Properties of Ohmic Contacts to Cadmium Sulfide Single Crystals

The usual materials and techniques for making electrical contact to CdS single crystals are not satisfactory. The $V-I$ characteristics are erratic and nonlinear, the photocurrent noise is excessive and there is a spurious photovoltaic effect, all associated with poor, high-resistance contacts. It has been found that In and Ga make ohmic contact to the crystals. With ohmic contacts the disturbing effects of the barriers are removed and measurements of the electrical characteristics of the crystals can be made with confidence. In addition, maximum performance can be obtained from the crystals. From the measurements it is shown that there can be large volume photosensitivity in CdS, and that the surface and volume (dark) conductivity of a crystal can differ greatly. Typical $V-I$ characteristics are presented demonstrating: (1) The improved performance obtained from a photosensitive CdS crystal with In contacts compared with that obtained with the Au contacts; (2) broad-area CdS crystal rectifiers. One was made from highly conducting crystals from which a forward current density ∼1 amp/${\mathrm{cm}}^2$ was obtained at 1.5 volts with a front to back current ratio of ${10}^6$. The other, a rectifier-photoconductive cell, was made from an insulating photosensitive crystal. In the forward direction the gain was >${10}^4$ at 6 volts (corresponding to ∼10 amp/lumen). (3) Space-charge-limited current in an insulator. The current is time dependent and increases more rapidly with voltage than the square-law theoretically predicted for a perfect insulator. The effect of crystal imperfections in determining the $V-I$ characteristics is indicated.