Computer simulation of instability and noise in high-power avalanche devices

The operation of avalanche transit-time devices at or near their output saturation point is often accompanied by instabilities and a deterioration of the spectral purity of the output signal. This can be attributed to a number of nonlinear effects that give rise to additional static or dynamic negative resistance. Depending on the external RF and bias circuit conditions, spurious oscillations, discontinuities, and quasi-random modulation of the carrier due to non-linear signal interaction can result. Prior to the onset of instabilities, low-frequency avalanche-noise components are selectively amplified, up-converted, and appear as noise sidebands of the output signal. These effects and their dependence on external circuit conditions are examined with the aid of a computer, simulation program. The features of this particular program are high computational speeds flexibility in the assumption of input data, and the inclusion of random noise in the simulation. The results of the computations are compared with analytical expressions and experiment. It is shown that the basic excess noise contribution at high signal levels stems from the "buildup" noise of the avalanche and increases approximately inversely proportional to the conduction current minimum. Instabilities are due to negative resistance components induced either by parametric signal interaction or rectification effects of the diode. Under high-power operation the spectrum of negative induced resistance extends from dc to the transit-time signal frequency. Strong parametric oscillations occur at subharmonic frequencies of the output signal while the period of dc block oscillations is primarily determined by the growth rate of the RF signal and the time constants of the external circuit elements. Under unfavorable bias and RF circuit conditions the initiation of lower frequency instabilities can lead to very high peaks of the current-density that are believed to be the cause of the frequently observed "anomalous" burnout of high-power diodes. The deterioration of the noise performance and stability at low temperatures results from the combined effect of the temperature dependence of effective reverse saturation currents and breakdown field. An example of a simple TRAPATT oscillator simulation is given and it is shown that the AM and FM noise spectra depend strongly on the magnitude of the minority saturation currents.