Determination of physical parameters of diffusion and drift transistors

Abstract

The dynamic properties of drift and diffussion transistors are studied under both low- and high-level injection conditions in terms of the excess carrier charge in the base region. Subject to the assumptions of effectively one-dimensional device geometry with exponential impurity grading and collector conductivity much greater than that of the base, methods of determination of the main physical and high-frequency equivalent-circuit parameters of transistors are presented, utilizing the bias dependence of excess carrier charge in the base and space charge in the collector-depletion region. It is shown that measurement of the base transit time and the junction capacitances under low-level injection conditions enables the following physical parameters to be determined: base field parameterm = \DeltaV/(kT/q), base width, base impurity distribution, emitter area, collector-depletion layer width. All the measurements are carried out at relatively low frequencies. Solutions for the base charge distribution, and hence charge-defined transit time, have been derived for the cases of exponential and erfc impurity grading under high-level injection conditions, assuming one-dimensional device geometry, constant base cross-sectional area, andDand\microindependent of injection level. Experimental results have been found to differ greatly from the theoretical expectations based on these assumptions, the drift transistor showing greater and the diffusion transistor less than the predicted dependence on injection level. Explanations for the observed effects have been put forward in terms of reduction of emitter area and ofDand\microat high injection levels.