Excess noise in semiconducting devices due to fluctuations in their characteristics when signals are applied

This paper describes the frequency spectra of hitherto unobserved excess noise obtained when small signals are applied to transistors and semiconducting diodes and resistors. It is found that this excess noise can be divided into two kinds, one, similar to shot noise, being more or less uniformly distributed over a wide band of frequencies, the other being concentrated in ranges near the frequency of the applied signal. The excess noise is expressed as a ratio of the difference between the noise with and without signals to the noise without signals, Δθ/θ0. The reference quantity θ0 is in most cases independent of frequency, being thermal or shot noise, and even in transistors does not vary greatly with frequency. The ratio Δθ/θ0, plotted as a function of the difference Δf between the applied signal frequency and the frequency at which noise measurements are made, is found to vary in a manner very similar to the flicker effect, varying as 1/Δf. It is also found to vary as the square of the signal strength. It is considered that the excess noise is due to fluctuations in the characteristics of the device under measurement. To enable measurements to be made close to the signal frequency it was necessary to balance out the signal by the use of various bridge circuits. The signal frequency used was 84 kc/s, high enough to be above the frequency at which flicker noise is usually observed and low enough to facilitate the construction of bridge circuits.