Patent GB-991462-A (GB)
Title: Infrared detector
Inventor: GARBURY MAX
IPC: G01J5/28
Filed: 1961-09-11
Abstract: 991,462. Measuring infra red wavelengths. WESTINGHOUSE ELECTRIC CORPORATION. Sept. 11, 1961, No. 32578/61. Heading G1A. [Also in Division H1] An infrared detector capable of distinguishing between different wavelengths comprises a doped semi-conductor target member and means associated with the target member to provide a signal indicative of the intrinsic photo-conductivity and a second signal indicative of the extrinsic photoconductivity of the member. The invention is based on the fact that radiation of energy higher than a particular value (i.e. less than a certain wavelength) is necessary to effect intrinsic conductivity, the energy of radiation below this level being sufficient to provide extrinsic conductivity only, by raising electrons from the valence band to an intermediate level or from an intermediate level to the conduction band intrinsic and extrinsic conductivities may be distinguished by utilising the fact that a magnetic field provided perpendicular to the direction of current reduces intrinsic conductivity but not extrinsic conductivity. A second method involves comparing the conductivity of a layer adjacent the radiation impinged surface with that of the bulk; since the material has a higher absorption coefficient for radiation with energy above the intrinsic threshold value, the intrinsic conductivity effect is confined to this surface layer. A third method utilises the fact that the rise and decay time constants of intrinsic photo conductivity are larger than those of extrinsic photo conductivity. Fig. 1 shows a semi-conductor target member 12 of germanium or silicon doped for example with gold, zinc, copper, platinum or manganese, having an end surface 17 arranged to be submitted to radiation through a transmitting window 20 of silver chloride or barium fluoride fitted in an evacuated enclosure. The member 12 is maintained at a low temperature to prevent thermal generation of carriers, by means of a cooling coil 16 containing liquid nitrogen, helium or hydrogen. The member may be supported by an electrically insulating, thermally conducting insert 28 made for example of sapphire. A transverse magnetic field is provided by magnet 24. Current is passed through member 12 from source 18 to amplifier 22 which supplies one output corresponding to the conductivity of member 12 with the magnetic field applied and another to the conductivity with no magnetic field. The two outputs are applied to the inputs of a multi-gun colour television tube so that the proportion of say red to blue in the display corresponds to the proportion of high energy to low energy in the radiation applied to member 12. In a modification a similar amplifier and display arrangement is utilised and member 12 is provided with an electrode at each end and a third electrode which is intermediate the other two and closely spaced to the electrode at surface 17. Conductivity of the surface layer which contains the intrinsic photo conductivity effect is thus compared with the conductivity of the remainder of member 12 which arises from extrinsic conductivity. Another embodiment has a chopping wheel to interrupt the radiation applied to surface 17 and the photo current is passed to a pair of bandpass filters which select pulses of a rise time of the order of 10 Á secs. and 1/10 Á secs. respectively corresponding to the intrinsic and extrinsic outputs; the outputs are applied to a colour television tube as in the previous embodiments. The invention enables the approximate temperature of the radiation source to be assessed.