Nonlinear transport phenomena and mode locking in millimetre-wave Gunn-instability structures

Abstract

The nonlinear behaviour of drift-diffusion transport model of the 3 µm GaAs Gunn-effect structure is analysed here. It is shown that the correct choice of the v(E) form and its exact approximation is necessary. Increase of the v(E) dependence steepness in the region above the threshold leads to dissolution of the dipole domains before anode contact. A local extremum appears on the dependence of the transit frequency on the applied dc voltage. This complicates mode-locking processes when a driving microwave signal is applied. The effective length of the transit region quasiperiodically changes, similarly to threshold modulation in relaxation oscillator models. The nonlinear interaction of oscillations leads to multiplication of the current oscillations period and complication of the terminal current shape. The quasiperiodic microwave signal outside the phase-locking regions has different upper and lower envelopes, leading to the appearance of an additional high-frequency component in the signal spectrum. The principal phase-locking region boundaries are defined by the golden mean proportions. Some results of the experimental investigations of Gunn oscillator nonlinear dynamics in the 40–55 GHz frequency band are also given here.