Solid-state photodetection: A comparison between photodiodes and photoconductors

Abstract

PIN photodiodes and bulk photoconductors used as direct demodulators and as heterodyne mixers of coherent optical radiation are analyzed by taking the Fourier transform of the impulse current response produced by a single photoexcitation. This method allows the presentation of the fundamental physical processes in a manner which emphasizes the similarities rather than the differences between these two photodetectors. At microwave frequencies, the available power from the two devices is shown to have the same dependence upon frequency, and optical and circuit parameters. Further similarities are revealed by the noise analysis; in each case the limiting mechanisms are generation-recombination and thermal noise. The noise performance of the photodiode is found to be superior to that of the photoconductor at low light intensities. At high light levels, the noise performance of both devices is similar. The physical embodiment of these devices is such that the photodiode is generally a superior low-level detector (i.e., at light levels in the milliwatt range), whereas the photoconductor gives comparable or better results at high light levels (i.e., in the watt range). It is found, however, that the product of the bandwidth and fraction of the available power reaching the load is limited in the case of the photodiode because of the finite junction capacitance, but is essentially unbounded for the bulk photoconductor.