Holes in semiconductor Why do we introduce the concept of holes in semiconductor physics?
Why don't we just describe hole current as of the current due to the electrons in the valence band.
is the concept of holes necessary?
 A: Electrons and holes are treated on the same footing, they are both quasi-particles in semiconductors, we should say quasi-electrons to be precise. The concept of quasi-particles (electrons and holes) in semiconductors can be useful for several reasons and in different contexts.

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*Quantum-field approach to the excited states in the system with many-body interactions.

In this case, you compute a propagator for a quasiparticle associated with propagating of an electron or its absence for a particular band: quasi-electrons for conduction band, holes for the valence band, while the ground state is treated as the vacuum state. We call them quasiparticles because their propagation is affected by the many-body interactions with other surrounding electrons and nuclei.
Those quasi-particles can interact with each other (form excitons for instance), however, instead of a highly correlated system, the quasi-particle concept offers a model system that can be treated in a low-density limit.


*Effective mass theory

For the low-energy excitations in most semiconductors, the excited state physics is similar to the physics of a free propagating particle with renormalized mass. However, the effective mass of electron in conduction band and the effective mass of an excitation represented by the removal of an electron are different.
A: To be short, the electrons in the valence band can not move when you apply reasonably small electrostatic fields, only the electron in the conduction band or its absence in the valence band has non-zero mobility.
