January 30, 2019


Photo-diode: It is define as P-N junction diode which converts light energy into electrical energy. It is connected in reverse bias configuration. It has small amount of reverse saturation current (in nano-amperes) which is produced due to thermally energy (light energy).


Types of Photo-diode

  1. PN photo-diode
  2. Schottky photo Diode
  3. PIN photo-diode
  4. Avalanche Photo-diode

Important features of Photo-diode are following:

  1. It can turn its current ON and OFF in nanoseconds. It is one of the fastest photo detectors.
  2. Operational sound is low
  3. Conversion from light energy to electrical energy is precise
  4. Robust construction and long life

Construction of Photo-diode

P-type and N-type semi-conductor are diffused then ohmic junction (has some resistance) formed. Above this junction a lens has been used in the cap of the unit to focus maximum light on junction. It is mounted on TO-5 packages and has active diameter of 2.5 mm.

Working of Photo-diode

When light energy strikes on lens of the photo-diode. Lens concentrates light on ohmic junction due to which electrons and holes are generated at junction, because of this the resistance of junction decreases. Then, holes of the region start move toward the anode, and electrons move toward the cathode which generates photo-current.

V-I Characteristics of Photo-diode

A photo-diode operates in a reverse bias mode. We get V-I characteristics of photo-diode by connecting the photo-diode in reverse bias. The current that flows when no light is incident are refers to dark current. The characteristics show that for a given reverse voltage increases with increase in the level of illumination. As optical power rises the photo current also rises linearly.

At zero luminance, the photo-current is almost zero excluding for small dark current.

At 10,000 luminance, the photo-current is almost 100 micro amps.


Applications of Photo-diode

  1. Detection of visible as well as invisible lights.
  2. For switching electronic devices.
  3. Used in logic circuits that requires stability and high speed.
  4. It is used in optical communication equipment.


Leave a Reply