Currents as high as 20 amperes per ${\mathrm{cm}}^2$ 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 Böer and Kümmel.

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.

• Received 21 September 1954

DOI:https://doi.org/10.1103/PhysRev.97.1531