Nonlinear space-charge domain dynamics in a semiconductor with negative differential mobility

On a semiphenomenological level, the explanation of the Gunn effect is one in terms of a time-independent, negative differential, bulk conductivity. This mechanism is based on the conduction-band structure of GaAs, which provides for two kinds of electrons, light and heavy ones. Light electrons dominate at low fields, heavy ones at high fields. Since the mobility of the heavy electrons is much lower than that of the light ones, there is a range of current decrease with increasing field, i.e., a negative conductivity. This negative conductivity leads to an electrical breakup of the crystal into alternating traveling domains of high and low fields, accompanied by alternating current. In a "mathematically perfect" crystal this instability would take the form of traveling negatively charged electron accumulation layers, separating the domains of high and low fields. In real crystals the inevitable spatial fluctuations in the impurity distribution lead to the experimentally observed dipole mode, wherein both negatively charged electron accumulation layers and positively charged electron depletion layers occur.