Optical Diodes: Understanding LEDs, Photodiodes, and Laser Diodes, Lecture notes of Electronics

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 Light-Emitting Diode (LED)

 Quantum Dots Diode

 Photodiode.

Optical Diodes

• When recombination takes place, the recombining electrons

release energy in the form of photons.

• The emitted light tends to be monochromatic (one color) that

depends on the band gap (and other factors).

Light-Emitting Diode (LED)

• A large exposed surface area on one layer of the semiconductive material permits the photons to be emitted as visible light.
• This process, called electroluminescence, is illustrated in Figure

Light-Emitting Diode (LED)

Electroluminescence in a forward biased LED.

• The first visible red LEDs were produced using gallium arsenide

phosphide (GaAsP) on a GaAs substrate.

• The efficiency was increased using a gallium phosphide (GaP) substrate,

resulting in brighter red LEDs and also allowing orange LEDs.

• Later, GaP was used as the light-emitter to achieve pale green light.
• By using a red and a green chip, LEDs were able to produce yellow light.
• The first super-bright red, yellow, and green LEDs were produced using

gallium aluminum arsenide phosphide (GaAlAsP).

• By the early 1990s ultrabright LEDs using indium gallium aluminum

phosphide (InGaAlP) were available in red, orange, yellow, and green.

LED Semiconductor Materials

• Blue LEDs using silicon carbide (SiC) and ultrabright blue LEDs

made of gallium nitride (GaN) became available.

• High intensity LEDs that produce green and blue are also made

using indium gallium nitride (InGaN).

• High-intensity white LEDs are formed using ultrabright blue GaN

coated with fluorescent phosphors that absorb the blue light and

reemit it as white light.

LED Semiconductor Materials

• The LED emits light in response to a sufficient forward current, as

shown in Figure (a).

• The amount of power output translated into light is directly

proportional to the forward current, as indicated in Figure (b).

LED Biasing

• An increase in IF corresponds proportionally to an increase in light output.
• The light output (both intensity and color) is also dependent on temperature.
• Light intensity goes down with higher temperature as indicated in the figure.

LED Biasing

• The graphs in Figure show typical radiation patterns for small LEDs.
• LEDs are directional light sources (unlike filament or fluorescent bulbs).

LED Biasing

• The radiation pattern is generally perpendicular to the emitting surface; however, it can be altered by the shape of the emitter surface and by lenses and diffusion films to favor a specific direction.
• Directional patterns can be an advantage for certain applications, such as traffic lights, where the light is intended to be seen only by certain drivers. Figure (a) shows the pattern for a forward- directed LED such as used in small panel indicators.
• Figure (b) shows the pattern for a wider viewing angle such as found in many super-bright LEDs.
• A wide variety of patterns are available from manufacturers; one variation is to design the LED to emit nearly all the light to the side in two lobes.

LED Biasing

• LEDs for lighting are available in a variety of configurations, including even flexible tubes for decorative lighting and low-wattage bulbs for outdoor walkways and gardens.
• Many LED lamps are designed to work in 120 V standard fixtures.
• A few representative configurations are shown in Figure(b).

LED Biasing

• Typical small LEDs for indicators are shown in Figure (a).
• In addition to small LEDs for indicators, bright LEDs are becoming popular for lighting because of their superior efficiency and long life.
• Now, a typical LED for lighting can deliver 100–120 lumens per watt, which is approximately five times greater efficiency than a standard incandescent bulb.

LED Biasing

LED Applications (LED Displays )

• Two types of LED circuit arrangements are the common anode and common cathode as shown.

LED Applications