the hemts' exceptional carrier mobility and switching speed come from the following conditions: the wide band element is doped with donor atoms; thus it has excess electrons in its conduction band. these electrons will diffuse to the adjacent narrow band material’s conduction band due to the availability of states with lower energy. the movement of electrons will cause a change in potential and thus an electric field between the materials. the electric field will push electrons back to the wide band element’s conduction band. the diffusion process continues until electron diffusion and electron drift balance each other, creating a junction at equilibrium similar to a p-n junction. note that the undoped narrow band gap material now has excess majority charge carriers. the fact that the charge carriers are majority carriers yields high switching speeds, and the fact that the low band gap semiconductor is undoped means that there are no donor atoms to cause scattering and thus yields high mobility.