The energy deposited in a silicon detector by a heavy charged particle, such as an alpha-particle, creates a large amount of electron-hole pairs. Under the influence of an electric field, the carriers drift towards the corresponding electrode. Due to diffusion, the charge carriers are spread. Lateral spreading depends on the collection time hence it is expected to be smaller for larger fields. In the case of pixellated detecting structure, this lateral spread will cause a sharing of the charge between the electrodes and many pixels will have a signal. Then, charge carriers generate a cluster of adjacent pixels. Also influencing the charge collection and its spread is the large amount of electron-hole pairs generated locally by the alpha particle, which creates distortions of the electric field along the ionizing path, giving rise to the plasma effect and the so-called funneling effect. The results of the charge sharing effect measured in the Medipix2 pixel detectors is shown as a function of the alpha particle energy and applied bias voltage. A model describing the effects of plasma and diffusion on the charge collection and charge sharing is presented.