A feedback method for investigating carrier distributions in semiconductors

The capacitance-voltage relationships of abrupt reverse-biased p-n junctions have long been used to investigate semiconductor doping distributions. Such investigations usually employ accurate point by point measurements or else RF harmonic generation to provide the required detailed connection between a small change in junction voltage and the associated quantity of charge flowing around the external circuit. The method described here differs in that a feedback loop is used in such a way as to provide a controlled motion of the low-field boundary of the depletion layer. This motion can be one of two kinds, either that of a constant amplitude modulation of the depletion layer width or else a motion corresponding to a constant amplitude modulation of the electric field. In the former case the information obtained corresponds to the carrier density , while in the latter it is its reciprocal. In practice this approach provides a number of advantages. Among these is the ability to mount the sample at the end of coaxial cables of essentially arbitrary length. This has allowed such applications as on-line profiling during accelerator implantations and profiling devices in cryostats to obtain information on free-carrier concentrations as a function of temperature. In the present paper the basis of the method and its implementation are outlined and some representative results are given.