# Direct and Indirect bandgap

How can I know which one is direct bandgap material and which one is indirect?

Why does Germanium have an indirect bandgap? How can be it become a direct bandgap material?

Here, I have attached a picture where I have calculated the direct and indirect bandgap of Germamium.

I increased its pressure by reducing the cellsize.

Is there any way of understanding, which one has a direct bandgap and which one has an indirect band gap?

• Do you mean how to determine whether a material has direct band gap without actually doing a calculation, or how to determine whether a material has direct band gap from calculation results? Commented Sep 10, 2022 at 15:57

Then, you look for the smaller $$\Delta E$$ between the bottom of the conduction band (BCB) and the top of the valence band (TVB). If this $$\Delta E$$ occurs for the same value of k, then you have a direct band gap material, otherwise, you will have an indirect band gap material.
The image bellow correspond to the Germanium1. As you can see, the smaller $$\Delta E$$ occurs between the BCB at k = $$\langle 111 \rangle$$ ($$L$$ point) and TVB at k $$= \langle 000 \rangle$$ ($$\Gamma$$ point).