as title. I read somewhere it was something about positron electron annilation but YT said that was rong.

Electron positron annihilation produces way too much enery. The resulting photon wouldn´t be visible to the human eye. The problem where to get the anti-matter from would be another reason why you wouldn´t use this effect.

 

I´m too overcharged with tasks right now (2 people on ICQ) to give further explanations. This link might help: http://electronics.howstuffworks.com/led1.htm

 

Found it on google, there are probably other and better pages to be found easily there.

 

EDIT: I remember having answered questions about photons and their production (photons are the particles of light, in case you don´t know that) in this forum before. Maybe a look in this thread might also help you: http://www.scienceforums.net/forums/showthread.php?t=4990