A?light-emitting diode?(LED) is a?semiconductor?device?that?emits ?light?when?current?flows through it.?Electrons?in the semiconductor ?recombine with?electron holes, releasing ener

A light-emitting diode (LED) is a semiconductor device that emits  light when current flows through it. Electrons in the semiconductor  recombine with electron holes, releasing energy in the form of photons.  The color of the light (corresponding to the energy of the photons) is  determined by the energy required for electrons to cross the band gap of  the semiconductor. White light is obtained by using multiple  semiconductors or a layer of light-emitting phosphor on the  semiconductor device.

LEDs – Light Emitting Diodes – Basic Introduction – YouTube 

 Links to an external site.  

Phototransistors are either tri-terminal (emitter, base and  collector) or bi-terminal (emitter and collector) semiconductor devices  which have a light-sensitive base region. Although all transistors  exhibit light-sensitive nature, these are specially designed and  optimized for photo applications. These are made of diffusion or  ion-implantation and have much larger collector and base regions in  comparison with the ordinary transistors. These devices can be either  homojunction structured or heterojunction structured

 Working principal of phototransistor – YouTube   Links to an external site.