Avalanche-initiated space-charge oscillations

Physical mechanisms are proposed for avalanche oscillations in n⁺-n-n⁺semiconductor structures. A linearized analysis is proposed for these mechanisms, the results of which agree well with the results of a large-signal computer simulation. The oscillation mechanism is dependent upon a large excess electron concentration that is present at high current levels in the n region of an n⁺-n-n⁺structure. This electron concentration causes a net negative space charge in the n region, which in turn causes the electric field to be nonuniform, peaking at the anode n⁺contact. At sufficiently high current densities, an avalanche zone will form at the anode contact. The resultant carrier generation in this zone creates a hole domain of density sufficient to quench the avalanche. This hole domain then travels across the n region under the influence of the field. The positive space charge of the hole domain depresses the field sufficiently to prevent avalanche from recurring at the anode until the domain has extracted at the cathode. The field variation during this cycle causes transit-time terminal voltage oscillations. It is shown how, under proper conditions, a steady-state plasma region may be established over a substantial portion of the device length. This plasma region will cause the device to exhibit a negative differential resistance, and will also support relaxation oscillations at a frequency comparable to the reciprocal of its extraction time.