Noise figure of UHF transistors as a function of frequency and bias conditions

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Abstract
It is shown that the observed values of the minimum noise figureF_{\min}of UHF transistors in common base connection can be explained in terms of the device parameter(1-\alpha_{dc}) r_{b'b}/R_{e0}and fαfor frequencies up to 1000 MHz. An interesting collector saturation effect is observed that gives a strong increase in UHF noise figure at high currents. Many features of the dependance ofF_{\min}on operating conditions can be explained by this effect. The current dependence ofF_{\min}for large values of |VCB| and high currents suggests a distribution in diffusion times through the base region. At intermediate frequencies, the noise figure increases with increasing collector bias |VCB| due to an increase inr_{b'b}, which in turn is caused by the dependence of the base width on |VCB|.

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