Since the effective mass of electrons in a conduction band decreases with increasing curvature of the band according to eq. (3-3), comment on the electron effective mass in the Valley of GaAs compared with the indirect X or L valley. How is this effective mass difference reflected in the electron mobility for GaAs and GaP shown in Appendix III? From Fig. 3-10 what would you expect to happen to the conductivity of GaAs if valley electrons drifting in an electric field were suddenly promoted to the L valley?