The presence of a natural superlattice in silicon carbide polytypes is shown to introduce a miniband structure into the conduction band, which leads to a number of effects in processes of impact ionisation namely to suppressing of electron heating and prevalence of holes in the impact ionization in a wide range of fields. These effects are observed when the electric field is parallel to an axis of natural SiC superstructure and they are absent when the electric field is perpendicularly to this axis. Notice that the parallel field effects are explained by a single Wannier-Stark localisation process. Owing to monopolarity of the impact ionisation and to the most high measured coefficients of impact ionisation among known semiconductors fabricated 6H-SiC avalanche photodiodes showed a low level of avalanche noise and a high photocurrent multiplication.