Conductors always exhibit a “small” resistivity, regardless of their size. Insulators, on the other hand, do not conduct electricity well, and hence exhibit a large resistivity. Semiconductors, then, are a unique class of materials exhibiting a range of resistivities between those of metals and insulators. Possessing an intermediate value of resistivity, in and of itself, suggests nothing with respect to why it is that semiconductors can be cleverly fashioned into useful devices. At the deepest level, the fundamental usefulness of semiconductors to realize a myriad of electronic and photonic devices is rooted in our ability to controllably manipulate its resistivity. There are both “elemental” and “compound” semiconductors, as well as semiconductor “alloys.” In a semiconductor alloy, however, the ratios between the constituent elements can be varied continuously, forming a solid “solution”. Semiconductors such as silicon can be grown in three very different physical forms depending on the exact growth conditions used: crystalline, polycrystalline, and amorphous.