KTU PHYsics S1 Physics for it_PHOTONICS.pptx

It is the 4th module of physics for informative science for 1st year s1 ktu students Group A.

1. PHOTONICS Module-5

2. INTRODUCTION It deals with the generation, control and detection of photons It deals with how light can be used for the effective transmission of information The word ‘Photonics’ is derived from the Greek word ‘Photos’ meaning light. Photonics developed with the invention of laser in 1960 Photonics→Photons+Electronics In photonics,photons have the same role as electrons in electronics Photonic devices have a number of advantages over electronic devices because of the very high speed of light.

3. Photonic Devices Photonic devices are components for creating, manipulating or detecting light. It includes Diode lasers, LEDs, Solar cells, Photodiodes etc Photonic devices are less sensitive to interference They have the advantage of energy saving and large communication distances This device can be found in many applications like military services, telecommunication and medical equipments .

4. Light Emitting Diodes(LEDs) It is a pn junction diode that emits light when it is forward biased. In other words, LED is an optical semiconductor device that converts electrical energy into light energy. Symbol

5. Direct band gap and Indirect band gap semiconductors Direct band gap semiconductors-13-15 th group elements Egs : GaAs,GaP,GaAsP Indirect band gap semiconductors-12-16 th group elements Egs : CdS,ZnS,CdSe

6. Direct bandgap semiconductors are used to make LEDs If the pn junction is made of direct bandgap semiconductors, the emitted energy falls either in visible or in infrared region. If the pn junction is made of Si or Ge, the emitted energy appears as heat. Therefore Si and Ge are not suitable for making light emitting diodes.

7. WORKING When a pn junction is forward biased, the electrons in the CB in n side move to p side and combine with the holes in the VB The energy level of free electrons in the conduction band is high compared to the energy level of holes in the valence band. Therefore, free electrons in the conduction band need to lose energy in order to recombine with the holes in the valence band. The emitted energy falls either in visible region or in infrared region

8. The colour or wavelength of light emitted depends on bandgap energy , λ = h-Planck’s constant,c -velocity of light, E g -bandgap energy The visible light LEDs have a bandgap between 1 .8eV and 2 .8eV  

9. I-V Characteristics It is similar to that of an ordinary diode except in the knee voltage(V F ) Knee voltage is different for different colours

10. ADVANTAGES Small size and long life Available in all colours Fast on-off switching ability Heat radiation is less Low power consumption Do not contain mercury as in CFL

11. Applications Remote controls and burglar alarms Seven segment display Power Indicators Flash lamps in mobiles and cameras Optical communication system Stage decoration Traffic Signals Head light and Indicator light of vehicles

12. PHOTODETECTORS It is a device which converts optical signals into electrical signals Photovoltaic Mode Here the junction is treated as an open circuit to measure the voltage across the junction Eg . Solar Cell Photoconductive Mode Here the pn junction is connected in the reverse bias and the current is made to flow externally to complete the circuit between the junction Eg.Photdiode

13. photodiode It is a pn junction device that generates current when exposed to light It operates in the reverse mode and converts light energy into electrical energy Si and Ge are used to make photodiodes Symbol of photodiode

14. Types of photodiodes A. JUNCTION PHOTODIODE It is a reverse biased pn junction. It consists of a thin p type layer on an n substrate

15. A. JUNCTION PHOTODIODE - Working of Photodiode Due to reverse biasing, the minority charge carriers are attracted towards the junction –leakage current or dark current Incident light causes the creation of a large number of electron hole pairs- Photocarriers Photocarriers produce a current called photocurrent in addition to the dark current. Width of the depletion region should be increased to absorb a large quantity of light

16. APPLICATIONS Cd players Smoke detectors Remote control devices Security systems Optical communication

17. PIN(Positive Intrinsic Negative) PHOTODIODE It is a semiconductor device in which an intrinsic semiconductor is sandwiched between a p type layer and an n type layer

18. PIN PHOTODIODE

19. PIN PHOTODIODE-working When light is incident on the diode, electrons are excited from VB to CB. This produces a large number of electron hole pair carriers Because of the large width, the intrinsic layer absorbs a very large number of photons compared to the p and n regions This increses photocurrent

20. PIN PHOTODIODE-applications Photodetection Optical communication To measure extremely low light intensity Fast Switches Attenuators High power electronic Applications

21. Solar cell or photovoltaic cell It is basically a pn junction which converts sunlight into electrical energy

22. Solar cell-Principle It is based on the principle of photovoltaic effect Photovoltaic effect-It is the generation of voltage across the pn junction in a semiconductor due to the absorption of light radiation The  photovoltaic effect  was first discovered in 1839 by Edmond  Becquerel

23. Solar cell The semiconductor materials like silicon, arsenide, indium, cadmium, selenium and gallium are used for making the PV cells. Mostly silicon and selenium are used for making the cell. Symbol

24. Solar cell-working When sunlight falls on a pn junction the electrons which are very loosely attached to their parent atoms absorb photons from the incident light and get excited to higher energy levels. This produces a large number of electron hole pairs. The electrons have a tendancy to move to the p side and holes to n side. But the depletion region prevents the diffusion of these charge carriers

25. Solar cell-working As a result electrons get accumulated in the n side and holes to p side .Because of the difference in their charges,p.d exists across the junction called open circuit voltage. Solar cell-working

26. SOLAR CELL-I-V CHARACTERISTICS It can be determined by connecting a voltmeter, an ammeter and a load as shown in the figure. A graph is drawn between voltage and current

27. SOLAR CELL-I-V CHARACTERISTICS Open Circuit Voltage ( V oc ) of Solar Cell It is measured by measuring  voltage  across the terminals of the cell when no load is connected to the cell. Normally open circuit voltage of silicon solar cell nearly equal to 0.5 to 0.6 volt. Short Circuit Current ( I sc ) of Solar Cell It is measured by short circuiting the terminals of the cell . It is the current through the solar cell when the voltage across it is zero.

28. Maximum Power Point of Solar Cell The output power is maximum for a specific load resistance . If we draw the V-I characteristics of a solar cell maximum power will occur at the bend point of the characteristic curve. It is shown in the V-I characteristics of solar cell by P m . Current at Maximum Power Point The current at which maximum power occurs. Current at Maximum Power Point is shown in the V-I characteristics of solar cell by I m . Voltage at Maximum Power Point The voltage at which maximum power occurs. Voltage at Maximum Power Point is shown in the V-I characteristics of solar cell by V m . SOLAR CELL-I-V CHARACTERISTICS

29. It is a measure of the quality of the solarcell It is calculated by comparing the maximum power to the theoretical power FF= P max /P T The ratio between product of current and voltage at maximum power point to the product of short circuit current and open circuit voltage of the solar cell FF = = A typical fill factor is about 0.8 The higher the fill factor, maximum is the power output   SOLAR CELL-I-V CHARACTERISTICS

30. Applications Used in watches , calculators Only source power for artificial satellites Solar Automobiles Villages can be electrified with the help of solar cells in remote areas Street light Electric fences