Trap Density Determination by Space-Charge-Limited Currents

Trap depths and densities can be determined from the analysis of the dependence of space-charge-limited currents on applied electric field in insulators. Values obtained from space-charge-limited current measurements are compared with those obtained from thermally stimulated currents and from photocurrent decay on the same crystal of cadmium sulfide. The results indicate that gross errors in trap depth and density can be made if it is simply assumed that the rapid rise in current toward the trap-free curve with increasing applied voltage always occurs when the traps are filled. Although this may sometimes be true, it is also possible for such a rapid rise to result from field ionization of the traps, or even in certain cases from the onset of double injection. Even if the latter effects occur, however, proper analysis of the space-charge-limited current data permits an evaluation of trap depth and density which is completely consistent with independent measurements of thermally stimulated current and photocurrent decay.