Thomas-Fermi calculations of the hole subband structure in p-type ␦-doped Si and GaAs quantum wells are carried out for different values of impurity concentration. Results are compared with previous self-consistent calculations and with some experimental reports, and very good agreement is found. In particular, the result of hole ground level from this model is exactly equal to the value reported for the experimental system with the smallest impurity spreading that has been achieved. ͓S0163-1829͑98͒04507-X͔

We present a simple design of a field effect transistor based on graphene nanoribbons, taking advantage of the metallic and semiconductor nature of nanoribbons with different widths. Such device could be constructed by using lithography techniques. The conductance of the proposed device is obtained by using the Kubo formula, assuming a strong damping due to the substrate and imperfections of the lattice. By removing the control electrodes, the design could also be used as an electrical resistance.

The Thomas-Fermi approximation is implemented in two coupled n-type ␦-doped quantum wells in Si. An analytical expression for the Hartree-Fock potential is obtained in order to compute the subband level structure. The longitudinal and transverse levels are obtained as a function of the impurity density and the interlayer distance. The exchange-correlation effects are analyzed from an impurity density of 8 ϫ 10 12 to 6.5ϫ 10 13 cm −2. The transport calculations are based on a formula for the mobility, which allows us to discern the optimum distance between wells for maximum mobility.

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