Question

1-why the mobility of electron is higher than hole?

2-why effective mass of hole is greater than electron?

Answer 1

If you look at the band diagram of any material say Si, GaAs, GaN etc., the first thing you notice is that the curvatures of conduction band edge and valance band edge are different, they are not equal. The curvature of band determines the mass of electrons and holes of the corresponding material. High curvature leads to low mass and low curvature leads to high mass. This is called effective mass approximation. For most of the materials valance band edge has low curvature than conduction band edge and hence high effective mass for holes than electrons, which eventually results in low mobility for holes.If we look at the mobility equation, u=q<t>/m*, where <t> is momentum relation time or average time between scattering, which is associated with scattering,;and m* is the effective mass of carrier. We can see larger effective mass leads smaller mobility u, and holes has larger m* than electrons (almost).