Which of the following semiconductor devices requires a reverse bias to operate?

Statement $-1$: $A$ photodiode operates in reverse bias.
Statement $-2$: The fractional change due to the photo-effects on the minority carrier dominated reverse bias current is more easily measurable than the fractional change in the forward bias current.

Explain the construction and working of a photodiode.

$A$ photo cell is a device used to:

If the energy gap of a semiconductor used for the fabrication of an $LED$ is nearly $1.9 \ eV$, then the color of the light emitted by the $LED$ is

In an $LED$,visible light is produced by: