Shot Noise in Transistors

The shot noise generated in a transistor may be represented in an equivalent circuit by a current generator ip across the collector junction and an emf es in series with the emitter junction. The characteristics of ip have been established by measurements of output equivalent saturated diode current made with the input open. The noise is independent of frequency up to approximately ƒα√1-α₀, whereƒαis the α-cutoff frequency of the transistor. It increases sharply in the neighborhood of ƒα, but then levels off at higher frequencies. The collector saturated current shows full shot effect up to frequencies well beyond the α-cutoff frequency. Information about es and about the correlation between e_s and I_p has been obtained from measurements of the noise figure. At low frequencies (ƒ<<ƒα), e_s and I_p are partially correlated for some transistors, while in others there is little or no correlation. Except for very small values of the source resistance, the main contribution to the noise figure comes from i_pp rather than from e_s. The experimental results are interpreted in terms of a recent theory by van der Ziel. Good agreement has been obtained.