Complex sentence: As these high-speed electrons move through the material they inevitably strike atoms, but if their velocity is not sufficient for avalanche breakdown (because the electric field is not strong enough) they are absorbed by the atoms and the process halts.
Split into simpler sentences: As these high-speed electrons move through the material they inevitably strike atoms. If their velocity is not sufficient for avalanche breakdown (because the electric field is not strong enough) they are absorbed by the atoms and the process halts.