We describe our systematic study of the two-dimensional electron mobilities μ of n-type ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As/GaAs heterojunctions, in particular their dependence on the electron concentration $N_s$ and the temperatures T, in a variety of field-effect transistors in which the impurity locations are precisely controlled to vary μ over the wide range of 5×${10}^3$ to 1.5×${10}^6$ ${\mathrm{cm}}^2$/V s. It is found that μ increases with increasing $N_s$ in the temperature range of 10 to 300 K. The measured mobilities are compared with theoretical calculations. The observed dependence of μ on $N_s$ at low temperatures is shown to be in excellent agreement with the theory of ionized-impurity scattering, whereas the high-temperature data disagree with the existing theory of polar-optical phonon scattering. A quantitative study has been successfully made on the effect of an undoped ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As spacer layer, which enhances not only μ itself but also the slope in logμ-log$N_s$ characteristics. The presence of both positive and negative temperature dependences of μ at low temperatures (T<40 K) is noted, and its connection with the effects of nondegeneracy and lattice scattering is also discussed.

• Received 16 December 1985

DOI:https://doi.org/10.1103/PhysRevB.33.8291