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Definition of Photo Diode
Photo means light and diode means a device consisting of two electrodes.A photo diode is a light sensitive electronic device capable of converting light into a voltage or current signal. It works on the principle of photo generation.
Symbol of Photo diode
The symbol of photo diode is shown in figure
Photo diode has two terminals anode and cathode with the arrows indicating that the light rays falling on photo diode reflecting its significance as a photo detector.
Types of photo diodes
There are mainly three types of photo diodes
- PN junction photo diode
- Avalanche photo diode
- PIN photo diode
Normal PN junction photo diode is used in low frequency and low sensitive applications. When high frequency of operation and high sensitivity is needed avalanche photo diode or PIN photo diodes are used.
Physical Structure of photo diode
A normal PN junction photo diode is made by sandwiching a P type semiconductor into N type semiconductor. All the sides of PN junction diode is enclosed in metallic case or painted black except for one side on which radiation is allowed to fall.
Modes of operation of Photo diode
A photo sensitive diode can be operated mainly in two modes
- Photo conductive mode
- Photo voltaic mode
The photo diodes used as photo detectors are optimized (in the physical construction of the device itself) to have fast response times whereas the photo diodes used in electrical energy generation are optimized to have high efficiency of energy conversion. The photo detectors are operated in photo conductive mode. Solar cells are operated in Photo voltaic mode.
Principle of operation
When a PN junction is illuminated with light, it ionizes covalent bonds and new hole, electron pairs are generated in excess of thermally generated pairs. If the photo generation occurs at a distance of the diffusion length order or less depletion layer the photo generated electron hole pairs are swept across by the applied reverse bias field. This mode of operation of photo diode is called photo conductive mode. The photo current varies almost linearly with incident light flux or optical power. The mode of operation of photo diode is called photo conductive diode. The current equation of a Photo diode is given as
I = IS+ IO*(1-eV/ (η*Vt))
Where η is 1 for Ge and 2 for Si, Vt is voltage equivalent of temperature, Is the short circuit current proportional to light intensity, Io is reverse saturation current of diode, V is positive for forward bias and negative for reverse bias. Photo current is diffusion current.
There can be two types of photo excitations they are a) intrinsic excitations b) Extrinsic excitations
Intrinsic excitations occur when an electron in valence band is excited by a high energy photon to conduction band. Alternatively a photon may excite an electron in donor level to conduction band or a valence band electron may go into acceptor state. Such excitations are termed as extrinsic excitations.
The minimum energy of light required for photo generation due to intrinsic excitation is the forbidden gap energy EG. The wavelength associated with this critical energy is known as critical wavelength.
λ = 1.24/ EG (in eV)
Photons with wavelength greater than critical wavelength cannot generate new charge carrier pairs.
Responsivity, Quantum efficiency, Dark current of photo diode
Responsivity is defined as the ratio of photo generated current to incident light power.It is measured in units of amp/watt.
Responsivity R = IS/Pin
Where IS is the photo current
Quantum efficiency is defined as fraction of incident photons contributing to photo current. It is unit less as it is a fraction.
Where NE is the number of generated carriers/unit time
NPhotons is the number of incident photons/unit time
Dark current is the current through the diode for zero illumination. It will be non zero due to back ground radiation and thermally excited minority saturation current.
The relation between the Responsivity and quantum efficiency is given as
Q = R*h*ν/e and R = IS/Pin
Where R is Responsivity, Q is Quantum efficiency, h is Planck’s constant, ν is frequency of incident photon, Energy of h*ν photon = h*ν, IS is photo generated current, Pin is incident optical power.
Hence the current in a photo diode is given as
VI characteristics of photo diode
The VI characteristics of a photo diode is shown in the figure
A photo diode is always operated in reverse bias mode. From the photo diode characteristics it is seen clearly that the photo current is almost independent of applied reverse bias voltage. For zero luminance the photo current is almost zero except for small dark current. It is of the order of nano amperes. As optical power increases the photo current also increases linearly. The maximum photo current is limited by the power dissipation of the photo diode.
Disadvantages of normal PN junction photo diodes
- Normal PN junction photo diodes have very high response times.
- It has very low sensitivity
Applications of photo diodes
- Photo diodes are used as photo detectors
- Photo diodes are used in providing electric isolation using a special circuitry called as Optocouplers. Optocoupler is an electronic component which is used in coupling optically the two isolated circuits by using light. The two circuits are optically coupled but electrically isolated. It is a combination of light emitting diode and photo diode (or) avalanche diode (or) photo transistor. Optocouplers are faster than the conventional devices.
- They are used in consumer electronics
- They are used in cameras as photo sensors, Slotted optical switch, in scintillators e.t.c