What is photodiode

The photodiode is a device that operates in reverse bias. The photodiode has a small transparent window that allows light to strike the pn junction.

To make the concept more clearly about the photodiode, I tell about it in more precise method. Photodiode is a semiconductor device. It has two leads. It transforms the light energy into electric current.

Some types of photo-diodes are shown below

When reverse biased, a rectifier diode has a very small reverse leakage current.  The same is true for Symbol of Photodiode, Definition of Photodiode,photo-diode.  The reverse biased current is produced by thermally generated electron hole pairs in the depletion region, which are swept across the pn junction by the electric field created by the reverse voltage.  In a rectifier diode, the reverse leakage current increases with temperature due to an increase in the number of electron hole pairs.

A photo-diode differs from a rectifier diode in that when its pn junction is exposed to light, the reverse current increases with the light intensity.  When there is no incident light, the reverse current, Iλ, is almost negligible and is called the dark current.  An increase in the amount of light intensity, expressed as irradiate (mW/cm2) , produces an increase in the reverse current.

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Photo Diodes

Photo Diode Operation:

To learn about the basic operation and rules of photo diode, we assume that, we connect the two leads anode and cathode of photo diode with current meter. And we place the photodiode in dark. What we will see reaction on meter? We see that the current meter displays zero current flow and similarly when the photo diode is exposing to light, it acts as current source which cause current flow from the cathode to anode through current meter.

Another Example about the Operation and work of Photodiode:

In the below figure diagram we can see a thin N-type semiconductor sandwiched with thick P-type semiconductor. Where N Side is Cathode and P side is anode. On illumination, a number of photons move from N- side into P side of photodiode. Some photons making it into P side collide with bound electrons which result ejection and creation of holes. It such type of collisions are close to P and N interface then ejected electrons move or cross the junction, yielding extra electrons on the N- side and extra holes on the P-side.

Due to segregation of Positive and Negative charges, potentials difference is created across the junction. When a wire is connected between the cathode and anode, a conventially +ve current flow from the anode to cathode.

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Operation of Photodiode

Photodiode Applications – Photovoltaic Current Source

Photodiodes are used to convert the light energy into electric current. And we can measure this current by meter.

The input intensity of light and output current are about linear.

Applications of Photodiode




  • Often photodiodes have built in lenses and optical filter.
  • The response time of photodiode slows down the increasing surface area.
  • Photodiodes are more sensitive than Photo resistors.

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