The question of how the evolution of the nonequilibrium state of a highly photoexcited plasma in polar semiconductors (HEPS) affects the mobility of its hot carriers (photoinjected electrons and holes) is considered. General analytic expressions for the mobility, valid for a large class of experimental situations, are derived. That allows for an in depth analysis of transport properties of hot carriers in HEPS: It is demonstrated that, depending on initial conditions and experimental constraints, four different types of transient behavior may occur, namely, (i) structure (relative maxima and minima) without overshoot and a near ohmic regime in the steady state at low field intensities, (ii) structure with overshoot and non-ohmic regime in the steady state at intermediate field intensities, (iii) overshoot (no structure) and strongly non-ohmic regime at intermediate to high fields, and, (iv) normal (monotonicly increasing current) with ohmic behavior in the steady state at high fields. A general criterion for the existence of structure and/or overshoot during the mobility transient is established. Numerical calculations appropriate for electron transport in the central valley of GaAs are presented. © 1988.313-4497499Algarte, Luzzi, Time Evolution of Nonequilibrium Photoexcited Plasma in Polar Semiconductors (1983) Phys. Rev., 27 B, pp. 7563-7574Hammond, Electron-Velocity Overshoot Observed in an Impulse-Excited GaAs Semiconductor (1985) Physica, 134 B, pp. 475-479Luzzi, Vasconcellos, Relaxation Processes in Nonequilibrium Semiconductor plasma (1985) Semiconductors Probed by Ultrafast Laser Spectroscopy, 1, pp. 318-331. , R.R. Alfano, Academic, New York, Chap. 5Shank, Fork, Greene, Reinhard, Logan, Picosecond Nonequilibrium Carrier Transport in GaAs (1981) Appl. Phys. Lett., 38, pp. 104-105Zubarev, (1974) Nonequilibrium Statistical Thermodynamics, pp. 237-445. , Plenum, New York, Chap. 4