The millimeter and sub-millimeter microwave ranges are very important for applications in communications, radar, meteorology and spectroscopy. However, the structure of semiconductor devices (transistors, diodes, etc.), required for such short wavelengths becomes very complex, which makes its fabrication difficult and expensive. One potential alternative to exploring this range of the electromagnetic spectrum resides in the use of non-linear wave interaction in active media. For example, the space charge waves in thin semiconductor films, possessing negative differential conductivity (InP, GaAs, GaN at 300K and strained Si/SiGe heterostructures at 77K), propagate at frequencies that are higher than the frequencies of acoustic and spin waves in solids. This means, for example, that an elastic wave resonator operating at a given frequency is typically 100 000 times smaller than an electromagnetic wave resonator at the same frequency. Thus attractively small elastic wave transmission components such as resonators, filters, and delay lines can be fabricated.
The study of non-stationary effects of the space charge in semiconductor structures, applied to solid-state microwave devices using the negative differential conductivity phenomenon, will be one of the most relevant topics in microelectronics and communications in coming years, due to the potential it represents in terms of amplification of micro- and millimeter waves. However, in order to understand the behavior of non-stationary effects, special attention must be paid to the inhomogeneous fluctuations of the carrier density in the plane of the film. For example, our recent work confirms the propagation and amplification of space charge waves in n-GaAs thin films with negative differential conductivity, which is demonstrated by the spatial distribution of the alternative part of the electron concentration in the film.