Conductivity of semiconductors (electron density / mobility)

I am preparing for an exam about sensors and one chapter is about temperature sensors/semiconductors.
To prepare, I have a bunch of multiple choice question and one is:

What is important for the conductivity of semiconductor-thermistors?

The correct answer is only the electron density within the conductance band. Another (wrong) answer is e.g. the mobility of electrons in that conductance band.

I wonder why? Why doesn’t the mobility play a role?