In this lecture we are going to learn about the Light Emitting Diode, the Working principle of LED, the Construction and working of LED, advantages, and applications of LED in a very detailed manner. So let us start with the basic knowledge of what is LED.
Light Emitting Diode (LED)
- The LED is an optical diode, which emits light when forward biased. Figure. 1 shows the symbol of LED which is similar to the p-n junction diode apart from the two arrows indicating that the device emits light energy.
Basic Operation of Light Emitting Diode (LED)
- Whenever a p-n junction is forward biased, the electrons cross the p-n junction from the n-type semiconductor material and recombine with the holes in the p-type semiconductor material. The free electrons are in the conduction band while the holes represent the valence band. Thus the free electrons are at a higher energy level with respect to the holes. When a free electron recombines with the hole, it falls from the conduction band to a valance band. Thus the energy level associated with it changes from a higher level and a lower level is released by an electron while traveling from the conduction band to the valance band.
- In the normal diodes, this energy released is in the form of health. But LED is made up of some special material that releases this energy in the form of photons which emit light energy. Hence such diodes are called Light Emitting Diode. This process is called electroluminescence.
- The above figure shows the basic principle of this process. The energy released in the form of light depends on the energy corresponding to the forbidden gap. This determined the wavelength of the emitted light. The wavelength determines the color of the light and also determines whether the light is visible or invisible(infrared). The various impurities are added during the doping process to control the wavelength and color of the emitted light energy corresponding to that of the infrared light spectrum hence in the normal diodes the light is not visible. The infrared light is not visible.
Materials and Colors of Light Emitting Diode(LED)
- The LEDs use mixtures of Gallium (Ga), Arcesinc (As), and Phosphorous (P).
- The color of the emitted light is decided by its wavelength which depends on the forbidden energy gap. THis gap is different mixtures. Hence different mixtures give different colors.
|Sr. No.||Mixture Used||Symbol||Color|
|1.||Gallium arsenide||GaAs||Infrared, invisible|
|2.||Gallium phosphide||GaP||Red or green|
|3.||Gallium arswenide phosphide||GaAsP||Red or yellow|
Construction of LED
- One of the methods used for LED construction is to deposit three semiconductor layers on the substrate as shown in the figure below.
- In between the p type and n type three exist an active region that emits light when an electron and hole recombine. When the diode is forward biased, holes from p-type and electrons from n-type both get driven into the active region. And when recombine the light is emitted.
- In this particular structure, the LED emits light all the way around the layered structure. Thus the basic layered structure is placed in a tiny reflective cup so that the light from the active layer will be reflected towards the desired exit direction. This is shown in the below figure. while the symbol of LEd indicating identification of anode and cathode is shown in the figure below.
Biasing of LED
- Consider a source connected to LED and a resistor as shown in the figure.
- The outward arrows associated with a diode indicate that it is LED.
- The resistor Rs is the current limiting resistor. Due to this resistor, the current through the circuit is limited and prevented from exceeding the maximum current rating of the diode.
Let, Vs = Supply Voltage and VD = Drop across LED
- Applying the KVL to the circuit we can write,
V_S=I_SR_s + V_D
- When forward biased, the voltage drop across conducting LED is about 2 to 3 V which is considerably greater than the across a normal silicon or germanium diode. The current range of commercially available LEDs is 10 to 80 mA. Unless otherwise specified, while analyzing LED circuits, the drop across LED is considered as VD = 2V.
- The reverse breakdown voltage of LED is much less than the normal diode, which is about 3V to 10V.
Spectral Output Curves for LED
- The visibility of her light is decided by its wavelength ( \lambda ) . The graph of the output light of LED against the wavelength ( \lambda ) gives the various curves called spectral output curves for LEDs.
- it can be seen that wavelength is expressed in nm (nanometers). The normalized output for visible light shows a peak at 460 nm for blue, 540 nm for green, 590nm for yellow, and 660 nm for red. The infrared invisible light output shows a peak at 940 nm.
Radiation Pattern of LED
- LED is a directional light source. It emits the light in a particular way which has a typical radiation pattern. Maximum emitted power is perpendicular to the emitting surface. The figure shows a radiation pattern for a typical LED, which shows that most of the energy is emitted within 20o of the direction of maximum light.
- Some LEDs use plastic lenses to spread the light for a greater angle to increase visibility. The colored lenses are also used to enhance the color.
Output Characteristics of LED
- The amount of Power output translated into the light is directly proportional to the forwarded current If. More the forward current If, the greater the output light. The graph of forwarded current and output light is mW is shown in the figure. These are called output characteristics for LED.
- When forward biased, the voltage drop across conducting LED is about 2 to 3 V which is considerably greater than that across a normal silicon or germanium diode. The current range of commercially available LEDs is 10 to 80 mA. Unless otherwise specified, while analyzing the LEd circuits, the drop across lED is considered as VD=2V.
- The reverse breakdown voltage of LED is much less than the normal diode, which is about 3V to 10V.
Important Parameter of LED Characteristics
- The two important parameters related to LED characteristics are,
1. Radiant Intensity:
- The LED output power per steradian is called axial radiant intensity. The symbol for radiant intensity is Ie. The steradian (Sr) is the unit of solid angle. The unit of radiant intensity is W/Sr or mW/Sr.
- The power per unit area at a given distance from the LED source is called irradiance. It is denoted as H and expressed in W/cm2 or mW/cm2. Mathematically irradiance can be calculated as,
where, Ie= Radiant intensity, and d= Disntace form LED source in cm.
Advantages of LED
- The various advantages of LED are,
- LEDs are small in size, and hence can be regarded as a point source of light. Because of their small size, several thousand LEDs can be packed in one sq. meter area.
- The brightness of light emitted by LED depends on the current flowing through the LED. hence the brightness of the light can be smoothly controlled by varying the current. This makes it possible to operate LED displays under different ambient lighting conditions.
- LEDs are fast operating devices. They can be turned on and off in time less than 1 microsecond.
- The LEDs are light in weight.
- The LEDs are available in various colors.
- The LEDs have a long life.
- The LEDs are cheap and readily available.
- The LEDs are easy to interface with various other electronic circuits.
- Some LEDs radiate infrared light which is invisible but still useful in some applications like burglar alarm systems.
- LEDs are useful for applications that are subjected to frequency on-off cycling. The fluorescent lamps burn out more quickly when cycled.
- LEDs can be easily dimmed using pulse width modulation or by controlling the forward current.
- LEDs are shock resistant and difficult to damage due to external shocks.
- LEDs do not contain toxic materials like mercury which is used in fluorescent lamps.
Disadvantages of LED
The various disadvantages of LED are,
- It draws considerable current requiring frequent replacement of battery in low power battery operated devices.
- The luminous efficiency of LEDs is low which is about 1.5 lumen/watt.
- The characteristics are affected by temperature.
- Need large power for the operation compared to normal p-n junction diode.
Applications of LED
Due to the advantages like low voltage, long life, cheap, reliability, fast on-off switching, etc, the LEDs are used in many applications. The various applications of LED are,
- All kinds of visual displays i.e. seven segment displays and alphanumeric displays such displays are commonly used in the watches and calculators.
- In optical devices such as optocouplers.
- As on-off indicator in various types of electronic circuits.
- Some LEDs radiate infrared light which is invisible. But such LEDs are useful in remote controls and applications like a burglar alarms.
Comparison of LED and P-N Junction Diode
|Sr. No.||LED||P-N Junction Diode|
|1.||It emits light when forward biased.||It does not emit light.|
|2.||It uses materials like gallium, arsenide, phosphide, and gallium phosphide.||It uses materials like silicon and Germanium.|
|3.||The drop across forwarded biased LEd is about 2V.||The drop across forwarding biased diode is about 0.7V, much less than that of LED.|
|4.||Reverse breakdown voltage is low, about 3V to 10V.||Reverse breakdown voltage is high, about 50V or more.|
|5.||Needs large power for the operation.||Needs less power for the operation.|
|6.||Draws considerable current from the battery.||Draws less current.|
|7.||The applications are optocouplers, seven segment displays, and alphanumeric displays.||The applications are rectifiers, clippers, clampers, voltage multipliers, and many other electronic circuits.|
Frequently Asked Questions:
What is LED?
A light-emitting diode (LED) is a semiconductor device that emits light when an electric current flows through it.
What is Light Emitting Diodes used for?
LEDs have a wide range of applications ranging from your mobile phone to large advertising billboards. They mostly find applications in devices that show the time and display different types of data.
How do LEDs work?
LEDs work on the principle of Electroluminescence. On passing a current through the diode, minority charge carriers and majority charge carriers recombine at the junction. On recombination, energy is released in the form of photons. As the forward voltage increases, the intensity of the light increases and reaches a maximum.
What is Electroluminescence?
Electroluminescence is an optical phenomenon, and electrical phenomenon where a material emits light in response to an electric current passed through it.
What are the advantages of LEDs?
LEDs consume less power, and they require low operational voltage. No warm-up time is needed for LEDs.
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