AC IMPEDANCE METHOD FOR HIGH-RESISTIVITY MEASUREMENTS OF SILICON

An ac impedance method for measuring the average bulk resistivity of ingots and slices of high-resistivity silicon is presented. Easily removable contacts, such as silver paint, are applied to the end faces of the sample, and the complex impedance of the resulting capacitive sandwich is measured as a function of frequency. The resistivity can be calculated from the frequency of the negative peak in the imaginary part of the impedance and from the absolute values of the real and imaginary parts at that frequency. The spectroscopic nature of the method gives an inherent separation of contact, surface region, and bulk effects as the respective responses usually occur at widely different frequencies. In addition to its intended application for measuring bulk resistivity, the method is useful for determining the quality of applied contacts and the effect of surface treatments which result in a significant depletion layer. Plots and the required data can be obtained very quickly with an appropriate microprocessor-based impedance analyzer. Extensive measurements of high-resistivity silicon were done to compare the method with the dc resistance, van der Pauw, and four-probe techniques. The agreement was within 5% for slices and ingot sections greater than 0.1 cm in length and resistivity above 5 k-OMEGA . cm.