---
title: "Short note: Light emitting diode"
subject: "Basic Electronics Engineering"
chapter: 2
key: "semester_2/basic_electronics_engineering:short-note-led"
source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/short-note-led
site: BCT Engineering
---

# Short note: Light emitting diode

Basic Electronics Engineering · Chapter 2: Diodes

- Asked in: 2073 Bhadra, 3 marks; 2068 Magh, 4 marks (asked 2 times)
- Video: [LEDs - Light Emitting Diodes - Basic Introduction](https://www.youtube.com/watch?v=Hl4v1aUFWrs) — The Organic Chemistry Tutor
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/short-note-led
- Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-2

## Question

Write a short note on light emitting diode (LED).

## Short answer

A light-emitting diode (LED) is a PN junction device that emits light under forward bias via electron-hole recombination in direct band-gap materials like GaAsP or GaN. Colour depends on bandgap. A series resistor limits current; typical forward voltage is 1.8-3.3V, current 5-20mA. Used for indicators, displays, lighting, and optical communication.

## Answer

A **light emitting diode (LED)** is a PN junction diode that emits light when forward biased. It is made from direct band-gap compound semiconductors such as gallium arsenide phosphide (GaAsP), gallium phosphide (GaP) and gallium nitride (GaN), not from silicon.

#### Principle
Under forward bias, electrons from the n-region and holes from the p-region cross the junction and recombine. In a direct band-gap material the energy released by recombination is given out as photons (**electroluminescence**). The photon energy equals the band gap: E = hf, so the colour (wavelength) depends on the material.

```text
 +Vcc o--[ R ]--|>|--o GND      Symbol: --|>|-- with
                 LED             arrows pointing out
```

A series resistor limits the current: R = (Vs − VF)/IF. Example: Vs = 5 V, VF = 2 V, IF = 10 mA gives R = 300 Ω.

#### Characteristics
- Forward drop 1.8 to 3.3 V (red about 1.8 V, blue/white about 3.2 V), typical current 5 to 20 mA.
- Low reverse breakdown voltage (about 5 V), so it must not be reverse biased.
- Fast switching (nanoseconds), long life, low power, small size.

| Material | Colour |
|---|---|
| GaAs | infrared |
| GaAsP | red/yellow |
| GaP | green |
| GaN/InGaN | blue/white |

**Applications:** indicator lamps, seven-segment displays, remote controls (infrared), optocouplers, lighting, traffic signals and fibre optic transmitters.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why is an LED made from direct band-gap compounds such as GaAsP, GaP or GaN rather than silicon?
   A. In a direct band-gap material recombination releases energy as photons
   B. Silicon has too low a forward voltage to switch on
   C. Compound semiconductors have no depletion region
   D. Silicon cannot be doped to make a PN junction
   Answer: A. In direct band-gap materials the recombination energy appears as photons, while silicon mostly gives heat. (Key point: LED material)

2. An LED with VF = 2 V and IF = 10 mA is run from a 5 V supply. What series resistor is needed?
   A. 500 Ω
   B. 200 Ω
   C. 300 Ω
   D. 150 Ω
   Answer: C. R = (Vs - VF)/IF = (5 - 2)/10 mA = 300 Ω. (Key point: Series resistor)

3. What decides the colour (wavelength) of light from an LED?
   A. The value of the series resistor
   B. The photon energy E = hf, set by the semiconductor band gap
   C. The supply voltage applied
   D. The size of the package
   Answer: B. Photon energy equals the band gap, so the material determines the colour. (Key point: Colour and band gap)

4. Why must an LED not be reverse biased?
   A. It then emits infrared instead of visible light
   B. Its reverse current becomes very large at once
   C. Its reverse breakdown voltage is low (about 5 V)
   D. The series resistor burns out
   Answer: C. LEDs have a low reverse breakdown voltage, about 5 V, so reverse bias can damage them. (Key point: Biasing)

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