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

It is shown that the observed values of the minimum noise figureFₘᵢₙof UHF transistors in common base connection can be explained in terms of the device parameter(1-αdc) rb'b/Rₑ₀and 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 ofFₘᵢₙon operating conditions can be explained by this effect. The current dependence ofFₘᵢₙfor 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 inrb'b, which in turn is caused by the dependence of the base width on |VCB|.