Why carbon is used in brushes instead of semiconductor material in motor?

There are a number of reasons, including:

1. Carbon is self-lubricating.

2. Carbon has a negative temperature coefficient of resistance, which means that its resistance falls as its temperature increases -which is opposite that of metals such as copper.

3. Carbon is softer than copper, so will not damage the commutator.

4. Carbon will acquire the shape of the commutator segments and, so, will ensure maximum contact with them.