Germanium is an indirect bandgap semiconductor, which means that the energy difference between the valence band and the conduction band is not direct. This means that when an electron transitions from the valence band to the conduction band, it must also emit a phonon, which is a quantum of lattice vibration. This process of phonon emission reduces the efficiency of LEDs made from germanium.

In contrast, direct bandgap semiconductors, such as gallium arsenide (GaAs), have a direct energy difference between the valence band and the conduction band. This means that electrons can transition from the valence band to the conduction band without emitting a phonon, making LEDs made from these materials more efficient.

For these reasons, germanium is not used as an LED material, while direct bandgap semiconductors such as GaAs are commonly used.