Heavy Doping Effects in Silicon

Abstract

This chapter presents the high doping effects in silicon, and discusses the changes in electronic and impurity energy levels occurring in a semiconductor when the impurity doping level is increased. Emphasis is placed on the qualitative description, mathematics being kept to a minimum. Only the effects that can be described without quantum-mechanical analysis are treated in some detail. The importance of the resulting band-gap narrowing is emphasized. In this chapter, different experimental methods, explained in the literature, to determine the effective band-gap narrowing are described and explained. A new measurement technique developed by the authors is outlined. The results obtained by the different methods are compared. Measured data for the diffusion length and the minority carrier lifetime at high doping levels are discussed. The chapter also presents conclusions about the nature of the recombination process at high concentrations. The modifications in the minority carrier transport equations caused by the band-gap narrowing present at high doping levels are described. The importance of and the difficulties associated with the choice of the suitable statistics describing the occupancy of the majority carrier band are illustrated. The limitations, introduced by heavy doping effects, on the current gain and the gain-bandwidth product of a bipolar transistor are described. The deterioration of the speed performance of an integrated injection logic gate is illustrated as well as the reduction of its upward current gain. The decrease in the efficiency and open-circuit voltage of a solar cell resulting from heavy doping is described.