Why the LED emitting light but P-N junction diodes are not?

Answer 1

LEDs are built from binary semiconductors like: gallium arsenide phosphide (GaAsP) for red, aluminium gallium indium phosphide (AlGaInP) for green, silicon carbide (SiC) for blue, etc. instead of the silicon or germanium used in ordinary junction diodes. This has the following effects:

1. when electrons and holes recombine at the P-N junction of an LED the bandgap is large enough to produce visible light photons, but in silicon and germanium the bandgap is only large enough to produce far infrared photons
2. the binary semiconductors used to make LEDs are transparent to visible light so the photons produced escape the device, silicon and germanium are opaque and immediately absorb the far infrared photons they produce which only heats the device slightly