Subharmonic generation and the trapped-plasma mode in avalanching silicon p+-n-n+junctions

Experimental evidence is presented which distinguishes two mechanisms whereby an avalanche diode can efficiently generate microwaves at transit angles below those of classical transit-time oscillations. Measurements of external current waveforms and subsequent calculations of terminal voltage waveforms demonstrated that efficient generation (η ∼10 percent) of microwaves was possible at sub-harmonics of transit-time excitations. This subharmonic generation occurred when various harmonic components interacted such that the peak total voltage was delayed with respect to the fundamental. In such cases of multiharmonic synergetic excitations, the peak displacement currents were less than 0.25 of that required for avalanche shock-front initiation. The distinguishing features of high-efficiency operation (η ∼50 percent) were found to be completely consistent with a trapped-plasma mode interpretation. Experimental external current and dV/dt waveforms have substantiated Evans' circuit analysis [28]. dV/dt waveforms indicated peak displacement currents at breakdown which were on the order of 1.5 times that required for shock-front initiation. Approximate conduction current waveforms exhibited larger currents during the shock-front transit than during the extraction period. General features were consistent with calculations made by Cottam [29]. Circuit characteristics for both subharmonic generation and the trapped-plasma mode were similar. Circuit analysis also indicated the critical importance of each separate harmonic of a trapped-plasma mode fundamental. Harmonic generation utilizing the trapped-plasma mode enabled an extension of the upper bound of the frequency of efficient generation.