mobility and transit time

Consider the 2.0micrometer long GaAs device where the E-field is 5kV/cm and Mn*=.067M0.

(a) Calculate the transit time of an electron through the device if the mobility is 8000 cm^2V-s.

(b) The mean free path of an electron (average velocity x scattering time) is the average distance an electron travels between two consecutive scattering events.  Calculate the mean free path of a GaAs electron at room temperature.

(c) How does the mobility of a semiconductor depend on T? and why?

How did you do it, and what are the answers?

Thank you.