Noise Characterization for Heterojunction Bipolar Transistors (HBTs)

A commonly used noise model for the bipolar junction transistor is the one developed by Hawkin who derived BJT noise model for minimum noise figure (F
_min
) by taking the emitter junction capacitance (C
_Te
) into account and by neglecting the bias dependence of the different transistor parameters. Although such a noise model was developed for homojunction transistors, it has been frequently used to calculate F
_min
for HBTs. It is observed that for a given frequency, the agreement in the medium range of bias current is acceptable as compared to low and high bias current where the two differ typically as high as 0.6 dB. This requires appropriate modification of Hawkin's formula when it is applied to HBT. In the present paper, a simple noise model for Fmin expression is derived by taking c
_De
> and C
_bc
into account, as well as the bias dependence of α
₀
. The effect of recombination current is included through the use of the ideality factor of the emitter-base junction diode. It is observed that the inclusion of these into the noise model provides a better explanation between experimental and theoretical calculations.