Active solid-state devices (e.g. – transistors – solid-state diode – Regenerative type switching device – With means to increase breakdown voltage
Patent
1992-01-24
1993-08-03
Mintel, William
Active solid-state devices (e.g., transistors, solid-state diode
Regenerative type switching device
With means to increase breakdown voltage
257185, 257186, 257199, 257438, H01L 2714
Patent
active
052332098
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to beryllium guard ring structures and fabrication thereof, and in particular to guard ring structures for avalanche photodiodes.
BACKGROUND OF THE INVENTION
In photodiodes (APD's), avalanche photocurrent is multiplied by impact ionisation of primary photogenerated carriers accelerated by a high field. In III-V APD devices for use at wavelengths in the regions of 1.3 and 1.5 microns a material such as indium gallium arsenide or suitable quanternary compounds may be used for the region where photogeneration occurs. If the multiplication also takes place in this material, which has a comparatively small band gap, high dark currents resulting from tunnelling occur at the high fields required for multiplication, giving rise to excessive noise. To avoid this problem the carrier pairs are photogenerated in indium gallium arsenide and then the holes are swept into a wider band gap indium phosphide layer containing the pn junction where avalanche multiplication takes place. This type of device is known as the separate absorption and multiplication (SAM) structure. A further modification is the SAGM which has a graded layer of intermediate composition (usually a quaternary) between the indium phosphide and indium gallium arsenide layers. The thickness and doping level of the indium phosphide multiplication layer has to be carefully controlled in order to achieve the correct field for avalanche multiplication near the pn junction, with a sufficiently low field at the interface with the indium gallium arsenide layer to avoid tunnelling. Both planar and mesa APD structures are known, and in general planar structures have proved superior, and are therefore preferred.
In planar APD devices the pn junction, which is generally formed by p+ implantation or diffusion into a nominally n type layer, will have a curved edge and is therefore subject to edge beakdown unless the boundary shape is modified to minimise the effects. Thus it is usual for a p type guard ring, also diffused or implanted, to be fabricated around the outer edges of the p type junction region. The structure and doping concentrations within the guard ring influence the extent to which edge breakdown is prevented.
SUMMARY OF THE INVENTION
Accordingly the present invention provides an avalanche photodiode comprising a semiconductor substrate, as n type light absorption layer, an n type indium phosphide multiplication layer, a p type region formed on the multiplication layer and providing an abrupt pn junction with the multiplication layer and a graded p type guard ring surrounding said p type region, characterised in that the guard ring comprises ion implanted beryllium at an implantation dosage of at least 5.times.10.sup.14 per cm.sup.2.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is now described by way of example with reference to the accompanying drawings in which:
FIG. 1 is a schematic cross section through a SAGM APD with a guard ring, and
FIG. 2 is a graph showing the dopant profile of the guard ring.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring to FIG. 1, a preferred planar SAGM, APD structure is shown which comprises an n+ indium phosphide substrate 1, doped for example with sulphur to a level of 8.times.10.sup.18 per cc. Over the substrate there is a 0.5 micron n type indium phosphide buffer layer 2, doped for example to a level of 10.sup.17 per cc, a 3 micron absorption layer of nominally undoped indium gallium arsenide 3 which in fact has a residual n type level of 10.sup.15 per cd and a 0.1 micron layer 4 of indium gallium arsenide phosphide with a 1.3 micron equivalent band gap. The layer 4 serves to grade the band gap between the layer 3 and the multiplication layer 5 while maintaining lattice match. Layer 5 comprises sulphur doped 10.sup.16 per cc indium phosphide and, overlying layer 5, is a final layer 6 of nominally undoped, actually 10.sup.15 per cc n type, indium phosphide. The indium phosphide layer 6 is of the order of 1 to 2 microns thick, and into its top surface a diff
REFERENCES:
patent: 4415370 (1983-11-01), Kagawa et al.
Taguchi et al., "Planar InP/InGaAs Avalanche Photodiodes with Preferential Lateral Extended Guard Ring," IEEE Electron Device Letters, vol. EDL-7, No. 4, Apr. 1986, pp. 257-258.
Feng et al., "Be-Implanted 1.3-.mu.m InGaAsP Avalanche Photodetectors", Appl. Phys. Lett. 34(9), May 1, 1979, pp. 591-593.
Shirai et al., "1.3 .mu.m InP/InGaAsP Planar Avalanche Photodiodes", Electronics Letters, Oct. 29, 1981, vol. 17, No. 22, pp. 826-827.
Kol'tsov et al., "Electrophysical and the Photoelectric Characteristics of P-N Junctions Formed by Implantation of Beryllium in GaAs.sub.1-x P.sub.x ", Sov. Phys. Semicond. 21(10); Oct. 1987, pp. 1152-1153.
Sakurai, "Optical Semiconductor Devices Operating in the 1 .mu.m Wavelength Region," Fujitsu Sci. Tech. J., 21, 1, pp. 19-30 (Mar. 1985).
Rimington Julie J.
Robertson Michael J.
Rodgers Paul M.
BT&D Technologies Ltd.
Mintel William
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