MOBILITY

in solid-state physics, measurement of the ease with which a particular type of charged particle moves through a solid material under the influence of an electric field. Such particles are both pulled along by the electric field and periodically collide with atoms of the solid. This combination of electric field and collisions causes the particles to move with an average velocity, called the drift velocity. Their mobility is defined as the value of the drift velocity per unit of electric field strength. The mobility can be determined experimentally. The mobility of a particular type of particle in a given solid may vary with temperature. Particles of the same type may have slightly different mobilities, in which case the average of their mobilities is measured. The charge carrier in most metals is the negatively charged electron. Only some of the electrons in the metal are free to move through the solid. The ease with which a metal carries an electric current, called its conductivity, is equal to the number of these free electrons present multiplied by their charge and by their mobility. Some metals carry electric currents by the motion of positively charged particles called holes, each of which corresponds to the absence of an electron. In materials called semiconductors, both electrons and holes are present, a condition that complicates the determination of their separate mobilities.