Theory of unipolar ballistic and quasiballistic transit-time oscillators for a terahertz range

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.