The document discusses semiconductor materials and their electrical properties. It explains that semiconductors have electrical conductivity between metals and insulators. The electrical properties of semiconductors can be understood using band theory, where electrons occupy allowed energy bands separated by forbidden gaps. At low temperatures, semiconductors behave as insulators as their valence bands are full and conduction bands are empty, but at room temperature some electrons gain enough thermal energy to cross the gap. This allows both holes and electrons to contribute to electrical conduction in semiconductors.