Here's why:

* Conductors: Allow electricity to flow freely through them due to a large number of free electrons.

* Insulators: Resist the flow of electricity due to a very limited number of free electrons.

* Semiconductors: Have a conductivity level between conductors and insulators. Their conductivity can be controlled by factors like temperature, impurities, and electric fields.

Germanium's conductivity falls in the middle, making it useful in electronics for applications like transistors and diodes.