Test: Light Emitting Diodes - Electronics and Communication Engineering (ECE) MCQ

The LED is a specialized form of PN junction that uses a compound junction. The semiconductor material used for the junction must be a compound semiconductor. The compound semiconductor commonly used along with the color of the light emitted is as shown:

• GaP = Green, Red
• GaN = Blue
• GaAs = Red, Infrared

GaS (Galium Selenide) is said to have the potential for optical applications but the exploitation of this potential has been limited by the ability to readily grow single crystals.

The bandgap of LED is related to its wavelength of light as:

Hence, as λ increases, the bandgap, and the build-in potential decreases, i.e.
For λ_R > λ_G > λ_B, the bandgap of the respective diodes will be:
⇒ V_R < V_G < V_B

The correct answer is e/u.

When a light ray travels from one medium to another the speed of the light in each medium changes.

Light Emitting Diode (LED):

• A Light Emitting Diode (LED) is a semiconductor unit that emits light when an electrical current flows through it.
• Light is produced when the particles that carry the charge (known as electrons and holes) combine together within the semiconductor material. 
• It uses materials like gallium and arsenide.
• Since light is generated within the solid semiconductor material, LEDs are described as solid-state devices.
• LEDs are utilized in applications as diverse as aviation lighting, automotive headlamps, advertising, general lighting, traffic signals, camera flashes, lighted wallpaper, horticultural grow lights, and medical devices.​
• It needs small power for operation.

PN Junction Diode:

• A unilateral device is a device that conducts only in one direction.
• p-n junction diodes conduct only when the p region is connected to a higher voltage and the n region is connected to lower voltage.
• When reverse biased, it acts as an open circuit.
• The symbol for a diode is as shown:

Zener diodes 

• Zener diodes are normal PN junction diodes operating in reverse-biased conditions.
• The working of the Zener diode is similar to a PN junction diode in the forward biased condition, but the uniqueness lies in the fact that it can also conduct when it is connected in reverse bias above its threshold/breakdown voltage.
• Zener diodes also called avalanche diodes or Breakdown diodes  are the heavily doped P – N junction diodes
• It is operated in a breakdown region.

The symbolic representation of the Zener diode is as shown:

Photo-diode:

• It is a light-sensing device that is used to sense the intensity of light
• Some of the examples are the smoke detector, a receiver in tv for converting remote signals, etc.

The symbolic representation of the Photodiode is as shown:

A light-emitting diode (LED):

• The device which is used to produce the different intensities of light and different colors depending upon the types of mater used in making it is called LED.

The symbolic representation of the LED is as shown:

Varactor diode:

• It is represented by a symbol of diode terminated in the variable capacitor as shown below:

Generally, optical devices are fabricated using direct bandgap semiconductors like GaAs and normal PN diode is fabricated by using Si. The most basic material for the fabrication of an LED is GaAs (Gallium Arsenide). 

Using GaAs as basic material, GaAs produces LEDs of different colors. This is as shown:

Concept:

When LED conducts, it is replaced by its cut-in voltage with a limiting resistor connected in series with it.

where, V_S = Source voltage
V_γ = Cut-in voltage
R = Limiting resistor 
I = Current

Calculation:
Given, V_S = 10V
Vγ = 2V
I = 20mA

Light Emitting Diode (LED):

LED stands for light-emitting diode. An LED is a semiconductor device, generating light through a process called electroluminescence. When we pass an electric current through semiconductor material, it emits visible light. As such, an LED stands in exact contrast to a photovoltaic cell, which is the cell used in solar arrays to convert visible light into electricity

Advantages:

• Energy-efficient: LED’s are now capable of outputting 135 lumens/watt.
• Long Lifetime: 50,000 hours or more if properly engineered.
• Rugged: LED’s are also called “Solid-State Lighting (SSL) as they are made of solid material with no filament or tube or bulb to break.
• No warm-up period: LED’s light instantly in nanoseconds.
• Not affected by cold temperatures: LEDs “like” low temperatures and will startup even in subzero weather.
• Directional: With LED’s we can direct the light where you want it, thus no light is wasted.
• Excellent Color Rendering: LEDs do not wash out colors like other light sources such as fluorescents, making them perfect for displays and retail applications.
• Environmentally friendly: LED’s contain no mercury or other hazardous substances.
• Controllable: LEDs can be controlled for brightness and color.
   

Disadvantage

• Blue hazard: There is a concern that blue LEDs and cool-white LEDs are now capable of exceeding the safe limits of the so-called blue-light hazard as defined in eye safety. 
• Light quality: Most cool-white LEDs have spectra that differ significantly from a black body radiator like the sun or incandescent light. However, the color rendering properties of common fluorescent lamps are often inferior to now available in state-of-art white LEDs.
• Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment.
• Blue pollution: Cool-white LEDs (i.e., LEDs with high color temperature) emit proportionally more blue light than conventional outdoor light sources.
• Voltage sensitivity: LEDs must be supplied with a voltage above the threshold and a current below the rating. 
• High initial price: LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than most conventional lighting technologies.
• Area light source: LEDs do not approximate the “point source” of light, but rather a Lambertian distribution. LEDs are not suited for large-area displays.

Advantages of LCD:

• They have low power consumption; A seven-segment display requires about 140 μW (20 μW/segment); This is a great advantage over LEDs which require about 40 mW per numeral
• They have a low cost

Disadvantages of LCD:

• LCDs are very slow devices; The turn-on and the turn off times are quite large
• The turn-on time is typical of the order a few milliseconds while the turn off time is ten milliseconds
• When used on DC their span is quite small, therefore, they are used with AC supplies having a frequency less than 500 Hz
• They occupy a large area
• They require an additional light source