Methodology for calculating turn-off transient of photoconductive circuit elements in picosecond optoelectronics

A methodology for calculating the transient following sudden removal of photon or mass-particle excitation is developed for semiconductor devices in which high hole-electron recombination rates, rather than short transit times, yield picosecond response time. We suggest the name photoconductive circuit element (PCE) for these optoelectronic transducers to emphasize their potential application to diverse circuit and system functions. The device consists of a nearly intrinsic active region that contains extremely high concentrations of deep-level states acting as recombination centers. Regions of semiconductors highly doped with shallow donor or acceptor atoms connected to metallic contacts border this active region. In calculating the turnoff transient, we set forth criteria that define nearly ideal ohmic-contact systems and the quasi-neutrality regime of the intrinsic region. If these criteria are met, the turn-off transient becomes a problem in photoconductive decay. For incremental variations in the electron and hole quasi-Fermi voltages much less than the thermal voltage, two characteristic times, assigned to each recombination-center energy level, rather than a single lifetime, describe electron-hole recombination. A computer algorithm accounting for incremental variations within suitably small time intervals then enables calculation of observables that may vary by many orders of magnitude, InP:Fe photoconductive circuit elements containing eight different Fe concentrations, excited by 780 and 600 nm laser-light pulses and 6 MeV electron pulses, showed a (
1/e
)-fall-time versus Fe-concentration in general accord with results calculated by the methodology. The capture cross sections and the energy distribution of the recombination-center energy levels receive attention. The InP:Fe transducers showed (
1/e
)-fall-times in the sub- 100-ps regime.