How Does a Light Emitting Diode (LED) Work? (Band Gap & Exam Questions)
A Light Emitting Diode (LED) is a special type of semiconductor diode that directly converts electrical energy into light. Unlike regular signal diodes, LEDs are specifically designed to emit visible or infrared light when a forward current passes through them.
LEDs are widely used as indicators, displays, and lighting sources due to their efficiency, longevity, and compactness. An LED operates on the same basic principle as a p-n junction diode. When forward biased, electrons from the n-type semiconductor recombine with holes from the p-type region at the junction.
This recombination releases energy in the form of photons, producing light. The thin, heavily doped semiconducting layer in an LED allows these photons to escape and create a visible glow, making LEDs essential components in electronic devices.
Construction and Structure of LEDs
- LEDs are constructed with a thin active semiconductor layer, usually enclosed in a transparent plastic epoxy resin shell.
- This shell not only protects the LED from mechanical damage and vibrations but also acts as a lens, focusing the light upwards.
- Most LEDs have two legs; the longer one (anode) is positive, while the shorter one (cathode) is negative.
- The semiconductor materials used in LEDs are not silicon or germanium (as in standard diodes), but compound semiconductors like Gallium Arsenide (GaAs), Gallium Phosphide (GaP), or Gallium Nitride (GaN).
- These materials are chosen for their ability to emit light at specific wavelengths (colors) depending on their composition and dopants.
How LEDs Emit Different Colors
The color of an LED depends on the wavelength of light it emits, controlled by the band gap of the semiconductor used in its construction. Different materials produce photons of different energies, resulting in various colors visible to the human eye. The color is not determined by the color of the LED's plastic body, but by the internal materials and structure.
| Material | Wavelength | Color | Typical Forward Voltage |
|---|---|---|---|
| GaAs | 850–940 nm | Infrared | 1.2 V |
| GaAsP | 630–660 nm | Red | 1.8 V |
| GaAsP:N | 585–595 nm | Yellow | 2.2 V |
| AlGaP | 550–570 nm | Green | 3.5 V |
| SiC | 430–505 nm | Blue | 3.6 V |
Key Features and Types of LEDs
LEDs come in various shapes and package types for different applications. The most common are small, round indicator LEDs, but multi-color LEDs (bi-color and tricolor) and special LED displays (like 7-segment displays) are also widely used. These can be combined to create alphanumeric displays and even full-color screens.
Multi-color LEDs often contain more than one diode chip connected in specific configurations inside a single package, allowing two or more colors to be displayed from the same component based on the direction or ratio of current flowing through them.
LED Circuit Parameters and Calculations
An LED must always be operated with a current-limiting resistor in series to prevent excessive current which can instantly damage the device. The forward voltage drop (VF) varies by color, and typical operating currents are in the range of 10–30 mA.
| Parameter | LED | Ordinary Diode |
|---|---|---|
| Material | Compound semiconductors (e.g. GaAs, GaN) | Silicon, Germanium |
| Operation | Emits light when forward biased | No light emission |
| Typical VF | 1.2 – 3.6 V (varies with color) | 0.7 V (Si), 0.3 V (Ge) |
| Application | Indicators, displays, lighting | Rectification, switching |
Calculating Series Resistance for an LED
To choose the correct series resistor (RS) for safe operation, use Ohm’s Law:
RS = (VS - VF) / IF
Where VS = supply voltage, VF = LED forward voltage, IF = desired forward current. For example, to power a 2V LED from a 5V supply at 10mA:
RS = (5V - 2V) / 0.01A = 300 Ω. Choose the nearest higher standard value, e.g., 330 Ω.
Formulas Linked to LED Light Emission
| Formula | Application |
|---|---|
| E = hν = hc/λ | Determines energy (E) of a photon; h is Planck’s constant, c is speed of light, λ is wavelength |
| E (in eV) = 1240 / λ (in nm) | Finds the photon energy from emitted light's wavelength |
Example: Calculating LED Light Wavelength
Suppose an LED has a band gap of 2 eV. The wavelength of light emitted is:
E = 2 eV, λ (nm) = 1240 / E (eV) = 1240 / 2 = 620 nm
So, this LED emits orange-red light.
Common Applications and Advanced Uses
LEDs are used as visual indicators in appliances, automotive lights, large screen displays, optical communication, and opto-couplers (for electrical isolation). Multi-segment and full-color displays utilize arrays of LEDs. Modern control techniques, such as Pulse Width Modulation (PWM), adjust brightness without reducing device life.
Practice Problems for Students
-
Calculate the series resistor needed for a red LED (VF = 1.8V) connected to a 9V supply at 15mA.
Solution: RS = (9 - 1.8)/0.015 = 480 Ω
-
If an LED emits blue light at 470 nm, what is the band gap energy?
Solution: E (eV) = 1240 / 470 ≈ 2.64 eV
-
Why must you not connect an LED directly to a battery without a resistor?
Excess current will destroy the LED due to overheating.
Further Learning and Vedantu Resources
- Read more on Photodiodes: Physics and Uses
- Explore daily life applications at Uses of LED in Everyday Life
- Review and practice common LED problems and circuit challenges
Understanding how Light Emitting Diodes function, their key materials, circuit calculations, and applications strengthens your grasp of semiconductors and their role in modern technology. Regular practice and problem-solving will help you master this concept for exams and real-world electronics.
FAQs on Light Emitting Diode (LED): Principle, Structure, and Applications
1. What is a Light Emitting Diode (LED)?
A Light Emitting Diode (LED) is a special type of semiconductor diode that emits light (visible or infrared) when current passes through it in the forward direction. LEDs convert electrical energy into light energy due to electron-hole recombination in the P-N junction, releasing photons corresponding to the material's energy band gap.
2. How does a Light Emitting Diode (LED) work?
An LED works when forward bias is applied, allowing current to flow across its P-N junction. Electrons recombine with holes and release energy as photons (light). The colour and wavelength of the emitted light depend on the LED's band gap energy and the semiconductor material used. Thus, LEDs convert electrical current directly into light energy.
3. What materials are used to make LEDs?
LEDs are made from various compound semiconductors such as:
- Gallium Arsenide (GaAs)
- Gallium Phosphide (GaP)
- Gallium Arsenide Phosphide (GaAsP)
- Gallium Nitride (GaN)
- Aluminium Gallium Phosphide (AlGaP)
- Gallium Indium Nitride (GaInN)
4. What is the difference between an LED and an ordinary diode?
The main differences between an LED and an ordinary diode are:
- LED emits light in forward bias, while an ordinary diode does not emit light.
- LEDs use compound semiconductors (like GaAs, GaP), whereas ordinary diodes use silicon or germanium.
- LED forward voltage drop is typically higher (1.2–3.6 V).
- LEDs are used for indication and displays; diodes are for rectification and switching.
5. Why does an LED emit different colours of light?
LEDs emit different colours based on the energy band gap of the semiconductor material. Each material has a unique band gap that causes emission of photons at specific wavelengths (colours) when electron-hole recombination occurs. For example, GaAs (infrared), GaP (green), and GaN (blue) each correspond to a different emitted colour.
6. What is the schematic symbol of an LED?
The LED symbol is similar to a regular diode symbol, but with two small arrows pointing away from the diode, representing emitted light.
- The longer lead is the Anode (+)
- The shorter lead (marked or notched) is the Cathode (−)
7. Which formula relates band gap energy and emitted light wavelength in an LED?
The wavelength (λ) of light emitted by an LED is related to its band gap energy (Eg) by:
Eg (eV) = 1240 / λ (nm)
Alternatively,
E = hc/λ
Where:
- E = photon energy (J or eV)
- h = Planck's constant (6.626 × 10−34 Js)
- c = speed of light (3 × 108 m/s)
8. What are the main applications of LEDs?
LEDs are widely used in:
- Display panels and TV screens
- Indicator and signal lamps
- Digital clocks, calculators, and remote controls
- Opto-isolators and optical sensors
- Traffic lights and automotive lighting
- Medical devices and therapy (LED therapy)
9. Why is a resistor needed in series with an LED?
A series resistor is required with an LED to limit the current and prevent damage. LEDs are current-dependent devices and can burn out if excessive current flows through them. The resistor ensures safe operation by controlling the forward current to within the rated limits of the LED.
10. Why is it difficult to manufacture blue LEDs?
Blue LEDs are difficult to produce because:
- They require large energy band gap materials (like Gallium Nitride, GaN), which are challenging to synthesize and process.
- Early development faced issues with material stability and efficient light emission.
- Precise control over crystal defects and doping was needed for high-brightness blue emission.
11. How can you calculate the forward current for an LED circuit?
To calculate the forward current for an LED:
- Subtract the LED forward voltage (VF) from the supply voltage (VS).
- Divide by the series resistor value (RS):
IF = (VS − VF)/RS
12. What are opto-isolators and how do LEDs work in them?
Opto-isolators (opto-couplers) are devices that use an LED and a photosensor (like a photodiode or phototransistor) enclosed in one package. The LED converts an electrical signal into light, which crosses an electrically-isolated gap and is detected by the photosensor, enabling signal transfer without direct electrical connection. They are used for safety and isolation between circuits.
