Pure silicon contains approximately 1 × 10^16 free electrons per cubic meter

Pure silicon contains approximately 1×1016free electrons per cubic meter.

(a) Calculate the mean free time, ττ, for silicon at room temperature.
(b) The answer in part (a) is much greater than the mean free time for copper ( 2.5×10−9 s). Why, then, does pure silicon have such a high resistivity compared to copper?

This video explains it! https://youtu.be/ogUuS3uId4A