Understanding Band Gap in Simple Terms

Bandgap is the term we use to describe the energy difference between the valence band (where electrons are typically found) and the conduction band (where electrons need to go to conduct electricity). When you’re working with semiconductors or learning about electronic materials, it’s important to know that the band gap tells us how much energy you must supply to an electron so it can jump from the valence band into the conduction band.

Valence Band and the Conduction Band

In solid materials like semiconductors or insulators, the valence band represents the energy band containing electrons with the highest energy levels at absolute zero temperature. On the other hand, the conduction band is located just above the valence band and consists of vacant energy states that electrons can freely move into.

Insulators: Large Bandgap

Insulators possess a relatively large bandgap, indicating that a substantial amount of energy is necessary for electrons to jump from the valence band to the conduction band. Consequently, insulators have very few electrons in the conduction band at normal temperatures, resulting in poor electrical conductivity.

Semiconductors: Smaller Bandgap

In contrast to insulators, semiconductors have a smaller bandgap. This smaller gap enables electrons to overcome the energy barrier and transition from the valence band to the conduction band more easily, particularly at higher temperatures. Such properties make semiconductors highly useful for diverse electronic applications.

Conductors: Minimal or Zero Bandgap

Conductors, in contrast, possess a very small or even zero bandgap. The valence and conduction bands overlap in conductors, allowing electrons to move freely between the two bands. This characteristic grants conductors high electrical conductivity.

Band Gap Energy for Photonic Devices

When we talk about photodiodes and semiconductor detectors, they work efficiently only if the photon energy is greater than the band gap energy. That’s the condition needed for proper light absorption, which leads to a usable photocurrent. If the incoming light has photon energy close to or below the band gap, the responsivity drops significantly due to fewer available states for absorption.

In solar cells (photovoltaics), it’s the same story. You and I can only use sunlight with photon energies above the band gap. If the light has too long a wavelength, it won’t generate electricity. While we could pick a material with a very small band gap to capture more light, it would give us low voltage output, reducing efficiency. That’s why choosing the right band gap is always a compromise—between capturing a wide spectrum of sunlight and achieving good power output.

Take silicon, for example—it’s the most commonly used material in solar panels with a band gap of 1.12 eV, which is decent but not perfect. To boost efficiency, we can use tandem solar cells, layering silicon with materials like GaAs or perovskite that have higher band gaps. This way, we capture more of the solar spectrum. And if we’re aiming for even better performance, multi-junction cells are possible, though they can be complex and expensive to produce.