Explanation
The electrons (or holes) in solids will collide with impurities, defects or other carriers. The average value of the free distance passed between two successive collisions is called the average free path, expressed as l, and the corresponding time experienced is called the average free time, or the relaxation time t, v0 is the real speed of the carrier, which is different from the drift speed. The more impurities and defects, the smaller the average free path of crystal electrons.
Classification
Fundamental of Material Science -> Physical and chemical basis of materials