Small-signal noise measure of avalanche diodes

Abstract

A study of small-signal noise measure of avalanche diodes is useful for establishing a reference for large-signal noise theories as well as to determine conditions for low noise operation in low power applications. Such a study depending largely on numerical calculations was presented by Haus et al. Similar results are derived by analytic techniques through the use of a second-order approximation for the avalanche current accounting for transport delay of the carriers in the avalanche region. A uniformly avalanching PIN diode is studied to derive the properties of the avalanche region. In the lossless case the noise measure has a minimum for a carrier transit angle of approximately 2π, while it is inversely proportional to frequency squared for smaller transit angles. Operation close to the avalanche frequency is favorable in a practical case because of parasitic series resistance. The effects of a drift region is included to obtain reasonable modeling of practical avalanche diodes. In the lossless case the impedance transformation and space-charge smoothing in the drift region cause a strong modulation of the noise measure with minima for drift transit angles π < θ d < 2 π and 3 π < θ d < 4 π. The noise measure decreases for increasing avalanche widths. Series resistance limits the practical upper drift angle to approximately 1.2π due to the decrease of the negative resistance with frequency. Similar calculations have been carried out for double-drift-region diodes, and show that these in general are noisier than the corresponding single-drift structures in the small-signal limit.