Demodulation of low-level broad-band optical signals with semiconductors

An experimental survey of the photoconductive performance of various high-purity semiconductors mounted in a re-entrant microwave cavity shows a significant increase in SNR over conventional infrared detectors. Outstanding examples are provided by germanium for wavelengths less than 1.5 microns and information bandwidth to 30 MHz, indium arsenide to 3.4 microns and the same bandwidth, and indium antimonide to 5 microns and hundreds of MHz. Theoretical analysis of the photocurrent gain-bandwidth product shows it to be controlled by two important parameters: the ratio of electric field in the sample to the root of the microwave energy stored in the cavity, and the saturated drift velocity in the semiconductor. Measured performance is in quantitative agreement with the theoretical expression. The system requires only a few milliwatts of microwave power to drive the carriers to saturated velocity; the signal is extracted with a conventional microwave receiver. A retrieval efficiency beta is introduced to compare the performance of broad-band detectors, whose significance for solid-state detectors is analogous to that of the quantum efficiency alpha for a photomultiplier. Large beta means increased SNR; when beta reaches alpha, the device can count individual quanta. Measured values of beta for the devices studied reach 10 percent for germanium under certain conditions, a value approached only by the avalanche diode and more than two orders of magnitude higher than has been reported for other solid-state detectors. Comparison of the values of NEP and of (D
^*
) with those of other infrared detectors shows a very large improvement in NEP and an important increase in (D
^*
). The latter ranges from 3 × 10
¹⁴
for germanium to almost 10
¹²
(BLIP condition) for indium antimonide, and NEP ranges from 2 × 10
^-17
for germanium to 2 × 10
^-14
for indium antimonide. For both materials and also silicon, the gain-bandwidth product is about 10
¹⁰
; for indium arsenide the screening by free carriers reduces it to 10
⁹
Hz. Noise studies reveal a large 1/f component. While its source has not been defined completely, the indication is that it can be significantly reduced by improvements in the microwave circuit. There seems to be no serious barrier to constructing photoconductive detectors which are shot-noise-limited and can count photons with high efficiency.