Electrical Fluctuations in Silicon Carbide Junctions
The parameters describing electrical fluctuations in junctions were taken to be the relative spectral density of nominal noise power (noise coefficient) and the spectral density of the short-circuit noise current. Measurements of the spectral density of fluctuations were made in the audiofrequency range by comparison with a noise generator. An increase of the forward voltage was accompanied by a nonmonotonic variation of the noise current and noise coefficient of a diode. The minimum noise intensity occurred at forward voltages between 1.9 and 3 V. This effect could be explained qualitatively by a decrease of the differential resistance when the voltage increased. When a junction was reverse-biased, the spectral density of the noise current varied very rapidly with the reverse current. This behavior was typical of the avalanche breakdown region of the current—voltage characteristic. The observed current—voltage characteristic was typical of surface breakdown. The occurrence of low-frequency noise in surface breakdown of silicon carbide junctions was in agreement with corresponding results for germanium diodes. When the reverse voltage was increased, the noise coefficient rose much more slowly than the spectral density of the noise current; this was due to the decrease of the differential resistance of the junction when the reverse voltage was increased. A study of the frequency dependence of the noise coefficient showed that, in the forward — bias case, there was either a typical low-frequency junction noise or a generation—recombination noise of the spreading resistance. In the reverse connection, some samples gave typical low-frequency fluctuations. In the majority of diodes, a generation—recombination noise component appeared against the lowfrequency noise background. With both forward and reverse connection, the time constant of the generation—recombination noise was about 10-5 sec. The nature of the fluctuations having this time constant is not yet known.
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