Space-Charge-Limited Currents Injected from a Point Contact

The theory of one-carrier, space-charge-limited currents has been studied for a spherical-flow geometry. The outstanding result is that the initial space-charge-controlled regime in the current-voltage characteristic, following the Ohm's law regime which always obtains at sufficiently low voltages, is universally a $\frac{3}{2}$-power law: $I=K{V}^{\frac{3}{2}}$. The coefficient $K$ depends on the density and energy distribution of traps, but is independent of both the point-contact radius and the anode radius. This result suggests the possible use of space-charge-limited currents injected from a point as a probe to measure the density of traps locally, i.e., in the neighborhood of the point. Experimental evidence for the $\frac{3}{2}$-power law, obtained with single-crystal CdS, is presented.