic spectrum wavelength is from 400nm to 1700nm.
• Indium gallium arsenide - Range of electromagnetic spectrum wavelength is from 800nm to 2600nm.
• Lead (II) sulfide - Range of electromagnetic spectrum wavelength is from <1000nm to 3500nm.
• Mercury cadmium telluride - Range of electromagnetic spectrum wavelength is from 400nm to 14000nm.

The noise generated by silicon-based photodiodes is less as compared with the germanium-based photodiodes. The reason behind this is that they have greater bandgaps.

Comparison with Photomultipliers

A photomultiplier is a device that can generate an electrical signal from the incident photons.

Advantages of photodiodes over photomultipliers:

• Low noise.
• Light in weight and compact.
• It has a longer lifetime.
• High voltage is not needed.
• Quantum efficiency is high.

Disadvantages of photodiodes over photomultipliers:

• The area is small.
• Response time is slower.
• Overall sensitivity is much low.

This was all we have regarding photodiodes. Hope you enjoyed going through it. Brands that manufacture them are Osram, First Sensor, and Vishay. Make sure to follow them using the links as provided.

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