Question

The light emitting diode (LED) is

• A. a heavily doped p-n junction with no external bias
• B. a heavily doped p-n junction with reverse bias
• C. a heavily doped p-n junction with forward bias
• D. a lightly doped p-n junction with no external bias
• E. a lightly doped p-n junction with reverse bias

Hint:

Remember that LEDs convert electrical energy directly into light. They are always used in forward bias to allow the recombination process that produces photons.

Answer 1

The Correct Option is C

Concept: A Light Emitting Diode (LED) is a semiconductor light source that emits light when current flows through it. It is a specialized p-n junction diode designed to release energy as photons.
• Doping: LEDs are made from heavily doped semiconductors to ensure a high concentration of charge carriers (electrons and holes) for efficient recombination.
• Biasing: Light emission occurs only when the diode is forward biased, allowing majority carriers to move toward the junction and recombine.

Step 1: Analyze the requirement for light emission. When a p-n junction is forward biased, electrons from the n-side and holes from the p-side move toward the junction. In the depletion region, these charge carriers recombine. As an electron "falls" from a higher energy level (conduction band) to a lower energy level (valence band), it releases energy.

Step 2: Evaluate the doping and biasing conditions. In semiconductors used for LEDs (like Gallium Arsenide), this energy is released in the form of photons. To maximize this effect, the junction must be heavily doped. Furthermore, light is only emitted during forward bias; in reverse bias, the depletion layer widens and prevents current flow and recombination.