Modelling of the active defects influence on the electrical characteristics of an SiGe-HBT

In this paper, a new conduction model for SiGe heterojunction bipolar transistors (HBTs) is proposed. The combined effect of the granular nature of the polycrystalline emitter and the defects caused by the etching process are introduced to explain the electrical conduction properties. This model is based on solving the ambipolar transport equation while considering the quantum effects present at the grain boundaries. The simulation allowed us to obtain the two-dimensional distributions of the minority and majority carriers in the structure and showed the limiting effect of the diffusion of free carriers by grain boundaries, which is especially significant for the minority free carriers. It was observed that at low and middle injection levels, the deep level-defects caused by the etching process lead to a decrease in the current gain. However, the defects present in the grain boundaries are able to limit this reduction. At high injection levels, the electrical characteristics are very close to those obtained in the absence of defects at the interfaces.