The contributions of nonlocal mechanisms to nonlinear transport in semiconductors, with special emphasis on hot-electron emission at heterojunctions and its variations which are now commonly termed real-space transfer effects, are reviewed. The goal is to equitably account for and bring together the body of literature that has developed, often independently, in the U.S. and the former Soviet Union as well as in Europe and Japan.

It is shown that the stationary distribution of ballistic current carriers moving across a thin doped base is unstable if there exists a negative effective mass (NEM) part in the carrier dispersion law. Under such a condition, a regime with a quasistationary current oscillations is established for a wide range of voltages across ballistic diode. The oscillation frequency and amplitude depend on the base length, doping concentration, and applied voltage. The current oscillations take place in a short-circuit regime (in absence of an external resonator). We consider asymmetric double quantum wells and/or composite ΓX quantum wells as possible structures allowing for the required dispersion relation with NEM part. Carrier dynamics in these structures are described quasiclassically and the validity of such a treatment is discussed.

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We have considered interactions between ballistic ͑or quasiballistic͒ electrons accelerated by a dc electric field in an undoped transit space ͑T space͒ and a small ultrahigh frequency ac electric field and have calculated the linear admittance of the T space. Electrons in the T space have a conventional, nonparabolic dispersion relation. After consideration of the simplest specific case when the current is limited by the space charge of the emitted electrons, we turned to an actual case when the current is limited by a heterostructural tunnel barrier ͑B barrier͒ separating the heavily doped cathode contact and the T space. We assumed that the B barrier is much thinner than the T space and both dc and ac voltages drop mainly across the T space. The emission tunnel current through the B barrier is determined by the electric field E(0) in the T space at the boundary B barrier/T space. The more substantial is, the tunnel current limitation the higher the electric field E(0) becomes. We have shown that for a space-charge limited current the change from parabolic dispersion to the nonparabolic branch induces narrowing and closing of the frequency windows of transit-time negative conductance starting with the lowest-frequency windows. These narrowing and closing frequency windows become effective only for very high voltages U across the T space: UӷmV S 2 /2e, where m is the effective mass for the parabolic branch and V S is the saturated velocity for the nonparabolic branch. For moderate voltages U, the effects of nonparabolicity are not very substantial. The tunnel current limitation decreases the space-charge effects in the T space and diminishes the role of the detailed electron dispersion relation. As a result, restoration of the frequency windows of transit-time negative conductance and an increase in the value of this negative conductance occur. The implementation of the considered tunnel injection transit time oscillator diode promises to lead to efficient and powerful sources of terahertz range radiation.