Wafers cut from undoped GaAs single crystals, grown by the liquid-encapsulated Czochralski (LEC) method, are used as semi-insulating substrates for the fabrication of planar, ion-implanted devices and circuits. As the dimensions of these structures are reduced, the effect and distribution of crystal defects and dislocations become relevant to device reproducibility and yield over the whole wafer.A nondestructive technique has been developed for the evaluation of sheet-resistance of commercially grown LEC GaAs semi-insulating wafers. The "dark-spot" technique, utilized by Blunt, has been modified to give a topographical representation of the sheet resistance distribution of semi-insulating GaAs wafers. This is achieved by illuminating the wafer with a thin light strip and defining a dark-spot with a cursor placed perpendicular to the strip. As the illuminated strip is believed to consist of material two orders of magnitude smaller in surface resistivity than the unlit portions, the measured resistance at the edges of the wafer will be determined by the resistance under the dark-spot. When the dark-spot is scanned over the entire wafer surface, measurement of the signal provides a topographical representation of the sheet-resistance of the wafer. Results were obtained on three 2-in.-diameter GaAs wafers grown by two different manufacturers. These results allow contours of isoresistive domains to be defined.The measurements confirm Jordan's theoretical model for the dislocation density distribution of LEC-pulled GaAs wafers, and thus give validity to the method. Because Jordan's thermodynamic model represents a theoretical lower limit for the dislocation distribution, the results obtained from the method developed here can give a measure of substrate quality and eventually determine guidelines in acceptance tests. This technique is capable of scanning large-area GaAs wafers at high speed and with good sensitivity. It may be, therefore, considered competitive with comparable X-ray evaluation methods when high resolution is not a requirement.