This document provides an overview of semiconductor fundamentals and classifications. It discusses the properties of metals, insulators, and semiconductors. Semiconductors are classified as elemental, compound, narrow band-gap, wide band-gap, oxide, magnetic, organic, and low-dimensional. Elemental semiconductors like silicon are widely used but have limitations for optoelectronics. Compound semiconductors like GaAs have advantages for applications requiring high speeds or light emission/detection.