Unit 2 Semiconductor Devices Lecture_2.5 Opto-Electronic Devices
Opto-electronics Opto-electronics is the study and application of electronic devices that interact with light. Electronics (electrons) Optics (light or photons) Optoelectronics
Opto-Electronic devices Light Dependent Resistor (LDR). Photo Diode. Photo Transistor. Solar cell. Opto coupler, etc
Photoconductive effect When radiation is incident on a semi-conductor, some absorption of light by the material takes place, and its conductivity increases. Energy content of a photon, E = hf h Planck s constant = 6.626 *10^-34 J s. f Frequency of the incident light. If f is low, E < E g, light passes through the material with very little absorption. If f is high, E E g, light is absorbed by the material creating electron-hole pairs; increased conductivity. Such a material is called photo resistor or photoconductor. The minimum frequency f c to cause photoconduction is, f c = E g h.
Light dependent resistor or Photoconductive cell Made up of a thin layer of semiconducting material like cadmium sulfide or lead sulfide, enclosed in a sealed housing. A glass window in the housing allows light to fall on the active semiconducting material of the cell. Its resistance decreases in the presence of light and increases in the absence of light. Acts as a conductor whose resistance changes when illuminated. Used in control circuits to control the current.
Light dependent resistor or Photoconductive cell
Photodiode Silicon photodiode is a light sensitive material photo detector. Converts light signals to electrical signals. A lens permits light to fall on the junction. When light fall on reverse biased PN junction diode, electron-hole pairs are created, whose movement causes current flow. Current is proportional to the intensity of light and affected by the frequency of light falling on the junction of the photodiode. The reverse current increases in direct proportion to the level of illumination.
Photodiode When no light is applied there is a minimum reverse leakage current flowing through the device dark current. Used as light detectors, demodulators, encoders, optical communication systems, switching circuits, etc
Photo transistor Current produced by photodiode is very low and cannot be used directly for control applications. Photo transistor is a light detector which combines a photo diode and a transistor amplifier. Upon illumination, permits a greater flow of current. Base-collector junction is illuminated with light focused through the lens. Base current is supplied by the current created by light illumination. Current is dependent mainly on the intensity of light and less affected by the external voltage applied.
Photo transistor
Photo voltaic cell A light-sensitive semiconductor device; produces a voltage when illuminated. `Consists of a piece of semiconductor material such as silicon, germanium or selenium bonded to a metal plate. Voltage increases as the intensity of light falling on the semiconductor junction increases. Used in low power devices like light meters. With improved efficiency, more power is produced as in solar cells.
Photo voltaic cell
Solar cell When sun light is incident on a photovoltaic cell, it is converted into electric energy. Consists of a single semiconductor crystal doped with both P and N type materials. About 0.6 V is developed by the solar cell in bright sun light. Amount of power depends on extent of active surface. 30 mw per sq. in. is produced by an average cell operating in a load of 4 ohm. To increase the power output, cells are used in series and parallel combinations. Solar cell or solar battery is used in satellites to provide electric power.
Solar cell
Optocoupler Solid state component which has light emitter, light path and light detector enclosed within a component and cannot be change externally. Provides electrical isolation between circuits opto isolator. Allows signal transfer without coupling wires, capacitors or transformers. Can couple both analog (variable signals) and digital (ON/OFF) signals.
Optocoupler For highest coupling, wavelength response of each device is made identical. Extent of isolation depends on kind of materials in light path and distance between light emitter and detector. High isolation resistance of 10^11 ohm and isolation voltage upto 2500 V. Used as an interface between high voltage and low voltage systems. Used in interfacing different types of logic circuits and in level-and-position sensing circuits.
Optocoupler
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