Movement of Carriers in Semiconductors

Electrons in the conduction band and holes in the valence band are considered "free" carriers in the sense that they can move throughout the semiconductor lattice that makes up the crystal structure of the material. A simple, but in most cases adequate description of carrier movement views each carrier as moving in a random direction at a certain velocity.

The carrier moves in this random direction for a distance called the scattering length before colliding with a lattice atom. Once the collision (scattering event) takes place, the carrier moves away in a different random direction.

A model of carrier movement through scattering from atoms

The velocity of the carriers is determined by the temperature of the lattice. The thermal velocity is an average carrier velocity. Carriers have a thermal velocity that is normally distributed around this average thermal velocity. Therefore,  some carriers having a greater velocity and some lower. 

Barring certain circumstances which will be discussed in the following sections, there is no net movement of carriers in any direction. Each direction of carrier movement is equally likely, therefore the motion of a carrier in one direction will eventually be balanced by the movement of the carrier in the opposite direction. In the following animation, a carrier moves a distance equal to the scattering distance in a random direction before it collides with a lattice atom (for clarity the lattice atoms are not shown). After scattering off the lattice atoms, the carrier again moves in a random direction. The following animation has 5000 scattering events.

Although carriers in a semiconductor are in constant random motion, there is no net motion of carriers unless there is a concentration gradient or an electric field, to be discussed in the next sections.