The paper describes measurements of the velocity of electrons at electric fields up to 9 kV cm-1 in GaAs and in GaAs/AlGaAs heterostructures. In order to avoid charge and field domain formation, this experiment uses a sinusoidally varying electric field with frequency of 35 GHz. The results of these measurements indicate that negative differential mobility is present in not only the undoped GaAs, but also in the heavily doped GaAs and in the selectively doped heterostructures. These results imply that such a measurement is necessary to correctly determine the velocity-field characteristics of electrons in this electric field range. The experimental results further indicate that in the case of the selectively doped heterostructures, the electrons have peak velocities lower than those found in undoped bulk GaAs and that the peak occurs at a lower electric field. At room temperature, this effect is primarily due both to the effective lowering of the Gamma -L energy difference which results in a more efficient k-space transfer of electrons and to real-space transfer of electrons into the AlGaAs. At 77 K, these mechanisms are still active; additionally, the two-dimensionality of the electrons results in an enhanced scattering of the electrons with the polar optical phonons. In the case of doped GaAs, because of the significantly increased ionised impurity scattering, the peak velocity is also lower than that found in undoped GaAs, but the peak occurs at higher electric fields.