The low-temperature electron mobility in 5-doped GaAs is calculated by using the Boltzmann equation and the relaxation-time approximation. It is assumed that the electrons are scattered from ionized impurities. Screening of charged impurities by electrons occupying several subbands is described with the help of (i) the random-phase approximation, (ii) the Thomas-Fermi method, and (iii) the bulk dielectric constant only. Among those methods mentioned above, the random-phase approximation has proved quite successful in studying the screening while the other two methods are inadequate. The mobility exhibits a drop when the excited subbands become occupied. It is shown, however, that as a consequence of the parity of the subband wave functions, the drop in the mobility when the Fermi level coincides with the bottom of the first excited subband is negligible.

We present a simple variational scheme for calculating the binding energies and the lowest dipole optical transition energies of D-impurities in the middle of quantum wells at moderate and high magnetic fields. The objective was to make the model and the method as simple as possible but to be able to explain many of the experimental features observed. The simple form of the electronic wavefunctions used enables physical insight into the properties of the D- system in quantum wells. The scheme, based on the parabolic approximation for the conduction band, gives results compatible with those of more sophisticated approaches. A simple renormalization procedure makes the results compatible also with the experimental results for GaAs quantum wells for which band non-parabolicity proved to be important. The energies for the lowest dipole optical transitions were calculated for a broad range of magnetic fields and well widths.

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We present the results of systematical variational investigations of the ground and states of centre in a rectangular quantum wire. The objective was to find the general trends for the donor binding energy and the dipole optical transition energy with changing transversal dimensions of the wire and the external magnetic field perpendicular to the wire. Simple Gaussian-like trial envelope functions and variable gauge of the vector potential were used in variational calculations. Besides, we investigated a donor impurity in quantum wires with parabolic confinements and the correspondence of such wires and rectangular quantum wires in the case of donor impurities.