Photodiode and LIGHT EMITTING DIODE

Presentation by JASWANT KUMAR ROLL NO.-12 IT(3rd SEM.)

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• A light emitting diode (LED) is essentially a PN junction opto-semiconductor that emits a monochromatic (single color) light when operated in a forward biased direction.

• LEDs convert electrical energy into light energy.

About LEDs (1/2)

LED SYMBOL

ABOUT LEDS (2/2)

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• The most important part of a light emitting diode (LED) is the semi-conductor chip located in the center of the bulb as shown at the right.

• The chip has two regions separated by a junction.

• The junction acts as a barrier to the flow of electrons between the p and the n regions.

LED CIRCUIT• In electronics, the basic LED circuit is an electric

power circuit used to power a light-emitting diode or LED. The simplest such circuit consists of a voltage source and two components connect in series: a current-limiting resistor (sometimes called the ballast resistor), and an LED. Optionally, a switch may be introduced to open and close the circuit. The switch may be replaced with another component or circuit to form a continuity tester.

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HOW DOES A LED WORK?

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• Each time an electron recombines with a positive charge, electric potential energy is converted into electromagnetic energy.

• For each recombination of a negative and a positive charge, a quantum of electromagnetic energy is emitted in the form of a photon of light with a frequency characteristic of the semi-conductor material.

Mechanism is “injection Mechanism is “injection Electroluminescence”. Electroluminescence”. Luminescence Luminescence part tells us that we are producing photons.part tells us that we are producing photons.

Electro part tells us that Electro part tells us that the photons are being produced the photons are being produced by an electric current.by an electric current.

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Injection tells us that Injection tells us that photon production is by photon production is by the injection of current carriers.the injection of current carriers.

Mechanism behind photon emission in LEDs?

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Producing photonElectrons recombine with holes.

CB

VB

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h

CB

VB

Energy of photon is the energy of Energy of photon is the energy of band gap.band gap.

holes

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• When sufficient voltage is applied to the chip across the leads of the LED, electrons can move easily in only one direction across the junction between the p and n regions.

• When a voltage is applied and the current starts to flow, electrons in the n region have sufficient energy to move across the junction into the p region.

How Much Energy Does an LED Emit?

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• The energy (E) of the light emitted by an LED is related to the electric charge (q) of an electron and the voltage (V) required to light the LED by the expression: E = qV Joules.

• This expression simply says that the voltage is proportional to the electric energy

• The constant q is the electric charge of a single electron, -1.6 x 10-19 Coulomb.

Colours of LEDs

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• LEDs are available in red, orange, amber, yellow, green, blue and white.

• LEDs are made from gallium-based crystals that contain one or more additional materials such as phosphorous to produce a distinct color.

• Different LED chip technologies emit light in specific regions of the visible light spectrum and produce different intensity levels.

LED Characteristics & Colours

• One of the major characteristics of an LED is its colour. The different LED characteristics have been brought about by a variety of factors, in the manufacture of the LED. The semiconductor make-up is a factor, but fabrication technology and encapsulation also play major part of the determination of the LED characteristics.

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Bargraph 7-segment Starburst Dot matrix

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Some Types of LEDs

LED Performance

• Color• White light• Intensity• Eye safety information• Visibility• Operating Life• Voltage/Design Current

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LED performance is based on a few primary characteristics:

Applications

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• Sensor Applications

• Mobile Applications

• Sign Applications

• Automative Uses

• LED Signals

• Illuminations

• Indicators

2. PHOTODIODE

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What is photodiode Photodiodes are semiconductor light sensors

that generate a current or voltage when the P-N junction in the semiconductor is illuminated by light

A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation. The common, traditional solar cell used to generate electric solar power is a large area photodiode.

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Symbol of photodiode

Principle of operation• A photodiode is a p-n junction or PIN

structure. When a photon of sufficient energy strikes the diode, it excites an electron, thereby creating a free electron (and a positively charged electron hole). This mechanism is also known as the inner photoelectric effect. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. This photocurrent is the sum of both the dark current (without light) and the light current, so the dark current must be minimized to enhance the sensitivity of the device.

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PHOTODIODE CROSS SECTION

Photodiode P-N junction state

Photodiode types

• PN photodiode • PIN photodiode • Schottky type photodiode • APD (Avalanche photodiode)

All of these types provide the following features and are widely used for the detection of the intensity, position, colour and presence of light.

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Features of photodiode

• Excellent linearity with respect to incident light • Low noise • Wide spectral response • Mechanically rugged • Compact and lightweight • Long life

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APPLICATION of photodiode

P-N photodiodes are used in similar applications to other photodetectors, such as photoconductors, charge-coupled devices, and photomultiplier tubes. They may be used to generate an output which is dependent upon the illumination (analog; for measurement and the like), or to change the state of circuitry (digital; either for control and switching, or digital signal processing).

Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, and the receivers for infrared remote control devices used to control equipment from televisions to air conditioners. For many applications either photodiodes or photoconductors may be used. Either type of photosensor may be used for light measurement, as in camera light meters, or to respond to light levels, as in switching on street lighting after dark.

Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a more linear response than photoconductors.

They are also widely used in various medical applications, such as detectors for computed tomography .

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Photodiode Alarm circuit

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This Photodiode based Alarm can be used to give a warning alarm when someone passes through a protected area. The circuit is kept standby through a laser beam or IR beam focused on to the Photodiode. When the beam path breaks, alarm will be gets active.

Working of alarm circuit• The circuit uses a PN Photodiode in the reverse bias mode to detect light

intensity. In the presence of Laser / IR rays, the Photodiode conducts and provides base bias to T1. The NPN transistor T1 conducts and takes the reset pin 4 of IC1 to ground potential. IC1 is wired as an Astable oscillator using the components R3, VR1 and C3. The Astable operates only when its resent pin becomes high. When the Laser / IR beam breaks, current thorough the Photodiode ceases and T1 turns off. The collector voltage of T1 then goes high and enables IC1. The output pulses from IC1 drives the speaker and alarm tone will be generated.

23IR Transmitter Circuit

A simple IR transmitter circuit is given which uses Continuous IR rays. The transmitter can emit IR rays up to 5 meters if the IR LEDs are enclosed in black tubes.

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Thank you for your Attention!