[Physics Class Notes] on Light Emitting Diode Pdf for Exam

The abbreviation for light-emitting diode is LED. In simple words, a light-emitting diode is a semiconductor device that emits light when current passes through it. When the particles (electrons and holes) within the semiconductor material carry the electric current, it produces light.

Led definition- Among all the different kinds of semiconductor diodes available today, light-emitting diodes (LEDs) is one that is most widely used. It emits either visible light or invisible infrared light when connected in forward biased. The invisible infrared light-emitting LEDs are used in remote controls.

When voltage is applied to an optical semiconductor device, and it emits light, it is known as Light Emitting Diode (LED). It is a device which converts electrical energy to light energy.

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Light – Emitting Diode Symbol

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The Led Symbol and normal p-n junction diode are similar, except that it contains an arrow pointing away from the diode that indicates the diode is emitting light.

LEDs come in various colors, and the most common colors of LEDs are red, yellow, green, and orange.

The color of the light being emitted by LED cannot be represented in its schematic symbol; rather the symbol is common for all colors of LEDs. So, based on the symbol, it is not possible to know the color of LED.

Types of LEDs

The basic types of LEDs are:

1. Through – Hole LEDs

This type of LED is available in different sizes and shapes, but the most common one is 3 mm, 5 mm, and 8 mm. They are available in different colors like red, blue, yellow, white, green, etc.

2. SMD LEDs

A special package of surface mount or SMD LEDs 9 Surface Mount Light-Emitting Diodes) can be easily mounted on the surface of PCB. Based on the physical dimensions, they are usually differentiated. E.g., the most common SMD LED is 3528 and 5050.

3. Bi – color LEDs

The LED that can emit two colors is known as Bi-color LEDs. They have three leads, two anodes, and a common cathode. Based on the configuration of the leads, the color gets activated.

4. RGB LED

Among the designers and hobbyists, RGB LEDs (Red-Blue-Green LED) are the favorite and most popular. Even among the technicians who build computers, it is popular for implementing RGB LEDs in motherboards, rams, computer cases, etc.

There are 3 LEDs in RGB LEDs on a single chip, and with the help of a technique called PWM (Pulse Width Modulation), the output of RGB LEDs to produce a wide range of colors can be controlled.

5. High Power LEDs

If the power rating of LED is greater or equal to 1 Watt, then it is called High Power LED as normal LEDs have power dissipation of few milli watts.

High-power LEDs are very bright and often used as flashlights, spotlight, automobile headlamps, etc.

Led Construction and Working

One of the processes used to construct LED is to deposit three semiconductor layers on the substrate. The three layers of semiconductor deposited on substrate are n-type, p-type, and active region. The region between n-type and p-type semiconductor layers is known as the active region.

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The brief working of led is shown here:

An LED can generate light because of the arrangement of two semiconductor materials located between its electrodes.

1. N-type- It is the semiconductor with extra electrons, also known as extra negatively charged particles.

2. P-type- It is the semiconductor with extra holes, also known as extra positively charged particles.

When N-type semiconductor is connected to negative electrode and P-type semiconductor to positive electrode, it activates the electron flow across the junction from negative to positive layer. As the negatively charged electrons move through the positive charged particles, they emit light.

The conductive material of LEDs is typically aluminum-gallium-arsenide (AIGaAs), however, other kinds are also available. These materials are specifically selected because they produce photons which will be released into the visible portion of the light spectrum. The type of and amount of material chosen alters the color of light since each material generates photons of different wavelengths, affecting how it appears to the human eye.

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Characteristics Curve of LED

The characteristics curve of LED shows that the forward bias of 1 V is sufficient to increase the current exponentially.

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The output characteristics curve shows that radiant power of LED is directly proportional to the forward current in LED.

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