why si is not used in the manufacture of leds?

 

Visible light isn't emitted. Different semiconductors and dopings are used for the different colors.

 

Actually they used to be, unless I'm mistaken, but better materials were found. The older LEDs were a lot less efficient, and dropped around 1.5V (for red).

If you're interested someone pointed an interesting site I bookmarked, LED Museum.

 

why si is not used in the manufacture of leds?

Silicon is an indirect bandgap semiconductor material and to emitt light both the proper energy and moment have to be given to an electron at the same time. This does not happen very often and thus silicon is inefficient for LEDs.

Have a look here:

http://en.wikipedia.org/wiki/Direct_bandgap