GB2136202A - Photodiode - Google Patents

Abstract

Selective detection of light signals of two different wavelength bands is accomplished by two separate pn junctions (13, 14) which are disposed on two different sides of a substrate (1) and are optically in series. The pn junction (13) lying in front in the light path responds to light of, e.g., 850 nm, but not to light in the range from 950 nm to 1,650 nm. The substrate acts as a filter and passes the light of longer wavelength while blocking the 850-nm light, 50 that the second pn junction (14) is only reached by light in the range from 950 nm to 1,650 nm. The device may be used in optical data communication. <IMAGE>

Description

SPECIFICATION Photodiode This invention relates to light-sensitive devices for detecting light signals of two different wavelength bands.

Such dual wavelength devices can be used in optical data communication if two independent data streams are to be transmitted over one data channel (e.g. optical fibre) in different wavelength bands and then are to be processed separately. In such systems, the two data streams are commonly separated by means of optical devices, such as prisms, colour filters or interference filters. A wavelength-demultiplexing light-sensitive device is described on pages 388 and 389 of the June 21, 1979 issue of the journal "Electronics Letters". There, a number of layers are deposited on a substrate using mesa technology to form two pn junctions which are optically in series, and a filter interposed between these pn junctions. The first pn junction responds only to photons of higher energy, i.e. to light of shorter wavelength, than the second.The intermediate filter is a semiconductor layer whose absorption edge lies between the two wavelengths to be detected separately.

An arrangement of this kind can detect light at different wavelengths without additional means, but it is difficult to build and not sufficiently selective.

The object of the invention is to provide a structure which is easier to manufacture and ensures higher selectivity.

According to the invention there is provided a photodiode structure for selectively detecting light signals of a first shorter wavelength band and a second longer wavelength band, the structure including a semiconductor substrate substantially transparent to said second wavelength band and provided with first and second pn junctions, wherein said first junction is responsive to said first band, wherein said second junction is responsive to said second band, and wherein said second junction is so disposed that, in use, light signals searching that junction are constrained to pass through a region of the substrate.

A photon falling on a pn junction forms an electron-hole pair if the energy of the photon exceeds a certain minimum, which is the case up to an upper cutoff wavelength. In the case of light of longer wavelength, the energy of the photons is too low for this. Accordingly, a pn junction is transparent to longer wavelengths. Due to the thinness of the pn junction, however, a few photons of sufficient energy also reach the region lying behind the pn junction. If they reach the second pn junction, they will generate electronhole pairs there and, thus, cannot be distinguished from photons of less energy, i.e. from light of longer wavelength. The proportion of those photons of higher energy reaching the second pn junction depends on the thickness of the intermediate layer acting as a filter.A layer of maximum thickness and with optimum filtering action is obtained if the two pn junctions are disposed on opposite sides of a substrate which is transparent to one of the two wavelength bands, and opaque to the other.

The structure may be used in the construction of an optical receiver device for use in an optical communications system.

An embodiment of the invention will now be described with reference to the accompanying drawing, which is a cross-sectional view of the dual wavelength light-sensitive device. The plane of section is normal to the light entrance aperture.

The device may exhibit rotational symmetry.

The support or substrate 1 for the device comprises n-type InP material. An n-type GalnAs Layer 2 is deposited on one side of the substrate 1. In this layer 2 and on the other side of the substrate, a p-type GalnAs region 4 and a p-type InP region 3, respectively, are formed by diffusion of acceptors (e.g. Cd for InP). Contact is made to the p-type region 3 at the front by a contact 5 made of an Au-Zn alloy, and the p-type region 4 at the back carries an area contact 6, also made of an Au-Zn alloy. Contact to the two contiguous ntype layers is made at the back by a coating 7 of an Au-Sn alloy. An insulating ring 8 of SIN, covers the interface between the p-type region 4 and the n-type layer 2 from external influences and insulates the area contact 6 from the contactmaking coating 7.Similar protection is provided by a cover 9, also of SiN,. A window 10 of siO2 or SiN, is designed to reduce reflection losses and protect the region 3. The whole arrangement may be encapsulated in the known manner (not shown).

A light beam 11 with a wavelength of about 850 nm enters through the window 10 and generates electron-hole pairs in the pn junction 13 produced between the substrate 1 and the region 3. The resulting voltage appears between the ringshaped contact 5 and the contact-making coating 7.

The light beam 1 1 is not completely absorbed in the space-charge region formed by the pn junction 13. A small portion penetrates further into the substrate 1 but is highly attenuated there, so that only a very small portion of the light beam 1 1 will reach the pn junction 14 formed between the layer 2 and the region 4. A second light beam 12, which geometrically coincides with the light beam 11, has a wavelength of about 950 nm-1 ,650 nm. To this beam, InP is transparent; the first pn junction 13 does not respond to light of this wavelength. The light beam 12 thus reaches the second pn junction 14 virtually unattenuated and generates electron-hole pairs there. The voltage thus produced can be picked off between the area contact 6 and the contactmaking coating 7.

It will be clear that the arrangement is not limited to the semiconductor materials described above. Thus, dependent on the two wavelengths to be detected, materials of appropriate band gap will be employed.

Claims (8)

1. A photodiode structure for selectively detecting light signals of a first shorter wavelength band and a second longer wavelength band, the structure including a semiconductor substrate substantially transparent to said second wavelength band and provided with first and second pn junctions, wherein said first junction is responsive to said first band, wherein said second junction is responsive to said second band, and wherein said second junction is so disposed that, in use, light signals searching that junction are constrained to pass through a region of the substrate.

2. A photodiode structure as claimed in claim 1, wherein the light path to said second junction includes a p-type layer of a first semiconductor material, an n-type layer of this first semiconductor material, an n-type layer of a second semiconductor material, and a p-type layer of the second semiconductor material.

3. A photodiode structure as claimed in claim 2, wherein the substrate forms one of the two n-type layers.

4. A photodiode structure as claimed in claim 2 or 3, wherein the first and second semiconductor materials are InP and Gaxinl~xAsyPl~y, respectively, wherein said first wavelength band is shorter than 920 nm, and wherein said second wavelength band lies between 950 nm and 1,650 nm.

5. A photodiode structure as claimed in any one of claims 2 to 4, wherein the two p-type layers are regions formed by diffusing acceptors into the respective semiconductor material.

6. A photodiode structure substantially as described herein with reference to the accompanying drawing.

7. An optical receiver device incorporating a photodiode structure as claimed in any one of claims 1 to 6.

8. An optical communications system incorporating a plurality of receiver devices as claimed in claim 7.