WHY DIODES ARE BLACK?? [BASIC CONCEPT BEHIND THE COLOR OF DIODE]
Channel: Complete Physics
Date: 2020-05-04

Why all the diodes are black?
Doping of semiconductor material increases the conductivity of material more than 100,000 times. After doping the charge careers need very less energy to reach in conduction band as they reach in donor energy level.
Energy required by "Ge" is 0.01eV and for "Si" is 0.05eV.
This small energy can be provided by sunlight also. In that case, the electrons in donor energy level can receive energy and popup in to the conduction band. 
Thus device can conduct at zero external voltage applied.
So to prevent this phenomenon, diodes are covered by black coating.