Electron velocity and negative differential mobility in AlGaAs/GaAs modulation-doped heterostructures

We have measured the electron velocity in low-doped GaAs and in AlGaAs/GaAs modulation-doped heterostructures at electric fields up to 8000 V/cm at both 300 and 77 K. In order to avoid the charge domain formation which occurs in dc measurements at these fields, this measurement uses 35 GHz radiation to supply the electric field. These measurements indicate that the peak velocity for electrons in the heterostructures is lower than for electrons in bulk low-doped GaAs. This result is explained in terms of modified intervalley transfer, real space transfer, and an enhanced scattering with polar optical phonons.