Effective Mass (solid-state Physics)

Effective Mass (solid-state Physics)

In solid state physics, a particle's effective mass is the mass it seems to carry in the semiclassical model of transport in a crystal. It can be shown that electrons and holes in a crystal respond to electric and magnetic fields almost as if they were particles with a mass dependence in their direction of travel, an effective mass tensor. In a simplified picture that ignores crystal anisotropies, they behave as free particles in a vacuum, but with a different mass. This mass is usually stated in units of the ordinary mass of an electron m_e (9.11×10 kg). In these units it is usually in the range 0.01 to 10, but can also be lower or higher, for example reaching 1000 in exotic heavy fermion materials, and reaching 0 at the so-called Dirac points in graphene.

The effective mass has important effects on the properties of a solid, including everything from the efficiency of a solar cell to the speed of an integrated circuit.