Gallium doped Germanium (Ge:Ga) Photoconductive Detector

See below for notes on the configuration for the measurements listed above. 

Photoconductors can provide a greater speed of response than a composite bolometer, at the expense of a narrower range of operating frequencies. They operate at higher frequencies (shorter wavelengths) than hot electron bolometers.

Gallium doping in Germanium gives a useful photoconductive response out to wavelengths longer than any other combination of elements. In unstressed configurations, the cut-off wavelength is approximately 120 um. By applying stress to the detector crystal, this can be extended to beyond 200 um. The measured spectral responsivity of stressed and unstressed detectors is shown in the figure below.

The performance characteristics of the detector depend critically on the operating conditions. In low radiation background conditions such as satellite astronomy, detector NEP can exceed 1 fW Hz^-1/2, though the speed of response will be limited, perhaps to 20 Hz (– 3dB.) In higher background laboratory applications the detector may operate at 50 kHz (-3dB), albeit with much reduced sensitivity.

QMC Instruments Ltd. offers this versatile detector for all conditions, either as a stand-alone detector or configured into complete calibrated detector systems giving optimum performance for a particular set of experimental circumstances. As with all our cryogenic detector systems, cooling can be provided either by a mechanical (pulse tube) cooler or by liquid helium. Mechanically cooled systems are complete and - apart from a vacuum pump for initial evacuation of the system - require only an electricity supply for operation.

Our liquid helium bath cryostats are designed and built to our specifications by our sister company, Thomas Keating Ltd. These cryostats offer the convenience of very long run-times. Cryostat operation is straightforward, and detector systems come with a comprehensive range of safety devices as standard. Superior cryogenic performance allows significant long-term saving on the cost of liquid helium. Larger cryostats are suitable for multi-channel detector systems or when very long liquid helium hold times are required.

Configuration for performance measurements 

Test measurements made on a 4.2 K unstressed detector in relatively high radiation background. The source was a blackbody at 800 K.  Filtering comprises black polyethylene at 77 K and a 120 cm^-1 low-pass powder filter at 4.2 K.

• f/3 condensing cone with 3.5mm diameter aperture
• 1.2 THz signal bandwidth centred at 3THz
• 300 nW background power
• 0.5 nW signal power 

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