Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure
Patent
1997-03-13
1998-06-09
Jackson, Jerome
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
257103, 438 22, 438 45, H01L 3300
Patent
active
057639060
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The current invention is concerned with devices for producing radiation in the mid infrared region of the electromagnetic spectrum and has particular (though not exclusive) relevance to the problem of gas sensing.
2. Discussion of Prior Art
Light emitting diodes (LEDs) have been studied for a number of years, primarily for the purposes of displays and communications (see for example A A Bergh and P J Dean "Light Emitting Diodes", Clarendon Press Oxford 1976). Technologies based on GaAs, GaP and InP and alloys grown as thin layers on these materials (eg Al.sub.x Ga.sub.1-X As grown on GaAs) are well developed and provide devices producing radiation of wavelengths from 0.5.times.10.sup.-6 m to 1.7.times.10.sup.-6 m.
U.S. Pat. No. 4,144,540 discloses an infrared detector composed of four layers of epitaxially grown material that is doped for selective wavelength absorption. The device has a narrow band, tuneable response. The upper layers comprise a heavily doped n type layer over a p type layer. The cut-on absorption edge is determined by the amount of doping in the uppermost layer and the cut off absorption edge can be varied by applying a reverse bias voltage to the n-p junction formed by these layers.
The "Encyclopedia Of Semiconductor Technology" (Encyclopedia Reprint Series), ed. by Martin Grayson, John Wiley & Sons, New York, US, teaches the optimisation of the amount of generated radiation that can be emitted from a LED. Although this document refers to the use of the Moss-Burstein shift to render the substrate of a LED transparent to radiation produced therein, the reference is made in relation to GaAs devices in which the key physical mechanism of operation is, in fact, that the radiation generated in the p type material has energies below the bandgap of the material. Using n-type material to extract the radiation is advantageous because it eliminates the very strong free carrier absorption associated with p type material. The Moss Burstein shift in GaAs is small and it is unlikely that the effect would be of significant use in these devices.
Historically, requirements for LEDs with operating wavelengths of longer than 1.55.times.10.sup.-6 m have been much more limited and consequently such devices have been the subject of much less development activity.
There is currently a growing interest in the development of cheap, quantitative, selective and low power gas detection apparatus. Many gases 5!.times.10.sup.-6 m which provide a characteristic signature of that gas. An optical system based on an LED operating at the characteristic absorption wavelength has the potential to meet these system requirements. Moreover, such an LED based system would be more power efficient than the alternative system based on a thermal infrared source.
Gases of particular interest include methane with an absorption at 3.3.times.10.sup.-6 m, carbon dioxide with an absorption at 4.2.times.10.sup.-6 m and carbon monoxide with an absorption at 4.7.times.10.sup.-6 m.
In a conventional LED structure, an evaporated metal contact is applied to the top surface of a layer structure which includes a p-n junction. This contact can obscure a significant portion of the emitting area of the LED and hence lead to a reduction in external efficiency. Moreover, in thin layers of material, current crowding is likely to occur, that is the vertical current through the contact layer is largely constrained to flow in the area of the device under the top contact (see W B Joyce and S H Wemple, Journal of Applied Physics, 41, 3818 (1970)). This further reduces the light output from the top surface of the device. This effect is particularly severe in devices fabricated with p-type upper contact layers due to the low mobility of holes in III/V semiconductors.
In GaAs/AlGaAs IR LEDs these problems of light extraction have been met by inverting the device structure and extracting the light from the substrate side. It is necessary to either etch a window in the substrate to produce a Burrus structu
REFERENCES:
patent: 3930161 (1975-12-01), Ameurlaine et al.
patent: 4008485 (1977-02-01), Miyoshi et al.
patent: 4144540 (1979-03-01), Bottka
Physical Review, vol. 123, No. 5, 1 Sep. 1961 New York U.S., pp. 1560-1566, J.R. Dixon & J.M. Ellis, "Optical properties of n-type indium arsenide in the fundamental absorption edge region".
Boud John Michael
Kane Michael John
Lee David
Wight David Robert
Jackson Jerome
Kelley Nathan K.
The Secretary of State for Defence in Her Britannic Majesty's Go
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