Effective mass and group velocity electrons and holes

Physics Q&A
1

Why is the group velocity of holes and electrons the same and not opposite?

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I would expect the group velocity of holes and electrons to be opposite, because E_h = -E_e and the group velocity is the derivative of the energy, so I would say v_h = - v_e

2

The group velocity is the derivative of energy with respect to momentum, and momentum of a hole is opposite to that of an electron, so that the two minus signs cancel.