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I know it seems like a trivial question, but I can't seem to find the answer anywhere.

So - what is the amplifying medium in a laser diode?

Meaning: In a classical laser resonator, there a substance (like HeNe gas) which undergoes a population inversion to allow lasing.

Every source I've read describes how a laser diode produces photons via recombination, and has metallic walls to allow photons to bounce back and forth inside the cavity, but no mention of an amplifying medium.

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The amplifying medium in a semiconductor diode laser is the semiconductor itself. More precisely, it's a thin layer of material surrounding the junction that separates the p-type side of the diode from the n-type. Application of an electric current through the diode allows a non-equilibrium condition to exist in that layer in which electrons and holes can exist in the same region of material ... briefly, until recombination occurs, and light is emitted. The current, however, continues to provide a fresh supply of electrons and holes. So a steady-state condition, not an equilibrium condition, exists where electrons and holes are constantly injected, electrons and holes constantly recombine, and light is constantly emitted.

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