Determination of the physical parameters of transistors of single- and double-diffused structure

Abstract

This paper is concerned with the estimation of physical parameters of the internal device structure of transistors in which the collector-base junction is of gradual (diffused) transition. The parameters evaluated are the dimensional quantities of base width, depletion layer width, and junction area, together with those that characterize the impurity density distribution in the base and other regions according to some representative functional expression. Previous determinations of such parameters have been restricted to devices of step (alloy) junction type, but here single- and double-diffused structures, in which the collector junction is graded, are considered. The estimation of physical parameters is carried out in terms of relatively simply measured properties of the transistor--in particular, the dependence of base transit time and junction capacitance on operating bias conditions. The theoretical basis of the determination is developed on the assumption of a simplified model of the device structure in which the major part of the base region and the collector are represented by an exponential distribution of impurity density, while a possible opposing field region near the emitter is considered to be of linear grading. In the analysis of this model, the position-dependence of the diffusion and mobility coefficients is taken into account. Experimental results show that consistent evaluation of the physical parameters of the device (viz, of its model representation) is possible for a range of structural types.