Band Theory And Energy Bands - SS3 Physics Lesson Note

In solid-state physics, band theory describes the behaviour of electrons in solids. According to band theory, the electrons in a solid are restricted to specific energy levels, which are formed into energy bands. These energy bands can be either filled with electrons (called valence bands) or empty (called conduction bands). The energy bands are separated by energy gaps, where no energy levels exist for the electrons.

The number of electrons in the valence band determines the electrical conductivity of the solid. In conductors, the valence band is only partially filled with electrons, allowing for some electrons to move freely in the conduction band, resulting in high electrical conductivity. In insulators, the valence band is completely filled, and the conduction band is empty, resulting in low electrical conductivity. In semiconductors, the energy gap between the valence and conduction bands is small, allowing for some electrons to jump from the valence band to the conduction band, resulting in moderate electrical conductivity.

The band theory helps in understanding the electrical and optical properties of solids, including the behaviour of semiconductors, conductors, and insulators. It also helps in the design and development of electronic devices, such as transistors and microchips.