Transient Behavior of the Ohmic Contact

Under certain conditions the response time of a solid for current transients may be determined by the properties of the injecting contact rather than of the bulk. In this case it is the time required for readjustment of the space-charge barrier at the contact for delivery of the new steady-state current. The critical parameter determining the response time of the contact is the total amount of excess charge within one Debye length of the potential minimum in the space-charge barrier. Trapped, as well as free, excess charge must be included in the calculation of the Debye length. The product of gain ($G$) and band width ($\frac{1}{{\tau }_0}$) for a photoconductor whose response time is contact-controlled is derived and expressed in the "universal" form $\frac{G}{{\tau }_0}=\frac{M}{{\tau }_r}$, where ${\tau }_r$ is the dielectric relaxation time under operating conditions.