Correlation of diffusion process variations with variations in electrical parameters of bipolar transistors

In this paper we consider the problem of determining the effects of a nonuniformity in diffusion processes on the resultant electrical characteristics in semiconductor devices. A bipolar, planar silicon p-n-p transistor is considered as an example. A model is presented describing the current gain in terms of measurable diffusion parameters. These parameters are 1) the four-point probe reading of the diffused base, 2) the emitter-base junction depth, and 3) the collector-base junction depth or √Dt of the base. The model is used to predict the sensitivity of current gain to each of these diffusion parameters; it is shown that the sensitivity of current gain to the junction depth can be drastically reduced by a modification in transistor structure. Design criteria for making this structural change are presented as well as the results on actual devices.