Question

The active medium in a semiconductor laser is

• A. Insulator
• B. Metal
• C. p–n junction
• D. Dielectric

Hint:

In a semiconductor diode laser, the p–n junction serves a double purpose: it conducts the electrical injection current and acts as the active medium where electrons and holes recombine to amplify light.

Answer 1

The Correct Option is C

Concept: Every laser system requires an active medium containing energy states that can be populated non-equilibrium-wise to enable light amplification via stimulated emission. In solid-state semiconductor injection lasers, this active region is formed by the depletion layer of a heavily doped p–n junction.

Step 1: The operational role of the p–n junction as an active medium.
Let us trace what occurs within a semiconductor laser structure under operation:
• A p-type semiconductor (filled with holes) and an n-type semiconductor (filled with electrons) are brought into direct atomic contact, creating a interface junction. Crucially, direct bandgap materials like Gallium Arsenide (\(\text{GaAs}\)) are chosen so that electron-hole recombinations convert energy directly into photons rather than lattice vibrations (heat).
• When an external forward-bias voltage is applied across this device, electrons from the n-side and holes from the p-side are forced to flow directly into the narrow junction zone.
• This continuous injection creates an exceptionally high concentration of overlapping carriers within the active region, establishing a state of population inversion within the junction.

Step 2: Verification of given selections.

• Insulator Dielectric: These materials have wide bandgaps and lack free charge carriers. They cannot conduct current or inject electron-hole pairs to sustain continuous population inversion.
• Metal: Metals have overlapping conduction and valence bands and lack a definitive bandgap, meaning electrical injection cannot create a localized population inversion to emit discrete optical photons.
• p–n junction: This interface architecture enables carrier injection, population inversion, and light amplification, making it the definitive active medium of the device. Hence, Option (C) is the correct answer.