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.

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Interfacial Capacitance Ry J. KRUPSKIAn analysis is made of experimental results on the capacitance of thin-film condensers. To find the screened electrostatic potential in a three layer structure a specular electron reflection model is employed which takes into account quantum effects. The resulting interfacial capacitance is in semiquantitative agreement with the measured value for the thin-film Al-AI,O,-A1 condenser.Es wird eine theoretische Analyse yon experimentellen Resultaten durchgefiihrt, die mit Kapazitatsmessungen an Diinnfilmkondensatoren im Zusammenhang stehen. Dab& wird das Model1 der spiegelnden Elektronenreflexion verwendet, das auch Quanteneffekte beriicksichtigt, um das abgeschirmte elektrostatische Potential einer Dreischichtstruktur zu erhalten. Die gefundene Interfacekapazitlt stimmt in der GroIlenordnung mit dem a n einem Al-Al,O,-Al-Dunnfilmkondensator gemessenen Wert iiberein.