Étude des effets de la forte injection dans les structures bipolaires

Abstract

The purpose of thesis is the development of software capable to do a simulation of bipolar transistors taking into account both the type of component architecture, the level of carrier’s injection, the granular nature of the polysilicon forming the emitter and defects caused by the etching process of this material. To achieve this goal, we proceeded in progressive steps. First of all, we considered the classical semiconductor equations accounting for the displacement and the conservation of the carriers as well in the case of a homojunction as in the case of a heteronjunction. By the finite difference method, we have passed from the continuous case to the discrete case by establishing at the end of this operation, a system of coupled equations whose resolution makes it possible to calculate the potential, the density of the electrons and the holes in each point of the component and also to deduce other physical quantities. This step constitutes a first ideal approach of the transistor and a comparison of the simulation results with the experimental measurements shows a rather important difference. Thus, in the second phase of the work, we proceeded to a correction of the model and this, by introducing the quantum drift diffusion model (QDDM) which take count of the quantum effects and the defects that can exist in the bipolar transistors and which disrupt their operation very significantly. Finally, in the last step, we used this software to simulate two types of bipolar transistors: homojunction bipolar transistor (TB-Si) and heterojunction bipolar transistor with a base containing germanium (HBT - SiGe). The two types of components belong to the same 0.35µm BICMOS-die with polysicilium emitters, but made according to two different architectures. Simulation results showed that:  the first type of transistor, TB-Si was much more affected by the presence of etching defects than the second type (HBT-SiGe),  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.