The semiclassical model of electron dynamics sets out to describe the behaviour of Bloch electrons between collisions. In order to have energy En(k) that is approx constant over all Bloch levels in a wave-packet, the spreading of the wavevector must be small compared to the 1st Brillouin Zone: Δk << |G| ~ 1/a, where G is a reciprocal lattice vector and a is the lattice constant. This implies that Δk >> a, meaning the wave-packet is spread over many primitive cells in real space.
The semi-classical model looks at the effect on the electron dynamics of external electric or magnetic fields that are approximately uniform over the scale of one such wave-packet. The periodic potential considered in the Bloch model is responsible for the important distinction between the classical and semi-classical models (because the free electron approximation is no longer valid). This potential changes on a scale far smaller than that of the wave-packet, requiring a quantum mechanical treatment.
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