Demodulation of low-level broad-band optical signals with semiconductors: Part II—Analysis of the photoconductive detector

A fast response photoconductor with a high frequency bias supply is considered as an envelope detector for optical signals. The sensitivity is studied and compared with that of the diode demodulator. The ratio of SNR for the two devices is equal to the photocurrent gain. It is shown to be theoretically possible to achieve enough current gain to overcome the noise of the following amplifier. The current gain comes from two effects. One is a true photocurrent gain in the semiconductor itself, which can exceed unity if the microwave field reverses before the photocarriers are swept out. It can be as high as the number of times the photocarriers traverse the photoconductor before they recombine. The impedance transformation from the high resistance of the detector to the amplifier input gives additional current gain. Analysis of gain-bandwidth limitations reveals no restriction imposed by material parameters, in contrast to the case of dc bias. With RF bias, blocking contacts to the crystal are useable and the relationship between material resistivity and gain-bandwidth avoided. The limiting parameters are the signal bandwidth and the bias supply frequency, the device current gain being limited to the square of the Q value of the circuit. The effect of G-R noise is also considered and conditions derived under which it is unimportant. The two cases of a photoconductor in a rectangular waveguide and in a cavity are studied in more detail, and design equations relating sensitivity to the material and to the circuit parameters are deduced. For bandwidths up to at least 1 kMc, the photoconductor in the cavity can greatly outperform the photodiode. Its sensitivity can approach that of the photomultiplier with a high efficiency cathode, which opens the possibility of extending this high performance into the infrared.