Why is carbon not a semiconductor although it is in the same group as silicon and germanium?

"A semiconductor is a material with electrical conductivity due to electron flow (as opposed to ionic conductivity) intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10?8 siemens per centimeter."

Conductivity is one of the properties that increases as you go down a group, so carbon, at the top of its group, is usually described as a resistor (at least in most allotropes), and silicon and germanium are described as semi-conductors. Continuing down the group, tin and lead are conductors.

The conductivity of elements also decreases across a row, which is why the semiconductors shown at the second link make a diagonal line in the periodic table.

https://en.wikipedia.org/wiki/Semiconductor

https://en.wikipedia.org/wiki/Periodic_table