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We are the first to consider DC characteristics and a linear admittance of a very short undoped transit space (T-space) in the case of a ballistic transport of electrons injected into this space from a heavily doped cathode contact. We have considered both a classic case of a unipolar space-charge-limited injection current and a case of a unipolar tunnel injection through a rectangular or triangular heterostructure barrier. In both cases, the effective injection occurs in not very strong electric fields (⩽(1−3)×105 V/cm). To reach a high-energy ballistic electron transport, we have assumed highly positioned noncentral electron valleys over the bottom of a central Γ-valley and have selected the appropriate semiconductors for the T-space.For all the considered examples, we have obtained frequency windows of a negative conductance in a deep terahertz range. This means that such unipolar devices are perspective as terahertz oscillators. Numerical estimates are completed for the InP-T-space with the length of 70 nm. [Copyright &y& Elsevier]

Engineered energy-wave-vector dispersion relations of either electrons or holes hold great promise for realizing fundamental oscillators at terahertz frequencies if they contain sections with a negative effective mass (NEM) at appropriate energy levels. However, neither bulk semiconductor materials nor quantum wells or quantum wires exhibit such NEM sections in the dispersion relations at favorable energy levels. Therefore, the novel use of a nanostructure is proposed to create an NEM section of electrons at suitable energy levels. This structure utilizes a heterojunction with a quantum well (QW) channel grown perpendicular to a superlattice. At small values of the wave-vector k, the electron wave function Ψ resides mostly in the QW channel and, as k increases, Ψ extends further into the superlattice. This spread of Ψ induces an NEM section in the energy dispersion relation E(k). Several combinations of suitable material systems are considered. [Copyright &y& Elsevier]