The performance of photoconductor is analyzed, via the concept of the steady-state Fermi level, and shown to be limited by the injection of space charge. Using the gain-bandwidth product $\frac{G}{{\tau }_0}$ as a measure of performance, it is found that $\frac{G}{{\tau }_0}=\frac{M}{{\tau }_r}$ where ${\tau }_r$ is the dielectric relaxation time under operating conditions, and $M=\frac{{\mathfrak{N}}_A}{{\mathfrak{N}}_T}$, with $e{\mathfrak{N}}_A$ the total charge on the anode and $e{\mathfrak{N}}_T$ the total volume charge, free plus trapped, effectively in thermal contact with the free charge. Generally $M>\mathrm{}1\mathrm{}$ is achieved only concomitantly with space-charge-limited currents varying steeply with voltage. An important exception is the case where recombination centers control the onset of injected space charge.

• Received 29 September 1958

DOI:https://doi.org/10.1103/PhysRev.113.1227