What is an LED?

An LED, or light-emitting diode, is a semiconductor device that produces light due to the recombination of electrons with holes when an electric current pass through it. This solid-state device allows forward current flow while preventing reverse current flow. The color of the light emitted by an LED depends on the energy needed for electrons to traverse the band gap of the semiconductor. Unlike other lighting technologies that depend on heated filaments or gas discharge, LEDs and other solid-state lighting sources, such as OLEDs, generate light within the solid semiconductor material.

Symbol of an LED

Figure 1: Symbol of LED

The symbol of LED is shown in figure 1, which is similar to the symbol for a standard diode, but with the addition of two small arrows pointing outwards from the tip of the triangle, indicating the emission of light from the LED. 

Different colors of an LED

The semiconductor material of an LED contains energy bands that confine both electrons and holes. The bandgap, or separation of the bands, determines the energy of the photons emitted by the LED. Various types of semiconductor materials have distinct bandgaps that result in the production of different colors of light. By modifying the composition of the active region of the LED, the specific wavelength and corresponding color of the emitted light can be adjusted.

The color of an LED is determined by the material used in the semiconducting element, with the primary materials being aluminium gallium indium phosphide alloys and indium gallium nitride alloys. Aluminium alloys produce red, orange, and yellow light, while indium alloys produce green, blue, and white light. Slight changes in the composition of these alloys can alter the color of the emitted light. Light-emitting diodes are heavily doped p-n junctions that emit colored light at a particular spectral wavelength when forward-biased. The color of the emitted light is based on the semiconductor material used and the amount of doping. 

Simple LED Circuit

Figure 2: Schematic of an LED circuit

The schematic of an LED circuit is shown in figure 2. A basic LED circuit consists of an LED, a resistor, and a voltage supply. The voltage supply provides a source of electrical energy, which powers the LED. The resistor is used to limit the amount of current flowing through the LED, protecting it from damage and ensuring that it operates within its safe operating range. When the circuit is closed, current flows from the voltage supply, through the resistor, and into the LED. And as a result, the LED emits light. 

Working of an LED

A PN junction is formed in a single semiconductor material by doping one side of the material with acceptor impurity atoms making it P-type and doping the opposite side with donor impurity atoms making it N-type. The junction formed between these two is called the PN junction. Holes are the majority charge carriers and electrons are the minority charge carriers in p-type material. In n-type material, electrons are the majority charge carriers and holes are the minority charge carriers. 

Figure 3: Working of a p-n junction semiconductor in forward bias mode

The above image (Figure 3) shows the working of a p-n junction semiconductor in forward bias mode. In a p-n junction semiconductor device, forward biasing occurs when the p-side of the material is connected to the positive terminal and the n-side is connected to the negative terminal of a power supply. During forward bias, since the majority electrons from the n-region repels with the negative terminal of the power supply, they move towards the p-region. At the same time, holes from the p-region move toward the n-region since they repel with the positive terminal of the power supply. In this case, the depletion region becomes very narrow and the carriers cross the junction. Then, they recombine with carriers of the opposite type, releasing energy in the form of photons. 

Figure 4: Working of a p-n junction semiconductor in reverse bias mode

The above image (Figure 4) shows the working of a p-n junction semiconductor in reverse bias mode. During reverse bias, since the majority electrons in the n-region attracts with the positive terminal of the power supply, the minority holes move towards the p-region. At the same time, minority electrons from the p-region move toward the n-region since the majority holes attract with the negative terminal of the power supply. In this case, the depletion region becomes wider. As a result, no current flows through the p-n junction under reverse bias, and light emission does not occur.

Figure 5: Working of an LED

The schematic representation of the working of an LED is shown in figure 5. An LED is a device that operates in the forward bias mode. In forward bias, the positive terminal of the power supply is connected to the p-type region of the LED, which attracts the negatively charged electrons (minority carriers) in the p-type region. Similarly, the negative terminal of the power supply is connected to the n-type region of the LED, which attracts the positively charged holes (minority carriers) in the n-type region. At the junction between these regions, the concentration of majority carriers increases, leading to their recombination and then current flows. 

In a standard diode, this recombination process results in the release of energy in the form of heat. But, in an LED, the energy is released as photons and this process is known as electroluminescence. The occurrence of electroluminescence happens when a material releases light due to the presence of an electric current flowing through it. As the forward voltage across the LED increases, the intensity of the light also increases until it reaches its maximum. 

LED Materials

The composition of LEDs comprises of compound semiconductor materials that contain elements from both group III and group V of the periodic table, collectively known as III-V materials. Gallium arsenide (GaAs) and gallium phosphide (GaP) are III-V materials that are commonly used to create LEDs. Before the mid-90s, LEDs were limited to only a few colors and were not commercially available in blue or white. New color options and applications were made possible with the development of LEDs using the gallium nitride (GaN) material. 

The main semiconductor materials used to manufacture LEDs include:

• Indium gallium nitride (InGaN) for blue, green, and ultraviolet high-brightness LEDs
• Aluminum gallium indium phosphide (AlGaInP) for yellow, orange, and red high-brightness LEDs
• Aluminum gallium arsenide (AlGaAs) is used for producing red and infrared LEDs.
• Gallium phosphide (GaP) is utilized for creating yellow and green LEDs.

LED I-V Characteristics

Figure 6: I-V characteristics of an LED

The I-V characteristic of an LED is an important aspect of its operation and is shown in figure 6. When a forward bias is applied to an LED, a voltage is applied across its p-n junction. The current flowing through the LED increases as the voltage rises until it hits the threshold limit. Beyond this threshold value, the current increases rapidly with only a slight increase in voltage. This characteristic behavior is due to the intrinsic properties of the LED's compound semiconductor material.

The voltage at which the LED begins to conduct is called the "forward voltage". This value varies depending on the color of the LED and the material used to manufacture it. As the current through the LED increases beyond its threshold value, the LED's brightness also increases.

The LED's light output intensity is directly proportional to the forward current flowing through it. To protect the LED from excessive current flow, it should be connected in a forward bias condition across a power supply with a series resistor to limit the current. Connecting an LED directly to a battery or power supply can destroy it quickly due to excessive current flow. The forward voltage drop across the PN junction of each LED varies and is determined by the semiconductor material used.

Types of LED

• Miniature LEDs
• Flash LED
• High-Power LEDs
• Red Green Blue LEDs
• Alphanumeric LED
• Lighting LED
• Bi and Tri-Colour

Advantages of LED

• Consume less power
• No warm-up time needed 
• Monochromatic
• Highly energy efficient
• Long lifetime
• Durability
• Instant lighting
• Directional
• Environment friendly

Applications of LED

LEDs are used in a wide range of lighting applications, including residential, commercial, and industrial lighting. They are also used in street lighting, automotive lighting, and traffic signals. The energy efficiency and long lifespan of LEDs make them a popular choice for lighting applications, offering significant energy savings and reduced maintenance costs.

They are used in digital displays, such as billboards, electronic scoreboards, and large-screen video displays. LED displays offer high brightness, excellent color reproduction, and low power consumption, making them ideal for outdoor and indoor applications.

LEDs are also used in backlighting applications, such as LCD screens, keyboards, and televisions. The uniform light output and low power consumption of LEDs make them an ideal choice for backlighting applications, where high brightness and energy efficiency are essential.

They are used as indicators in electronic devices, such as power buttons, status lights, and warning lights. They are used in automotive lighting applications, including headlights, brake lights, and turn signals, in medical devices, such as photodynamic therapy and surgical lighting and also used in indoor horticulture applications, such as growing plants in greenhouses and vertical farms.