Semiconductors
Semiconductors can behave like an insulator or a conductor and the most popular semiconducting material is silicon. Silicon atoms have four electrons in its outermost shell, and one silicon atom will join up with four other atoms to share electrons. This arrangement is known as a crystal. Silicon is grown into large crystals and cut into wafers for electronic components.
Doping Silicon
Silicon on its own isn't very useful, so silicon makers add posphorus or boron, doping the silicon. When this silicon is grown into crystals, it has more useful properties. Phosphorus atoms have five electrons in its outer shell, and boron atoms only have three electrons. Silicon doped with extra phosphorus electrons is known as N-Type Silicon - N = Negative - and silicon with electron deficient boron atoms is known as P-Type Silicon - P = Positive.
Semiconductor Light Sources
Semiconductor materials will convert light, heat, electrons to infrared or visible light. The best semiconductor light sources are PN junction diodes. Read more about LED's.