Silicon Technologies: Ion Implantation and Thermal Treatment

Chapter 3

Dopant Diffusion: Modeling and Technological Challenges ¹

3.1. Introduction

Semiconductor doping is a key element for manufacturing electronic devices. Indeed, a pure (intrinsic) semiconductor is almost an insulator at room temperature, with a valence band almost entirely full, and a conduction band almost entirely empty. As an example, the intrinsic density of mobile carriers in silicon is only about 10¹⁰ cm^-3 at 300 K. The interesting electronic properties are obtained thanks to the possibility of “doping” the material by introducing adequate impurities (dopants), making it possible to introduce mobile carriers. In fact, the functioning of all the active electronic devices mainly relies, if not entirely, on the possibility of forming P/N junctions, i.e. to put in contact areas doped with an acceptor (impurity having an electron deficit compared to the matrix, involving a conduction by “holes” in the valence band) and areas doped with a donor (impurity having an electron excess compared to the matrix, involving “free electron” conduction in the conduction band). In silicon and germanium (elements of column IV of the periodic table), donors are elements of column V (the most frequently used being phosphorus and arsenic), while acceptors are elements of column III (the most frequently used being boron).

The detail of the electric behavior of a semiconductor device thus strongly depends on the dopant distribution inside the structure. For evidence, we need to compare, for ...