What is a material in which the forbidden gap is so narrow that it can be considered nonexistent?

The material in which the forbidden gap is so narrow that it can be considered nonexistent is best described as a conductor. In conductors, the conduction band and the valence band overlap, allowing electrons to move freely without the need for any additional energy to jump the gap. This offers a low resistance to the flow of electric current, as the electrons can easily flow under the influence of an electric field.

In contrast, semiconductors possess a small band gap that allows for electron transition at specific energy levels, while insulators have a larger band gap that prevents electron flow. Superconductors exhibit unique properties, such as zero electrical resistance below a certain temperature, but do not have a nonexistent band gap in the same context as conductors.

Understanding these characteristics helps in identifying the nature of different materials in electronics and their applications in various devices.