Determination of a collector junction model for planar bipolar transistors

This paper is concerned with the representation of the collector-base junction of planar bipolar transistors by a model capable of accurate characterization of junction capacitance and avalanche breakdown behavior. The model chosen consists of an exponential impurity density profile with cylindrical peripheral region approximation, and is regarded as the simplest representation suitable for the purpose. Objectives are the practical determination of the defining parameters (collector background, impurity density, exponential characteristic length, and collector junction depth) for such a model, and the demonstration of its accuracy for the physical characterization of the device structure. Model parameter determination is carried out in terms of junction capacitance and BVCBOmeasurements, by processes of computer fitting between model and measured data; the procedures used are nondestructive. It is shown, in particular, that the values obtained for the collector resistivity and junction depth are in very good agreement with those derived by 4-point probe measurement and by bevel and stain sectioning, respectively.