The noise performance of PNP and NPN SiGe structures is examined
by an experimentally verified hydrodynamic (HD) noise model.
This model is a hierarchical numerical noise model because all noise parameters required by this model are generated by full band Monte Carlo bulk simulations leading to the methodology of the hierarchical numerical noise simulation. The hierarchical HD noise model is applied to compare the performance of NPN and PNP SiGe HBTs. The simulations include AC, DC and noise characteristics like the minimum noise figure. A similar noise performance for both types of devices is found.
The accuracy of the SPICE and unified compact noise models
is assessed in the RF range by comparison with the hydrodynamic device model for a state-of-the-art SiGe HBT with a low base resistance. Despite the low base resistance, as a general result, it turns out that the noise is dominated by the thermal fluctuations of the holes within the base and the exact determination of the base
noise resistance is a prerequisite for accurate compact noise modeling. It is shown that the base noise resistance equals the base resistance and can be evaluated with standard parameter extraction schemes. Based on an accurate base resistance the SPICE model
yields good results as long as the frequency is considerably
below the peak cutoff frequency. The unified model, on the other hand, is found to yield good results even at frequencies comparable to the peak cutoff frequency. But this is achieved at the expense of an additional parameter which is difficult to determine without physics-based numerical noise simulation. Moreover, it is shown that the drift-diffusion model should not be used to assess the accuracy of compact noise models, because it yields erroneous noise results
for state-of-the-art SiGe HBTs.