Conductors, Insulators, Semiconductors — Definition

Definition

Imagine electrons in an atom as residing in specific energy levels, much like planets orbiting a star at different distances. When many atoms come together to form a solid material, these discrete energy levels broaden and merge into continuous 'energy bands' due to the close proximity and interactions between atoms. This concept, known as the energy band theory, is crucial for understanding how materials conduct electricity.

At the heart of this theory are two primary bands: the valence band (VB) and the conduction band (CB). The valence band is the highest energy band that is completely or partially filled with electrons at absolute zero temperature.

These are the electrons involved in chemical bonding. The conduction band, on the other hand, is the lowest energy band that is normally empty or partially filled with electrons. Electrons in the conduction band are free to move throughout the material and contribute to electrical current.

Separating these two bands is a region called the **forbidden energy gap ($E_g$)** or band gap. This is an energy range where no electron states can exist. For an electron to move from the valence band to the conduction band and become a free charge carrier, it must acquire enough energy to jump across this forbidden gap.

Based on the characteristics of these energy bands and the size of the forbidden energy gap, materials are broadly classified into three categories:

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Conductors — These materials, typically metals, have a very high electrical conductivity. In conductors, the valence band and conduction band either overlap significantly or the conduction band is partially filled even at absolute zero. This means there's no energy barrier for electrons to move into available higher energy states within the conduction band, allowing them to flow freely and constitute an electric current with minimal applied voltage.

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Insulators — These materials, like rubber, glass, or plastic, have extremely low electrical conductivity. In insulators, the valence band is completely filled, and the forbidden energy gap ($E_g$) between the valence band and the conduction band is very large (typically greater than 3 eV). This large gap means that electrons require a substantial amount of energy to jump from the valence band to the conduction band. At room temperature, thermal energy is usually insufficient to bridge this gap, so very few electrons are available in the conduction band, leading to negligible current flow.

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Semiconductors — These materials, such as silicon and germanium, have electrical conductivity that lies between that of conductors and insulators. In semiconductors, the valence band is filled, and there is a forbidden energy gap ($E_g$) between the valence band and the conduction band, but this gap is relatively small (typically around 0.5 eV to 1.5 eV). At absolute zero, semiconductors behave like insulators because the valence band is full and the conduction band is empty. However, at room temperature, some electrons can gain enough thermal energy to jump across the small band gap into the conduction band, leaving behind 'holes' in the valence band. Both these electrons and holes can act as charge carriers, allowing for a measurable, albeit limited, current flow. Their conductivity can be significantly altered by temperature changes or by introducing impurities (doping).