Complex sentence: The electrons dissipate energy in the form of heat for silicon and germanium diodes but in "Gallium arsenide phosphide" (GaAsP) and "Gallium phosphide" (GaP) semiconductors, the electrons dissipate energy by emitting photons.
Split into simpler sentences: The electrons dissipate energy in the form of heat for silicon and germanium diodes. But in "Gallium-Arsenide-phosphorus" (GaAsP) and "Gallium-phosphorus" (GaP) semiconductors, the electrons dissipate energy by emitting photons.