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When the light intensity getting increased the Photo Diode will be act like Zener diode. It starts to let the current to be flowed from cathode to anode.

For normal operation of photo diodes must be connectied to circuites with inverse polarity. If we connect photo diodes with normal polarity, it will act like normal diodes and we can not use it according to light intensity.

So we have to apply the negative(-) voltage to photodiodes anode and positive(+) voltage to photodiodes cathode.

Below image you can see the both connections on the simple circuit.

Basic Principle of PhotoDiodes

When we connect a photodiode with normal polarity, it will act like normal diode.

When We conect a photodiode with inverse polarity, it will not transmit energy until apply light. To activate this photodiode we have to apply a light.

Due to Inverse polarity on the photodiode, a neutral zone occurs with no load. When light applied on this zone, free electrons increase and with this current transmitting will be started on this area.

Below image you can see the simple current flowing with appliyng the light on photo diode.

Operational Modes of PhotoDiode

There are two operational modes of Photodiodes. These are

• Photovoltaic Mode This mode is also known as zero bias mode. No external voltage applied to photodiode in this mode. This mode is using generally at low-speed applications or to catch low intensity lights.
• Photoconductive Mode An inverse bias supplied to the photodiode in this mode. According to inverse bias the depletion area will be increased and decrease the capacitans. This will increase the operation speed and also make the photodiode sensitive to noise.

Photo Diode Types

As we call Photodiode there are 4 kind of photodiode exist. These types of photodiodes are in use at differant areas.

• PN photodiode This type of photodiode is first developed of photodiodes.
• Schottky photodiode This form of photodiodes are developed based on Schottky diodes. This type of photodiode has a small junction zone ant low capacitance. According to this specification it has very high speeds. With this specification it is using the ares which needs high speeds like fiber optic communications.
• PIN photodiode This type of photodiodes is the very efficient way to catch light photons. It has wider instrinct area to catch the light photons between P and N junctions. With this specification PIN photodiode has widely usage area. Also has a small capacitance between PN junction.
• Avalanche photodiode This type of photodiodes has high gain specification. So it is using under low intensity lights else it will has high noise values.

Performance parameters of a photodiode

There are some parameters to decide the performance of photodiode.

• Responsivity This parameter is refers the generated photocurrent according to light intensity. It is a ratio of generated photocurrent to light intensity.
• Quantum efficiency This parameter refers to generated electron-hole pairs according to incident photons. We can specify this ratio as photoelectrons/incident photons.
• Response time This parameter refers the time which takes the charge carring cross pn juntion with the generated light.

That is all in this article

Have a good photon catching.

Burak Hamdi TUFAN