Like their energy-efficient cousins, CFLs, LEDs don’t waste energy on heat so they don’t get especially hot. But unlike CFLs, LEDs are illuminated solely by the movement of electrons in a semiconductor material. A semiconductor is a material with electrical conductivity (meaning the ability to transfer electrical energy) between that of a conductor and an insulator (hence the prefix “semi”). Inside an LED, when an electrical current passes through the semiconductor material, electrons move through the material and drop to other energy levels, and in the process they emit photons of light. LEDs are becoming an increasingly important and common light source because of their high degree of energy efficiency.