We consider electron dispersion relations in a quantum well overgrown on a cleaved edge of a superlattice and covered by a top barrier containing a donor modulation-doping sheet. Parameters of this structure are selected so that the electron wave function, which is localized inside of the quantum well at kZ = 0 (kZ is a wave vector component along the superlattice vector), enters the superlattice deeper and deeper with increasing kZ. As a result of this penetration, electrons become heavier and heavier. Such a dynamic (quantum) real-space transfer (without any scattering) leads to the appearance of a negative effective mass section in the lowest quantization subband. The position of this section as well as the width of the subband itself and its distance from the next subbands are regulated by both the quantum well parameters and parameters of the superlattice. The latter has to be with maximally high barriers and a small spatial period. The limitation of transverse motion of electrons in the superlattice layers is accomplished by an electric field of a pn-heterojunction formed as a result of an acceptor doping of the superlattice barriers and the above-mentioned donor modulation doping of the top barrier. This field also effectively controls the dispersion relations. Diodes with negative effective mass sections in the dispersion relation promise to be efficient terahertz range oscillators.