It is shown that the small‐signal behavior ofn‐type GaAs can be described in the approximation of displaced Maxwellian distribution functions in terms of five field‐dependent relaxation constants &Ggr;kwhich are defined as the eigenvalues of the matrix controlling the change with time of the electron temperatures, the drift velocities, and the population ratio. For the frequency dependence of these quantities and the mobility, expressions of the formk=15Cki&ohgr;−&Ggr;kresult, where theCkdepend only on the bias field. Hence the frequency limit for negative differential mobility is derived, having a maximum value of 200 GHz. Furthermore the frequencies of the plasma modes are calculated as a function of the bias field and the electron concentration. Two of the six plasma modes are oscillatory (especially for large electron density); one of the relaxation‐type modes (the soft mode) is shown to give rise for current‐controlled instability of the infinitely extended semiconductor for bias fields which belong to negative slope of the static characteristic.