Multiple heterostructure photodetector

Details Multiple heterostructure photodetector Multiple heterostructure photodetector

1990-08-23

1991-11-26

Nelms, David C.

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

357 30, 2503384, 25037013, H01J 4014, H01J 2714, G01J 520

Patent

active

050685244

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a photodetector of the kind which is responsive to photon absorption.
Photodetectors are distinguished from other forms of radiation detectors in that they respond to photons received from a thermal scene. Thermal detectors such as pyroelectric detectors rely on the change in some physical property as a function of temperature to sense the temperature variation in a scene.
Photodetectors such as photodiodes and photoconductive devices are well known in the prior art. They are made of semiconducting material, and respond to photon absorption by creation of an electron-hole pair. In a photoconductor, an external bias voltage sweeps out such charge carriers, and photons are detected from increase in bias current. Alternatively, photodiodes may be operated in a photovoltaic mode without external bias. In this case, the internal electric field in the photodiode at its pn junction is responsible for charge separation.
In order to achieve high photodetector responsivity, the fractional change in charge carrier density produced by an incident photon flux should be as large as possible. Consequently, the minority carrier density in the absence of illumination should be as low as possible. This also maximises signal to noise ratio, since noise arises from fluctuations in carrier density and will be low if carrier density is low. In narrow bandgap semiconductor materials at moderate or high temperatures, the generation rate of minority carriers is itself proportional to the number of carriers. This is in fact the dominant source of noise in near intrinsic or n-type narrow bandgap materials. To improve responsivity and reduce noise, infrared photodetectors may be cooled to low temperature to reduce the free carrier concentration. This is particularly relevant to photodetectors of narrow bandgap semiconductor materials such as cadmium mercury telluride (CMT), i.e. Cd.sub.x Hg.sub.1-x Te where the value of x is adjusted according to the required detection wavelength interval. Detection in the atmospheric transmissive wavelength intervals 3-5 .mu.m and 8-12 .mu.m requires x equal to 0.28 and 0.205 respectively. An example of a CMT photoconductive detector is described in British Pat. No. 1,488,258 (U.S. Pat. No. 3,995,159). Such detectors are cooled to 190.degree. K. for operation at 3-5 .mu.m, and to 80.degree. K. for operation at 8-12 .mu.m. At ambient temperature, about 300.degree. K., they exhibit predominantly intrinsic conductivity; i.e. the charge carrier concentration is largely due to ionisation of valence electrons. When cooled to the appropriate operating temperature, the intrinsic contribution to conductivity is greatly reduced. Conductivity then becomes dominated by dopant impurities and is therefore extrinsic. As has been said, cooling to the extrinsic conductivity regime enhances photodetector responsivity and signal to noise ratio.
The requirement to cool CMT photodetectors to low operating temperatures is a severe disadvantage to their use. It is only tolerated because, when cooled, such detectors have excellent properties in other respects, e.g. high detectivity (D*), responsivity and quantum efficiency. However, it is necessary for conventional CMT photodetectors to be associated with refrigeration apparatus consisting of a cooling engine or Joule Thompson cooler. This adds greatly to the weight, cost and bulk of photodetection apparatus, and is a severe limitation to its use in man-portable systems in particular.
To overcome the cooling requirement problem, it has been proposed to achieve carrier concentration reduction by other means. This is described in European Patent Application No. EPA 0,167,305, and also by Elliott and Ashley in Electronics Letters, May 9, 1985, Vol. 21, No. 10, pp. 451-452. A photodetector is disclosed in which non-equilibrium effects are employed to reduce the carrier concentration. The device described is a three layer pp.sup.- n heterostructure of CMT materials. Here the subscript bar (-) and its absence under p or n indicates relatively wide and r

REFERENCES:
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patent: 4553152 (1985-11-01), Nishitani
patent: 4679063 (1987-07-01), White
patent: 4791467 (1988-12-01), Amingual et al.
Shanley et al., "Wide Bandwidth, High Sensitivity Hg.sub.0.8 Cd.sub.0.2 Te Photodiodes for CO.sub.2 Laser Applications," SPIE, vol. 227, CO.sub.2 Laser Dev. and Appl. (1980), pp. 123-132.
Stelzer et al., "Mercury Cadmium Telluride as an Infrared Detector Material", IEEE Transactions on Electron Devices, vol. ED-16, No. 10, 10/69, pp. 880-884.

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