[Physics]

LEDs - Light-Emitting Diodes

LEDs - Light-Emitting Diodes

Introduction

• LEDs (Light-Emitting Diodes) are semiconductor devices that emit light when an electric current flows through them.
• They are widely used in various applications, including displays, indicators, lighting, and optical communication.

Operation of LEDs

• LEDs are based on the principle of electroluminescence, which is the emission of light when electrons recombine with holes in a semiconductor material.

Structure of an LED

• An LED consists of a p-n junction diode connected to positive and negative terminals.
• The junction is encased in a transparent plastic, allowing the emitted light to escape.

How LEDs Work

• When the p-n junction of an LED is connected in forward bias (positive voltage applied to the p-side and negative voltage to the n-side), electrons and holes flow through the junction in opposite directions.
• Some of these electrons and holes recombine at the junction, releasing energy in the form of photons of light.

Energy and Band Gap

• The difference in energy level between the conduction band and valence band of the semiconductor determines whether an LED emits light.
• If this energy difference is sufficient, the energy is emitted as a photon of light, making it a light-emitting diode.

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Physics

LEDs - Light-Emitting Diodes

Wavelength and Colour

• The size of the bandgap between the conduction and valence bands affects the frequency and wavelength of the emitted light.
• A larger band gap results in higher-frequency light, which corresponds to shorter wavelengths.
• The colour of the light emitted by an LED is determined by the wavelength of the photons it emits.

Calculations for LED Characteristics

• The energy of the emitted photon can be calculated using the equation: E = hf, where E is the energy, h is Planck's constant (6.63x10^-34 Js), and f is the frequency of the emitted light.
• LED example: If an LED emits light with a frequency of 5 x 10^14 Hz:
  • (a) Calculate the wavelength of the emitted light: λ = c/f = 3 x 10^8 m/s / 5 x 10^14 Hz = 600 nm.
  • (b) The colour of the light emitted by this LED is in the orange range.
  • (c) The construction and materials of the LED determine the emitted light's colour, with different semiconductors producing different colours.