Noise resistance of heterojunction bipolar transistors

It is customary to use Hawkin´s model to calculate the noise performance of Heterojunction Bipolar Transistors (HBTs). As was already known, and also recently shown by Pucel and Rohde (1993), the noise resistance R_n increases monotonically with frequency. However, recent measurements have shown R_n vs. f behavior which is just the opposite and R_n decreases with frequency. This apparent inconsistency, between the theoretical prediction and the experimental measurement of R_n, is the topic of the present paper. In this paper, Hawkin´s isothermal model (device temperature~ambient temperature) is revisited in order to calculate the bias dependence of R_n in HBTs with varying frequency. The incorporation of the parasitic circuit elements along with a proper calculation of junction temperature will enable one to explain for the first time the high frequency noise behavior of HBTs. Though, as noted by some researcher, the contribution of base-collector capacitance is minimal in the noise figure calculation, it will play a vital role in the calculation of R_n versus f. Also the device self-heating, which makes the isothermal approximation invalid for HBTs, is included in the present work. The obtained results will be compared with the experimental measurement to verify the validity of the model