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Fig 7.3.3. V-I Characteristics Of photo transistor Applications:
Photo transistors can be used as light detectors. As light operated switches. In communication systems (demodulation of optical fibers). 7.4 Light emitting diode (LED):
LED is a forward biased p-n junction diode. When diode is in forward bias the electron in n-region crosses the p-n junction and recombine with holes. The free electrons in n-region are placed in conduction band. The holes in p-region are placed in valence band. Valence band has low energy than conduction band. When recombination takes place the recombining electrons release energy in the form of heat and light.
Fig 7.4.1 LED
In Ge and Si diodes, almost the entire energy is given up in the form of heat and no light is emitted. But in GaAs (Gallium arsenide) materials, the entire energy is given up in the form of light. Ga As LED produces red light. GaP (Gallium phosphide) produces green light. By varying the quantities of these elements, it is possible to produce light of different colors (wavelengths) like red, green, yellow and blue.
Fig 7.4.2 P-N junction diode
The fig.7.4.3 shows the schematic symbol for a LED. The arrows are shown as pointing away from the diode, indicating that light is being emitted by the device when forward biased. Although LED’s are available in several colors. Only important LED’s are red, green, orange and yellow.
Fig 7.4.5 Circuit for LED
Therefore voltage across Rs = Vs – Vd. Circuit current If = (Vs - Vd)/Rs. Advantages of LED:
The LED is a solid-state light source. The advantages of LED are low voltage, longer life (more than 20 years) and fast on/off switching(less than Nano seconds).
Applications of LED’s:
The LED is a low power device. The power rating of a LED is of the order of mill watts. This means that it is useful as an indicator but not good for illumination. The important application of LED’s is
- As a power indicator
- Seven segment display. 1. As a power indicator: A LED can be used to indicate whether the power is on or not. The fig. shows the simple use of LED as
a power indicator. When switch S is closed, power is applied to load. At the same time current also flow through the LED which lights, indicating power is on. The resistor Rs in series with the LED ensures that current rating of the LED is not exceeded.
2. Seven Segment Display: LED’s are grouped to form seven segments display. Fig (7.4.6) shows the front of seven segment display. It contains seven LED’s (A, B, C, D, E, F and G) shaped in fig of 8. LED’s A, B, C, D, E and F are arrange clock wise from the top with LED G in the middle.
Fig 7.4.6 Seven segment display
Each LED is called a segment. If a particular LED is forward biased, that LED or segment will light and produces a bar of light. By forward biasing various combinations of seven LED’s, it is possible to display any number from 0 to 9.For ex: if LED’s A, B, C, D and G are lit the display will show the number 3.
The fig.(7.4.6) shows the schematic diagram of seven segment display. External series resistors are included to limit currents to safe values. Here anodes of all seven LED’s are connected to a
- For on/off operations with low resistance relays.
- In industry for monitoring operations. Solar cell:
The most important photo voltaic device is the solar cell, which can convert solar radiation or sunlight directly into electricity with high efficiency (22%) by the photo voltaic effect.
Principle: When sun light falls on a p-n junction of diode more free electrons and holes are produced in this region than the illuminated region. These increased charge carriers disturb the electro static balance at the p-n junction. This causes an electric current to flow from the p- type to the n-type.
Fig 7.5.1 P-N junction diode in solar cell
Construction and Working:
Consider a p-n junction semiconductor diode. When p-n junction is formed electrons and holes are diffused across the junction. Then both sides of the junction positive immobile ions and negative immobile ions are created consequently a junction voltage is developed in the direction of n - region to p – region.
Fig 7.5.2 Circuit diagram for solar cell
Now sunlight is incident on the junction region, incident photons will break the covalent bonds of immobile ions and then electron – hole pairs are generated. Now these released electrons due to the attraction of positive immobile ions, they are pulled into n – region, similarly holes due to the attraction of negative immobile ions, they are pulled into p – region. The incident sunlight is continuously falling; there is a net negative charge on n – side and net positive charge on p – side. Consequently a potential is developed across the diode. This is directed from p – region to n – region. This voltage is in the opposite direction to junction voltage. When the magnitude of this produced voltage is equal to the magnitude of junction voltage both will cancel each other. Further voltage is developed from p – region to n – region which acts a battery.
corresponding voltage and current is called maximum power voltage Vmp and maximum power current Imp.
Efficiency of Solar cell: Solar cell energy conversion efficiency (n) is the percentage of power converted (from absorbed light to electrical energy) and collected, when a solar cell is connected to an electrical circuit. This term is calculated using the ratio of the maximum power point Pm, divided by the input light irradiance (E, in w/m 2 ) and the surface area of the solar cell A in m^2 n = Pm/E.A = Vmp.Imp/E.A
Solar cells are constructed with silicon, gallium arsenide and cadmium sulphide with many other semiconductors and in various device configurations.
The maximum efficiency for Si cell is about 22% and it depends on width of the depletion layer. The depletion layer width is low then Voc is low and Isc is high. Depletion layer width is high then Voc is high and Isc is low.
Advantages of Solar cells:
- Solar cells have long life.
- Pollution free.
- Maintenance free.
Applications:
- Solar cells are used extensively in satellites and space vehicles to provide power for running various electronic equipment.
- To run house hold devices, in calculators.
- Automatic switching on and off the street light. At night, when no sunlight falls on cell, the current stops and with the help of a relay system, the street light is automatically switched on.
- Large area arrays of solar cells are the most important for long duration power supply for satellites and space vehicles. Summary:
Opto electronic devices are the devices which converts the light energy into electrical energy. Electro optic devices are the devices which converts the electrical energy into light energy. The examples for opto electronic devices are photo diode, photo transistor, and solar cell The examples for electro optic devices are LED, LCD. Photo diodes are used in alarm circuits Solar cells have so many applications such as in storage of batteries, in calculators, in watches etc. LED’s are used in display devices in air ports and railway stations.
