An AC Bridge for Semiconductor Resistivity Measurements Using a Four-Point Probe

Abstract

A new direct-reading ac bridge circuit has been developed to measure semiconductor bulk and sheet resistivity, using a four-point (or other appropriate) probe. The range of resistivity which can be measured is from 0.001 to 10,000 ohm-cm. Resistivity is read directly from resistance decades and a ratio multiplier, eliminating voltmeter and ammeter errors — the final reading being the result of a bridge-balancing operation for each measurement. Stability and sensitivity provide better than 0.5 per cent electrical accuracy, with mechanical point spacing being the controlling limitation on the over-all accuracy of the measurement. The use of ac eliminates the influence of rectification, thermal, or contact potentials on the measurements, and also provides sensitivity more readily than with dc. The four-point probe and test specimen are the only nongrounded elements. An Appendix compiles four-point probe conversion factors for thin circular and rectangular slices of material. New tables are presented for slices having a continuous diffused skin all over, and thus also conducting across the back.