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

It is shown that the observed values of the minimum noise figure
F_{\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 f
α
for 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 of
F_{\min}
on operating conditions can be explained by this effect. The current dependence of
F_{\min}
for large values of |V
CB
| and high currents suggests a distribution in diffusion times through the base region. At intermediate frequencies, the noise figure increases with increasing collector bias |V
CB
| due to an increase in
r_{b'b}
, which in turn is caused by the dependence of the base width on |V
CB
|.