Semiconductor photocoupler

Abstract: A semiconductor photocoupler is composed of a light emitting element and a light receiving element. Wavelength of emitted light changes as a function of exciting current intensity of the light emitting element, and capacity of the light receiving element changes as a function of wavelength of receiving light and ceases the capacity change as the receiving light disappears. Signals are transmitted in current-light-capacity type transmission with memory action in the light receiving element.

Description: BACKGROUND OF THE INVENTION This invention relates to a semiconductor photocoupler. A semiconductor photocoupler is used for transmitting information between circuits electrically isolated from each other, and is usually called an optically coupled isolator. Optically coupled isolators on the market are of current-light-current type in information transmission, where an instantaneous magnitude of a current flowing in a phototransistor(or a photodiode) of the secondary circuit is proportional to that driving an LED(light emitting diode) of the primary circuit. The inventor of the present invention has disclosed another type of optically coupled isolator in his prior invention. The prior invention has been patented as a Japanese Patent No. 1,455,290 entitled "Semiconductor Photocoupler" and published as JP-A-59090967. In the prior invention, the type in information transmission is of current-light-capacity change, and the capacity (ie. capacitance) change ceases when the light received at a light receiving element of the secondary circuit disappears. In theory, the prior invention utilizes a deep impurity level in and around the neighborhood of the p-n junction of a light receiving element. Some impurities located deep in a semiconductor have a very large optical capture cross-section(σ n for electron and σ p for hole) for certain wavelengths. Some electrons (or holes) thus captured are seldom released by thermal excitation at room temperature. Therefore, such a trap center(capture cross-section) has an occupation rate of electrons(or holes) of a level dependent only on the wavelengths of the received light. When such a trap center is formed in a p-n junction, the junction capacity is dependent on the wavelength of the received light, and the junction maintains its capacity after the received light disappears. H. Kukimoto et al have disclosed in "Physical Review B vol.7 No.6 pp.2486∼2507" that oxygen doped in a p-n junction or in a p channel of a Zn, O doped GaP red light LED makes such a trap center. As for oxygen doped in GaP, the capture cross-section σ p for capturing valence band electron at the oxygen level begins at 1.4 eV of received light energy and increases as the received light energy increases to about 1.7×10 -16 cm 2 at 1.8eV(λ≒689nm), while the capture cross-section σ n for releasing electrons trapped at the oxygen level to conduction band begins at 0.8eV, increases as received light energy increases, reaches to a maximum of 3.4×10 -16 cm 2 at 1.2eV, and then decreases as received light energy increases. Therefore, this oxygen level will become neutral by received light of energy higher than 1.7eV because it captures electrons at the oxygen level, and will become positively charged by receiving light of energy level of about 1.2eV. Since the oxygen level is sufficiently deep(0.9eV from the conduction band), the trapped electrons are thermally released only in a small quantity. Therefore, the GaP red LED doped by Zn, O makes a light receiving element of a semiconductor photocoupler working on current-light-capacity in information transmission and having a memory action. In the prior invention, a semiconductor photocoupler is composed of a light receiving element of the GaP red LED doped by Zn, O, and two light emitting elements, one light emitting element being a GaP red LED(1.77eV), and the other light emitting element being a Si doped GaAs LED(1.29eV). The semiconductor photocoupler of the prior invention transmits a binary signal indicating which light emitting element of the two light emitting elements is energized as a binary capacity change in the light receiving element and maintains the capacity corresponding to the lastly received wavelength. This photocoupler can be used as a coupler of binary signals, but can not be used as a coupler of analog signals. As there is one-to-one correspondence between the current in the light emitting element and that in the light receiving element in a conventional optically coupled isolator, the isolator can be used as a coupler of analog signals. If the current-light-capacity type photocoupler is to be used as a coupler of analog signals, there must be one-to-one correspondence between the current in the light emitting element and the capacity of the light receiving element. SUMMARY OF THE INVENTION Therefore, a primary object of the preferred forms of the present invention is to obtain a semiconductor photocoupler of current-light-capacity type transmission having a one-to-one (ie. unambiguous) correspondence between the primary current and the secondary capacity. Accordingly, the present invention consists in a semiconductor photocoupler comprising a light emitting element for emitting light when excited by driving current, and a light receiving element for receiving light emitted by said light emitting element; characterised in that said light emitting element comprises means for varying the wavelength of the emitted light in response to the magnitude of said driving current, and said light receiving element comprises means for changing the capacitance thereof in response to the wavelength of said emitted light, said means ceasing to change the capacitance when said received light disappears. BRIEF DESCRIPTION OF THE DRAWINGS Further objects, features, and advantages of this invention will become apparent from a consideration of the following description, the appended claims, and the accompanying drawings in which the same numerals indicate the same or the corresponding parts. Figs. 1a-1d show an embodiment of this invention. Figs. 2a-2c show another embodiment of this invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1a, a light receiving element 1 and a light emitting element 2 are placed facing to each other. Both the light receiving element 1 and the light emitting element 2 have pairs of electrodes, but in Fig. 1a, only one electrode is shown for each element. As for the light receiving element 1 of Fig. 1a, a GaP red LED(pellet size of 0.5 mm□) doped by 2×10 17 cm -3 Zn and 5×10 16 cm -3 O in a p channel epitaxial layer is used, and as the light emitting element 2, an AlGaAs LED(pellet size of 0.4mm□) having a sloped ratio of crystal mixture is used. The light receiving element 1 and the light emitting element 2 are placed facing to each other as shown in Fig. 1a, sealed by a primary sealing of transparent resin, and then sealded by black resin covering the whole surface including the primary sealing to the surfaces shown by dotted lines in Fig.1a as a secondary sealing. Fig.1b shows a crosssectional view of the AlGaAs LED. This light emitting element 2 is fabricated by a process as described below. In a first process, Si doped p channel Al x Ga 1-x As epitaxial layer 3 is grown on an n channel(100) GaAs substrate by a conventional gradually cooled liquid phase epitaxy process. Then Te doped n channel GaAs epitaxial layer 4 is generated. The distribution of the ratio of crystal mixture of AlAs in the Al x Ga 1-x As epitaxial layer 3 is, as shown in Fig.1c, x=0.35 at the start of the growth(at the boundary between the GaAs substrate) and x=0 at the boundary between the GaAs layer 4. Then, the n channel GaAs substrate for the epitaxial wafer is eliminated, and a p side electrode 5 and an n side electrode 6 are formed. The AlGaAs LED thus fabricated emits an infra red light(λ≒930nm) as a Si doped GaAs LED when excited by a low intensity current in forward direction. But as the current intensity is increased, the injected electrons begin to contribute for light emission in a deep point of the p channel Al x Ga 1-x As epitaxial layer 3, resulting in a gradual increase of light emission at shorter wavelengths. When the current intensity is further increased, a component of a red light(λ=750∼690nm) is increased. For a combination of the light emitting element 2 shown in Fig.1b with a light receiving element 1, the relation between the forward current of the light emitting element 2 and the capacity change of the light receiving element 1 is shown in Fig.1d. The capacity change is measured at a frequency of 100kHz by a measuring equipment adapted to measure a fine capacity change by using capacitance bridge. The initial capacity when the driving current in the light emitting equipment is zero, is 120pF. As shown in Fig.1d, the capacity increment ΔC increases monotonously as the current in the light emitting element is increased, maintaining a one-to-one correspondence between the current in the primary circuit and the capacity in the secondary circuit. The characteristic shown in Fig.1d is thought to come from the following cause. That is, in a small current intensity range, components of infra red light of λ of about 930nm predominate and the oxygen level of the light receiving element 1 releases electrons at a larger rate to the conduction band than that of capturing electrons from the valence band at those wavelengths of the received light. As a result, only a small portion of the oxygen level is neutralized to make a very small capacity change. On the contrary, in a large current intensity range, components of red light of 750∼690nm wavelength range increase, resulting in a comparatively large amount of neutralization of the oxygen level to make a large capacity change. As explained in a photocoupler of this invention, it is apparent that the capacity change in the light receiving element 1 corresponds to the driving current in the light emitting element 2. And as is easily understood, the capacity of the light receiving element 1 is maintained after the received light disappears. When a new light is received, the capacity of light receiving element takes a value corresponding to the current intensity of the light emitting element 2 which emits the new light. Another embodiment of this invention is illustrated in Figs.2a∼2c. Fig.2a is a crosssectional view of a light emitting element used in this embodiment. The light receiving element used in this embodiment and the spacing of the light receiving element 1 and the light emitting element are the same as described in connection with Fig.1a. On an n channel GaAs substrate, a Si doped layer Al y Ga 1-y As is epitaxially and continually grown to form an n channel AlGaAs epitaxial layer 7 with a decreasing ratio of crystal mixture(maximum 0.45 and minimum 0) and a p channel AlGaAs epitaxial layer 8. Then, the GaAs substrate is eliminated and an n side electrode 9 and a p side electrode 10 are formed. Fig. 2b shows a distribution of the ratio of the crystal mixture of the epitaxial layers 7 and 8. In this embodiment shown by Figs. 2, contrary to the embodiment shown in Figs. 1, components of shorter wavelengths predominate in a small current intensity range, and components of longer wavelengths increase in a large current intensity range. Thus, as shown in Fig. 2c, the amount of the capacity change ΔC decreases with the increasing current intensity of the light emitting element. Still one-to-one correspondence between the current in the primary circuit and the capacity change in the secondary circuit is maintained. Heretofore, only preferred embodiments of this invention have been described, but it must be understood that this invention is not limited by the embodiments described. Any type of light emitting element 2 where the wavelength of the emitted light changes as a function of the driving current of the element, and any type of light receiving element where the capacity of the element changes as a function of the wavelength of the received light can be used in this invention.

Claims: A semiconductor photocoupler comprising a light emitting element (2) for emitting light when excited by driving current, and a light receiving element (1) for receiving the light emitted by said light emitting element; characterised in that said light emitting element comprises means for varying the wavelength of the emitted light in response to the magnitude of said driving current, and said light receiving element comprises means for changing the capacitance thereof in response to the wavelength of said emitted light, said means ceasing to change the capacitance when said received light disappears. A semiconductor photocoupler according to claim 1, wherein said light receiving element comprises a GaP red LED doped by Zn and O in a p-type epitaxial layer. A semiconductor photocoupler according to claim 1, wherein said light emitting element comprises a p-type Alx Ga1-x As epitaxial layer(3) having a sloped ratio of crystal mixture, and an n-type GaAs epitaxial layer(4) formed on said p-type Alx Ga1-x As epitaxial layer, said ratio of crystal mixture decreasing for Al as said p channel layer approaches the boundary between said n-type layer. A semiconductor photocoupler according to claim 3, wherein said light receiving element comprises a GaP red LED doped by Zn and O in a p-type epitaxial layer. A semiconductor photocoupler according to claim 1, wherein said light emitting element comprises an n-type AlGaAs epitaxial layer(7) and a p-type AlGaAs epitaxial layer(8) on said n-type layer, said n-type and said p-type layer having continuously sloped ratio of crystal mixture in which ratio of crystal mixture decreases for Al from said n-type layer to said p-type layer. A semiconductor photocoupler according to claim 5, wherein said light receiving element comprises a GaP red LED doped by Zn and O in a p-type epitaxial layer. A semiconductor photocoupler according to claim 1, wherein said light emitting element comprises a p-type Alx Ga1-x As epitaxial layer having a sloped ratio of crystal mixture, and an n-type epitaxial layer formed on said p-type Alx Ga1-x As epitaxial layer. A semiconductor photocoupler according to claim 1, wherein said light emitting element comprises a p-type epitaxial layer, and an n-type Alx Ga1-x As epitaxial layer having a sloped ratio of crystal mixture formed on said p-type epitaxial layer. A semiconductor photocoupler according to claim 1, wherein said light emitting element comprises a p-type epitaxial layer, and an n-type epitaxial layer having a sloped ratio of crystal mixture formed on said p-type epitaxial layer. A semiconductor photocoupler according to claim 1, wherein said light emitting element comprises a p-type epitaxial layer having a sloped ratio of crystal mixture, and an n-type epitaxial layer formed on said p-type epitaxial layer.

Inventors: KATO TETSURO, KATO, TETSURO, Kato, Tetsuro, c/o NEC Corporation
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1993
Main classification: H01L31
IPC codes: H01L31