A method for measuring the conductivity of high resistivity semiconductor materials

Abstract

A simple method of measuring the conductivity of semiconductor materials is studied both analytically and experimentally. The basic method utilizes two pairs of coils, operating as transformers, and excited with a source of sinusoidal voltage. The two primary coils are connected in series, so that they have the same current flowing through them; the secondary coils are connected in series, in such a manner that the induced voltages tend to cancel. An approximate null in the secondary circuit is produced by positioning of the coils. A sample (usually small) of semiconductor material is placed between one pair of coils. A voltage proportional to the bulk conductivity of the sample is induced in the secondary winding for that pair, disturbing the voltage balance in the secondary circuit. Because this voltage is 90 degrees out of phase with the original voltage, it may be detected by methods that take account of the phase relations. In the preliminary experimental studies reported in this thesis, the out-of_-phase voltage component was determined by simple trigonometric relations. A practical laboratory instrument would use a phase sensitive detector excited in such a manner that it responds to the desired component. The experimental study verified the essential correctness of the theoretical analysis and demonstrated the potential usefulness of a practical instrument based upon the principles utilized. Certain refinements and simplifications of design for a practical instrument are indicated.