Verification of the non-local avalanche current model in mextram for advanced SiGe HBTs

Abstract

To realize optimum performance, SiGe HBTs are typically designed with heavily doped implanted collectors. For practical circuits operating at either high collector current density (Jc) or high collector-base voltage (Vcb) avalanche multiplication is an important effects that must be accurately measured and modeled. For example in digital applications, the avalanche multiplication factor (M-1) determines the breakdown voltage, which in turn determines the maximum power supply for stable logic operation. In critical RF circuits such as power amplifiers (PA) and low noise amplifiers (LNA), the base-collector junction avalanche multiplication degrades the linearity of the circuit because of the resulting strong non-linear feedback from the output (collector) to input (base) is particularly the case for state-of-art high-performance transistors featuring high collector doping. Therefore the accuracy of avalanche multiplication models in different operational conditions is critical to devices design of high linearity LNA and PA circuits. In this thesis, the temperature dependence of the avalanche current in Mextram compact model is addressed through extensive DC measurements over temperature on advanced industrial SiGe HBTs, it was discovered that the current local-electric field based avalanche current model in the Mextram model is incapable of describing the avalanche current as a function of device temperature.This setback is the key motivation behind the work in this thesis. By employing the simplified energy-balance equation, the impact ionization rate was expressed in terms of the carrier (electron for NPN HBT) energy or temperature (Te). Here a triangular shaped electric field distribution corresponding to the normal forward operation regime was assumed. Taking the integral of the electron temperature dependent ionization rate over the epilayer yielded the non-local multiplication factor. The product of the multiplication factor with epilayer current gives the non-local avalanche current, which takes non-local avalanche effects into account. The compact formulation of this non-local avalanche current model was derived and implemented at Delft University of Technology in the in-house version of Mextram compact model. An extended experimental verification of the new compact model for the non-local avalanche current implemented in Mextram was carried out for different advanced SiGe HBT technologies; and the results are presented in this thesis. Verification results showed that the non-local avalanche current model can accurately describe the avalanche current as a function of temperature for different SiGe HBTs (both NPN and PNP). These results implies that the observed setback in Mextram model with respect to the temperature dependence of the avalanche current can be fully addressed by taking non-local avalanche effects into account.