Effect of Various Boundary Conditions on the Instabilities in Transferred Electron Bulk Oscillators

Abstract

Transferred-electron bulk-negative-differential-conductivity devices with nonuniform geometry, field, and doping distributions are analyzed. Numerical solutions of the conduction equations was possible due to the novel scheme introduced to account for the area variation in the direction transverse to the electron flow. The simulation results show that the space-charge travel can be controlled with the applied bias voltage for concentric and some other nonuniform geometries. This results in the possibility of tuning the frequency over two octaves. The nature of the frequency control characteristic is shown to be dependent on the device geometry. Dipole space-charge nucleates due to nonuniform geometry without any necessity of initialization or doping fluctuations. Multiple space-charge formation is observed to occur in an oscillation cycle as a result of sequential decay processes. Space-charge modes in nonuniform geometries display polarity dependence. Divergent and convergent geometries are shown to favor dipole and accumulation-type space charges, respectively. Analysis of devices with doping gradients demonstrate the equivalence of these gradients to the geometry nonuniformities. The analytical solutions of the conduction equations indicate the simple dependence of the space-charge behavior on the geometry. The space-charge nucleation, growth, propagation, and decay are related to the time variation of the cathode field and the domain voltage.