Photoconductor Performance, Space-Charge Currents, and the Steady-State Fermi Level

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 Gτ₀ as a measure of performance, it is found that Gτ₀=Mτᵣ where τᵣ is the dielectric relaxation time under operating conditions, and M=NANT, with eNA the total charge on the anode and eNT the total volume charge, free plus trapped, effectively in thermal contact with the free charge. Generally $M>1$ 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.