VELOCITY-FIELD CHARACTERISTICS OF SELECTIVELY DOPED GAAS/ALXGA1-XAS QUANTUM-WELL HETEROSTRUCTURES

Recent experimental observations indicating poor correlation between the saturation velocity and Ohmic mobility lead us to propose a theory for a velocity-limiting mechanism in quantum-well heterostructures. The theory is based on the distribution function which takes into account the electron-drift anisotropy introduced by the high electric field for a degenerately doped quantum well. The drift velocity is shown to be limited by the Fermi velocity; a result which indicates that the saturation velocity is independent of the low-field mobility which is strongly controlled by momentum-randomizing scattering events. The dominance of optical-phonon emission at high electric field lowers the saturation velocity below the Fermi velocity. Excellent agreement is obtained between the theoretical and experimental results on the velocity-field characteristics of GaAs/AlxGa1-xAs quantum-well heterostructures.