How Do Direct and Indirect Band Gaps Differ?

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Updated November 5, 2026

In a direct band gap an electron drops from a high energy level to a lower one with almost no change in momentum, so the spare energy readily leaves as a photon of light. In an indirect band gap, as in silicon, the electron must change momentum significantly and needs a phonon to help, so giving off light is much harder.

Contents

Horizontal position represents crystal momentum; vertical position represents energy. A direct transition barely changes momentum; an indirect transition exchanges momentum with the lattice.

Direct band gap: straight drop
Indirect band gap: diagonal drop

What you will learn

Tell a direct band gap from an indirect one.
Say why electrons in a direct material give off light more easily.
See why silicon is rarely used to make LEDs.

Before you start

It helps to know that a semiconductor has electrons at different energy levels. Nothing more is needed; new words are explained when they appear.

What is a direct band gap?

A direct band gap is one where the electron drops without changing momentum. According to RP Photonics' article on the band gap, in a direct band gap the conduction-band minimum and valence-band maximum occur at the same crystal momentum, while remaining separated in energy.

Momentum here can be read as the electron's "motion". Dropping without having to change it keeps things simple. RP Photonics says that in a direct band gap, emission usually occurs with photon energies close to the band gap energy. A photon is a packet of light.

Dropping straight down, the spare energy leaves as light.

Higher energy band
Lower energy band
Electron
Light given off

How is an indirect band gap different?

In an indirect band gap the electron must change its momentum as it drops. RP Photonics also says that in an indirect band gap, additional phonons must be involved in absorption and emission to provide the required momentum change of electrons. Silicon is a material of this kind.

Energy and momentum must both be conserved. The article says both cannot be met unless a phonon is also emitted. A phonon is a quantum of vibration of the atoms in a crystal. Phonon-assisted processes are possible but occur at much lower rates, so light emission is less likely.

A diagonal drop needs extra lattice vibration, so the energy struggles to become light.

Higher energy band, shifted sideways
Lower energy band
Electron
Heat vibration (phonon)

What does this difference mean for devices?

It decides which materials suit a lamp. RP Photonics says direct band gap materials are preferred for light-emitting diodes and laser diodes, while indirect band gaps pose challenges for silicon photonics. It gives gallium arsenide as an example of a direct gap and silicon as an indirect one.

The article adds that silicon's indirect band gap makes it very inefficient for light emission, making it unsuitable as the emitting material in conventional semiconductor LEDs and lasers. Silicon also absorbs light weakly near the band edge, so photodiodes and solar cells need thicker layers.

A direct band gap material makes a lamp glow; a silicon chip computes and is not used to give off light.

Glowing LED
Silicon chip

Real-world example

The small LED on a charger or a control panel uses a semiconductor that is good at giving off light. The silicon chip inside a phone does the computing and does not shine outwards. This is a short way to remember the difference; real devices involve many more manufacturing details.

Common misunderstandings

Silicon never gives off any light at all. The source says only that silicon is very inefficient at light emission, making it unsuitable as the emitting material in conventional semiconductor LEDs and lasers.

An indirect band gap means a bad material. Silicon is an indirect material and still underlies a huge number of chips. It is simply not suited to producing light.

A phonon is light. A phonon is a vibration of the atomic lattice; the photon is the particle of light.

Direct band gap: the electron drops with no change in momentum, so a photon is easily emitted.
Indirect band gap: the electron must change momentum and needs a phonon.
So direct materials suit LEDs and lasers, while silicon is poor at giving off light.

Terms in this article

English and Vietnamese terms, with the meanings used here. Hover, tap or Tab to a term with a dotted underline for an explanation as you read.

band gapvùng cấm: The range of energy an electron in a semiconductor must cross to move between two bands.
direct band gapvùng cấm trực tiếp: A band gap where an electron drops with almost no change in momentum, so a photon is easily emitted.
indirect band gapvùng cấm gián tiếp: A band gap where an electron must change momentum significantly as it drops, so a phonon is needed.
photon: A particle of light; a single packet of light energy.
phonon: A quantum of vibration of the atoms in a crystal, carrying both energy and momentum.

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