ELECTRONIC DEVICE HAVING LIGHT TRANSMISSION SYSTEM

Abstract: Within an electronic device having a plurality of circuit parts (7...9) - such as a three-dimensional device - a light transmission system which transfers signals between the circuit parts by the use of light is provided, which is formed of a light emitting source (10, 11) which emits light having a desired wavelength, a photoelectric conversion portion (14, 15) which absorbs the light and converts it into an electric signal, and a light travelling part (12, 13) which conveys the light emitted from the light emitting source (10, 11) to the photoelectric conversion portion (14, 15). Each of the light emitting source (10,11), light traveling path (12, 13) and photoelectric conversion portion (14, 15) is formed of a superlattice structure in which a plurality of materials of unequal energy gaps are accumulated.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method of optical signal transportation within an integrated circuit (IC), an electronic device, etc. More particularly, it provides a light transmission system for an electronic device in which a signal is transmitted to a desired layer even when layers having different functions are accumulated and integrated as in a three-dimensional integrated circuit. In the operation of an electronic device, internal information signals have important roles. Especially in the electronic device of a structure wherein parts of different functions are connected and integrated in parallel or in accumulation, the mutual transfer of information is indispensable. To this end, a method of electrically connecting both the parts is adopted most commonly. This is described in 'Japan Journal of Applied Physics', Vol. 20, No. 9 (1981), pp. L623 - L626. However, in such a case where the functional parts are arranged separately with an insulator or where they form a multilayer structure vertically, an electrical interconnection has been impossible or a complicated process has been required. To overcome the disadvantages of electrical connections, GB-A-2 113 912 uses optical connections between different circuit paths. An optical waveguide structure is also disclosed in US-A-3 785 717. In these known waveguide structures, a waveguid is only capable of transmitting a single optical signal between a pair consisting of a light source and an optical detector. The waveguides necessary for the transmission of a plurality of signals therefore require a large space in the device. SUMMARY OF THE INVENTION It is the object of the present invention to provide an electronic device in which the space required by optical connections is reduced. This object is solved by the device characterized in claim 1. The invention uses a waveguide for simultaneous transmission of a plurality of light signals. Each of the light emitting source, the light traveling path and the photo-electric conversion part is formed of a superlattice structure in which a plurality of materials of unequal energy gaps are layered. An example for the fabrication of a superlattice structure is described in US-A-4 205 329. Now, the superlattice structure will be described. In general, the transmittances of materials are inherent in the respective materials, and in case of mixing two sorts of materials, a transmittance peculiar to the mixture is uniquely determined. In recent years, however, it has been revealed that when semiconductors of unequal transmittances or energy gaps are regularly placed one over another as very thin layers, an energy gap unequal to those of both the semiconductors can be formed. Fig. 1 is a diagram for explaining such a phenomenon. In the figure, B indicates a material which is greater in the energy gap than a material A, and the combination of the materials consists of GaAs as A and GaAlAs as B by way of example. When the thickness of each layer of such an accumulated structure decreases down to the order of 10 nm, the quantum effect appears, and new energy levels are formed in the part A of narrow energy as shown in Fig. 1. In the figure, numeral 1 designates the conduction band of the material A, numeral 2 the conduction band of the material B, numeral 3 the valence band of the material A, and numeral 4 the valence band of the material B. Numerals 5 and 6 indicate the conduction band and valence band of the levels formed anew, respectively. The positions of the new levels are expressed as: Here, L denotes the thickness of the layer A, h Planck's constant, and m the effective mass of a carrier. n indicates the ordering number of levels, and a case of n = 1 is illustrated in Fig. 1. With increase in n , the energy level shifts to a higher energy side. When the value of an electron is selected as m , the position corresponds to the conduction band side, and when the value of a hole is selected, the position corresponds to the valence band side. Under such circumstances, the optical properties of this system are chiefly predominated by a new energy gap E c defined by the conduction band 5 and the valence band 6. By way of example, the wavelength of emission based on carrier injection becomes a value corresponding to E c , and also the absorbance is the greatest for the wavelength corresponding to E c . It is accordingly possible, for example, to change the optical properties by changing an energy confinement width L, as described in 'Japan Journal of Applied Physics', Vol. 20, No. 9, 1981, pp. L623 - L626. The above is the explanation of the superlattice structure. The inventors have devised a light transmission system which can guide light selectively by skillfully utilizing the phenomenon and which is able to be installed in an electronic device such as integrated circuit. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view of a structure in which materials A and B of unequal energy gaps are layered, as well as a diagram for explaining energy bands in the structure. Fig. 2 is a general view of an electronic device in which the light transmission system of the present invention is built. Fig. 3 is a diagram for explaining the energy bands of the light transmission system of the present invention. DESCRIPTION OF PREFERRED EMBODIMENTS Now, an embodiment of the present invention will be described. Fig. 2 shows an electronic device which consists of circuits 7, 8 and 9 having three sorts of different functions, for example, a sensing part, a data processing part and a memory part. In the figure, portions 10 thru 15 constitute the light transmission system of the present invention, in which the portions 10 and 11 are optical signal generation portions, those 12 and 13 are light conveyance portions, and those 14 and 15 are photoelectric conversion portions. Fig. 3 is an energy band diagram of this light transmission system. As shown in Fig. 3, the optical signal generation portions 10 and 11 belonging to the circuit 7 are of layered structures each of which is made of materials of unequal energy gaps. The material of narrower gap corresponding to the material A in Fig. 1 is thicker in the portion 10 than in the portion 11. Consequently, as understood from Eq. (1), an energy gap which is formed in the region 10 becomes narrower than in the region 11, and the emission wavelength of the region 10 is long, while that of the region 11 is short. Meanwhile, the light traveling paths 12 and 13 are formed of layered structures each of which is similarly made of different materials. The thicknesses of layers are selected so that the path 12 may have a wider energy gap than the path 13. As a concrete example, GaAs (energy gap: 1.43 eV) is chosen as the material A and AlAs (energy gap: 2.16 eV) as the material B, and the layered structures are formed by molecular beam epitaxy or MOCVD (CVD based on the plasma decomposition of organic metals). The width of the GaAs layer is set to 10 nm in each of the regions 10 and 13, and to 3.5 nm in each of the regions 11 and 12. The width of the AlAs is set to 10 nm in any of the regions. At this time, the effective energy gap becomes approximately 1.5 eV in each of the regions 10 and 13 and 1.65 eV in each of the regions 11 and 12. Now, when carriers are injected into the region 10 in the circuit 7, light is emitted at a wavelength of about 820 nm. This light is transmitted through the regions 11 and 12, and is absorbed by the region 13 to generate photocarriers. The photocarriers are derived as an electric signal through the region 15. On the other hand, when carriers are injected into the region 11, light is emitted at a wavelength of about 750 nm, and this light is absorbed by the region 12. In case of this wavelength, the light is absorbed also by the region 13. However, photocarriers can be prevented from flowing into the region 13 by such a method as increasing the number of wells in the region 12 or setting a low threshold level of detection for the region 13. In this way, when the optical signal generator 10 is driven, the signal is sent to the circuit 9, and when the optical signal generator 11 is driven, a signal is sent to the circuit 8. By flashing the optical signal generators, digital signals can also be transmitted. Although the present embodiment has illustrated the method in which both the light emission portions and the light conveyance portions are formed of the same combination of the materials, a similar effect is attained even when different materials are used for the individual light emission portions or when light emitting diodes or the likes are employed. In addition, although the signals are transmitted in one direction in the example, signals can also be emitted from the second layer 8 to both the first layer 7 and the third layer 9. The characterizing feature of the present invention consists in that a multilayer structure wherein the width of a well layer broadens gradually with increase in a distance from a light emitting portion generating a signal is formed, thereby to endow the absorbance with a wavelength dependency. It is also possible that, when one set of materials can no longer meet a requirement, a light conveyance path is formed from an intermediate position by the use of a different combination. Of course, the number of circuits is not restricted to three, the circuits may well be arranged laterally, and a light transmitting insulator film may well be interposed between the circuits. Although, in the present embodiment, the effect has been explained as to the compound semiconductor materials, other kind of materials may be selected according to the purpose of an electric device. As described above, according to the present invention, signals can be transmitted to desired places within an electronic device, so that information can be transferred very smoothly between different circuit components in the electronic device in which circuits having different functions are combined.

Claims: An electronic device having a light signal transmission system for interconnecting a plurality of circuit parts (7,8,9), comprising: a light emitting source (10, 11), a plurality of photo-electric detection portions (14, 15), and a light travelling path (12, 13) having its ends connected to said light emitting source (10, 11) and to said photo-electric detection portions (14, 15) respectively, wherein, the light emitting source (10, 11) emits a plurality of light signals of various wavelengths, each photo-electric detection portion (14, 15), detects a predetermined light signal among the light signals of various wavelengths, the light emitting source (10, 11) is formed of a superlattice structure, and the light travelling path (12, 13) is formed of a superlattice structure in which a plurality of materials with unequal energy band gap are layered and in which the thickness of the layers with narrow energy band gap increases with increase in the distance from the light emitting source (10, 11). The device of claim 1, wherein the materials are GaAs and AlAs.

Inventors: HIRAI, TADAAKI, ISHIOKA, SACHIO, MATSUBARA, HIROKAZU, SHIMOMOTO, YASUHARU, TAKASAKI, YUKIO, TSUJI, KAZUTAKA
Applicants: HITACHI, LTD.
Application year: 1986
Main classification: H01L31
IPC codes: H01L27, G02B6, H04B10, H01L31