Question

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

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

3-why are holes heavier than electrons?

4- Explain effective mass for electron and hole?

Answer 1

1] Mobility of electrons is higher than holes because electrons are atleast a 1000 times lighter compared to positive ions (holes) and so the acceleration produced in them is much large.

Since mobility = drift speed/Electric field strength

therefore, for the same electric field strength as that given to holes, the drift speed will be larger for electrons thus indicating that the mobility is large.

2] Effective mass of hole is greater than electron because the top part of the valence band is much flatter than the lowest part of the conduction band. Since the top part of the valence band is flatter, is small and so,

is small meaning m_h* is large.

3] Holes are essentially absence of electrons and since electrons have higher mobility, it is implied that the mass of the electrons is much less compared mass of holes [even though holes do not actually exist].

4] Effective mass is a concept which arises only when the ions are not free but inside a lattice. The electric fields due to lattice sites are such that it changes the motion of the charged particles. This leads to curvature of the valence and conduction band and due to the equation,

this manifests as a mass parameter [LHS] and so the effective mass of holes and electrons comes into being.