Demultiplexing Photodiode Sensitive to Plural Wavelength Bands

1. Field of the Invention

The present invention relates to a light-sensitive device for separately detecting light of two different wavelength bands.

2. Description of the Prior Art

In optical data communication systems, there are times when two independent data streams are to be transmitted over one data channel (e.g., optical fiber) 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, color 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.

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 electron-hole 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 drawing is a cross-sectional view of a light-sensitive device constructed in accordance with the invention.

An embodiment of the invention will now be explained in more detail with reference to the accompanying drawing. The plane of section is normal to the light entrance aperture. The device may exhibit rotational symmetry.

The support of the light-sensitive device is a substrate 1 of n-type InP material. An n-type Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y with selected values of x and y layer 2 is deposited on one side of the substrate. In this layer 2 and on the other side of the substrate, a p-type Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y with selected values of x and y 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 n-type layers is made at the back by a coating 7 of an Au-Sn alloy. An insulating ring 8 of SiN.sub.x 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 contact-making coating 7. Similar protection is provided by a cover 9, also of SiN.sub.x. A window 10 of SiO.sub.2 or SiN.sub.x 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 can be picked off between the ring-shaped contact 5 and the contact-making coating 7.

The light beam 11 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 11 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 contact-making coating 7.