Space-Charge-Limited Currents in Single Crystals of Cadmium Sulfide

Currents as high as 20 amperes per cm^2 can be drawn through thin insulating crystals of CdS in the dark. A series of experiments demonstrate with a high degree of certainty that these are space-charge-limited currents---the solid state analog of space-charge-limited currents in a vacuum. This conclusion is contrary to a recently published interpretation of similar observations on CdS crystals by B\"oer and K\"ummel. The use of pulsed voltages made possible the observation of currents close to those of a trap-free solid. The steady-state currents are many orders of magnitude lower than these but still many orders of magnitude higher than would be expected from the low-field resistivity of the insulator. The presence of traps determines the form and magnitude of the steady-state current-voltage curves. Conversely, these curves become a sensitive tool for the measurement of trap densities. Trap densities computed independently from space-charge-limited currents and from photoconductive currents show reasonable agreement.