What is the Difference Between Direct and Indirect Band Gap

The key difference between direct and indirect band gap is that the crystal momentum of electrons and holes is the same in a direct band gap for both the conduction band and the valence band, where an electron can directly emit a photon, whereas indirect band gap has different k-vectors where a photon cannot be emitted.

The terms direct and indirect band gap are described mainly in semiconductor physics. These categories are discovered depending on the minimal energy state in the conduction band and the maximal energy state in the valence band that are characterized by a certain crystal momentum in the Brillouin Zone. K-vector or wave vector is an important concept in band gaps. The k-vector of a plane wave is a vector (at least in the case of isotropic optical media) that points in the direction in which the wave propagates. This vector component is always perpendicular to the wavefronts.

1. Overview and Key Difference
2. What is Direct Band Gap
3. What is Indirect Band Gap
4. Direct vs Indirect Band Gap in Tabular Form
5. Summary – Direct vs Indirect Band Gap

What is Direct Band Gap?

Direct band gap is a band gap where the k vector is similar for the highest states in the valence band and for the lowest states in the conduction band. In other words, we can call a band gap a direct band gap if the crystal momentum of electrons and holes is the same in the conduction band and the valence band, and an electron can directly emit a photon.

Some examples of direct band gap materials include amorphous silicon and some III-V materials, including InAs and GaAs.

What is Indirect Band Gap?

Indirect band gap can be described as the phenomenon in which the maximum of the valence band as well as the minimum of the conduction band are at different values of k. For example, the maximum of the valence band is k=0, while the minimum in the conduction band is at a k vector on the Brillouin zone boundary. In this type of band gap, a photon cannot be emitted since the electrons have to pass through an intermediate state, transfering momentum to the crystal lattice.

Some examples of indirect band gap materials include crystalline silicon and Ge, as well as some III-V materials such as AlSb.

What is the Difference Between Direct and Indirect Band Gap?

Direct and indirect band gap are important terms in semiconductor physics. The key difference between direct and indirect band gap is that in direct band gap, the crystal momentum of electrons and holes remains the same in the conduction band and the valence band and an electron can directly emit a photon, whereas indirect band gap has different k-vectors where a photon cannot be emitted. Some examples of direct band gap materials include amorphous silicon and some III-V materials, including InAs and GaAs, while some examples of indirect band gap materials include crystalline silicon and Ge, as well as some III-V materials, such as AlSb. Moreover, a direct band gap is more efficient than an indirect band gap.

The below infographic presents the differences between direct and indirect band gap in tabular form for side-by-side comparison.

Summary – Direct vs Indirect Band Gap

Direct and indirect band gap are two important phenomena that are described together in similar contexts. The key difference between direct and indirect band gap is that in direct band gap, the crystal momentum of electrons and holes remains the same in the conduction band and the valence band and an electron can directly emit a photon, whereas indirect band gap has different k-vectors where a photon cannot be emitted.

Reference:

1. “Direct Band Gap Materials.” Direct and Indirect Band Gap Semiconductors, Department of Materials Science and Metallurgy – University of Cambridge.

Image Courtesy:

1. “Bulkband structure” By Saumitra R Mehrotra & Gerhard Klimeck – Bandstructure Lab on nanoHUB.org (CC BY 3.0) via Commons Wikimedia
2. “Indirect Bandgap” (Public Domain) via Commons Wikimedia