Patent ID: 12245459

DESCRIPTION OF EMBODIMENTS

Embodiment 1

Example 1

Throughout the present specification, the Roman numerals denoting groups of elements follow the defunct CAS numbering system, and the Arabic numerals denoting groups of elements follow the current IUPAC numbering system.

FIG.1is a schematic cross-sectional view of a light-emitting device1in accordance with the present embodiment.

Referring toFIG.1, the light-emitting device1includes a light-emitting diode2, a protection diode3, and a partition wall4separating the light-emitting diode2and the protection diode3. The light-emitting device1is typically disposed on an array substrate (not shown) and electrically connected to a TFT (thin film transistor) formed on the array substrate.

Light-Emitting Diode

The light-emitting diode2includes a light-emitting layer25that may contain quantum dots (semiconductor nanoparticles) or an organic light-emitting material in the present example of the invention.FIG.1represents an example where the light-emitting layer25contains QDs (quantum dots).

The quantum dot (QD) has a “core/shell structure” (not shown) including a core and a shell surrounding the core.

The light-emitting diode2has a structure including a stack of layers. Each layer may be formed by sputtering a material doped with a desirable impurity or by applying a colloidal solution of such a material prepared in nanoparticle form.

Specifically, as shown inFIG.1, the light-emitting diode2includes an anode electrode (positive electrode)21, a first hole transport layer22, a second hole transport layer23, a third hole transport layer24, the light-emitting layer25, an electron transport layer26, and a cathode electrode (negative electrode)27arranged in this order when viewed from the bottom layer toward the top layer inFIG.1.

The anode electrode21and the cathode electrode27contain a conductive material and are electrically connected to a first hole transport layer22and the electron transport layer26respectively.

The first hole transport layer22, the second hole transport layer23, and the third hole transport layer24transport holes from the anode electrode21to the light-emitting layer25. These first, second, and third hole transport layers may be collectively referred to as the hole transport layers when there is no need to distinguish between the layers. Each hole transport layer is, for example, an inorganic semiconductor layer.

The hole transport layers of the light-emitting diode2are a n+semiconductor layer (first hole transport layer22), a p+semiconductor layer (second hole transport layer23), and a p-type semiconductor layer (third hole transport layer24) arranged in this order when viewed from the anode electrode21as shown inFIG.1in the present example of the invention.

The n+semiconductor layer22contains either a Group 13 element and the same Group II-VI semiconductor as the shell of the quantum dot or a Group 13 element and a Group II-VI semiconductor that contains a Group II element that, in the periodic table, is placed in a period below the Group II element contained in the shell.

The p+semiconductor layer23contains either a Group 15 element and the same Group II-VI semiconductor as the shell of the quantum dot or a Group 15 element and a Group II-VI semiconductor that contains a Group II element that, in the periodic table, is placed in a period below the Group II element contained in the shell.

This Group 15 element has a concentration that is equal to, or of the same order as, the concentration of the Group 13 element in the n+semiconductor layer22.

The p-type semiconductor layer24contains either a Group 15 element and the same Group II-VI semiconductor as the shell of the quantum dot or a Group 15 element and a Group II-VI semiconductor that contains a Group II element that, in the periodic table, is placed in a period below the Group II element contained in the shell. The p-type semiconductor layer24has a lower Group 15 element concentration than the p+semiconductor layer23.

The electron transport layer26transports electrons from the cathode electrode27to the light-emitting layer25. The electron transport layer26may have a function of disrupting the transport of holes.

Protection Diode

The protection diode3enables the light-emitting diode2to discharge the excess electric charge accumulated therein to protect the light-emitting diode2in the present example of the invention. The protection diode3has reverse characteristics. The term, “reverse characteristics,” in this context indicates that the protection diode3has such a polarity that it allows a large electric current flow therethrough at low voltage when the p-n junction thereof undergoes a reverse breakdown and returns to zero flow condition when the voltage decreases.

The protection diode3has a structure including a stack of layers. Specifically, as shown inFIG.1, the protection diode3includes an anode electrode31, a first hole transport layer32, a second hole transport layer33, and a cathode electrode34arranged in this order when viewed from the bottom layer toward the top layer inFIG.1.

The anode electrode31of the protection diode3is formed integral to the anode electrode21of the light-emitting diode2as shown inFIG.1in the present example of the invention. The first hole transport layer32and the second hole transport layer33of the protection diode3are made of the same materials as the first hole transport layer22and the second hole transport layer23of the light-emitting diode2respectively.

Specifically, similarly to the n+semiconductor layer22of the light-emitting diode2, the n+semiconductor layer32of the protection diode3contains either a Group 13 element and the same Group II-VI semiconductor as the shell of the quantum dot or a Group 13 element and a Group II-VI semiconductor that contains a Group II element that, in the periodic table, is placed in a period below the Group II element contained in the shell.

The p+semiconductor layer33contains either a Group 15 element and the same Group II-VI semiconductor as the shell of the quantum dot or a Group 15 element and a Group II-VI semiconductor that contains a Group II element that, in the periodic table, is placed in a period below the Group II element contained in the shell.

This Group 15 element has a concentration that is equal to, or of the same order as, the concentration of the Group 13 element in the n+semiconductor layer32.

The cathode electrode34of the protection diode3and the cathode electrode27of the light-emitting diode2are separately formed as shown inFIG.1.

In other words, the first hole transport layer22and the second hole transport layer23of the light-emitting diode2are only separated by the partition wall4, which is in contact with the anode electrode31(21), and provided with electrodes, one on the top and the other on the bottom, to form the protection diode3in the present example of the invention.

This structure in accordance with the present example of the invention only requires an additional separation step when compared to the process of manufacturing the light-emitting diode2, to manufacture the protection diode3. The protection diode3is electrically separated from the light-emitting diode2, but located adjacent to, and connected in parallel with, the light-emitting diode2, which obviates the need for post-processing such as wire bonding. A specific manufacturing method will be described later in detail.

The structure can achieve efficient discharging of electric charge because the electrical interface between the same material as at least a portion of the hole transport layer and the electrodes has a low contact resistance. The structure also requires a fewer steps to manufacture the light-emitting device, hence allowing for low cost manufacturing of the light-emitting device, because the steps of manufacturing the light-emitting diode and the steps of manufacturing the protection diode can be performed in parallel.

FIG.2is a band structure diagram for the protection diode3in the light-emitting device1. Dotted lines EfinFIG.2indicate the Fermi levels of the first hole transport layer32and the second hole transport layer33(two simple substances) of the protection diode3.

FIGS.3to6are layered structure and band structure diagrams for the light-emitting diode2in the light-emitting device1.

Specifically,FIG.3is a layered structure diagram for the light-emitting diode2in the light-emitting device1. The light-emitting diode2has a layered structure that is, as shown inFIG.3, equivalent to the light-emitting device1shown inFIG.1minus the protection diode3and the partition wall4. Detailed description thereof is therefore omitted.

FIG.4is a band diagram for a hole transport layer that has an isolated three-layered structure shown inFIG.3. Dotted lines EfinFIG.4indicate the Fermi levels of the first hole transport layer22, the second hole transport layer23, and the third hole transport layer24(three simple substances) of the protection diode3.

FIG.5is a band diagram for the light-emitting device1in the absence of electric field.FIG.6is a band diagram for region A shown inFIG.5when the light-emitting device1is being driven. The electron transport layer26has a known structure and is therefore omitted inFIG.6.

FIG.7is a band diagram for the protection diode3in the light-emitting device1in the absence of electric field.FIG.7shows that the cathode electrode34of the protection diode3has a lower work function than the protection diode3because the cathode electrode34has the same structure as the cathode electrode27of the light-emitting diode2, and the junction between the cathode electrode34and the p+semiconductor layer33is a Schottky junction with a high barrier.

When this band structure is placed under forward bias, an additional voltage is needed that flattens the Schottky barrier between the p+semiconductor layer33and the cathode electrode34, so that the junction between the n+semiconductor layer32and the p+semiconductor layer33can undergo a reverse breakdown.

This voltage is equal to the difference between the work function of the cathode electrode34and the Fermi level Efof the p+semiconductor layer33. The operating voltage of the protection diode3is therefore higher than the drive voltage of the light-emitting diode2.

The present example of the invention causes a large electric current to flow through the protection diode3when the protection diode3is placed under an applied voltage that is greater than or equal to the reverse breakdown voltage thereof. This particular mechanism enables the light-emitting diode2to discharge the excess electric charge accumulated therein through the protection diode3, thereby protecting the light-emitting diode2.

Example 2

Process of Manufacturing Light-Emitting Device

FIGS.8to15are diagrams representing a process of manufacturing a light-emitting device in accordance with the present example of the invention. The light-emitting device in accordance with the present example of the invention has the same structure as the light-emitting device1in accordance with Example 1, except that the light-emitting diode2and the protection diode3share a single cathode electrode in the former.

The following will describe in detail a process of manufacturing the light-emitting device in accordance with the present example of the invention.

The anode electrode21is formed on an array substrate (not shown) by a common electrode forming method such as vapor deposition or sputtering as shown inFIG.8. The anode electrode21is preferably reflective to exploit the light output of the light-emitting diode2, but may be transmissive.

Next, the partition wall4is formed that separates the light-emitting diode2and the protection diode3. A hole transport layer doped with a high concentration of impurity is then formed in such a manner as to include the n+semiconductor layer22and the p+semiconductor layer23stacked in this order on the anode electrode21.

The hole transport layer may be formed by sputtering or if nanoparticles are used, by applying a colloidal solution. Table 1 below shows possible combinations of materials, impurities, and impurity concentrations for the n+semiconductor layer22and the p+semiconductor layer23.

TABLE 1Base Material(Isolated)(Isolated)Energy LevelEnergy LevelN-typeP-typeof Upper Endof Lower EndDopingDopingof Valenceof ConductionAmountAmountMaterialBand [eV]Band [eV]ElementAdded [cm−3]ElementAdded [cm−3]ZnS−5.2−3.2Vacant Al, In,Not less thanN, Li, F, Cl, INot less thanGa, O1E191E+21ZnSe−5.5−2.7Vacant Al, In,Not less thanN, Li, F, Cl, INot less thanGa, O1E191E+21CdS−6.2−3.7Vacant Al, In,Not less thanN, Li, F, Cl, INot less thanGa, O1E191E+21ZnO−7.2−4Vacant Al, In,Not less thanN, Li, F, Cl, INot less thanGa, O1E191E+21GaN−3.2−6.7Si, ONot less thanMg, Zn, BeNot less than1E191E+22AIN−1.3−7.5Si, ONot less thanMg, Zn, BeNot less than1E191E+22

Next, as shown inFIG.9, a part of the face of the stack up to the p+semiconductor layer23(to the left of the partition wall4inFIG.9) is covered with a resist or bank material (mask35) by photolithography. This covered region will be a part of the protection diode3when the protection diode3is completely manufactured.

Next, as shown inFIG.10, the p-type semiconductor layer24(36), the light-emitting layer25(37), and the electron transport layer26(38) are stacked in this order by a common method. The light-emitting layer25is made of quantum dots in the present example of the invention and may be formed by applying a colloidal solution or inkjet printing in this example.

The electron transport layer26may be made of an inorganic or organic material used in known QLEDs or OLEDs such as ZnO, GZO, ZAO, IZO, IGZO, TiO2, WO3, or MoO3. Table 2 below shows the electrical properties of ZnO-based materials.

TABLE 2μ [cm2/MaterialCBM [eV]VBM [eV]Eg[eV]Ef[eV]n [cm−3]V · sec]ρ [Ω · cm]IZO−4.4 to −4.8−7.2 to −7.92.8 to 3.1Up to CBM101860 to 9010−4GZO−4.2 to −4.6−7.3 to −7.73.1Up to CBM1020to 102116 to 252.8 × 10−4to8 × 10−4AZO−4.1 to −4.4−7.6 to −8.23.5 to 3.8Up to CBM1019to 102040 to 6010−4ZnO−4−7.23.2Up to CBM1018to 102030 to 4010−4

Next, as shown inFIG.11, the mask35is removed that covers the region destined to be the protection diode3.

Next, as shown inFIG.12, a new mask28(39) is prepared that has an opening that matches the shape of the cathode electrode27(34) for the light-emitting diode2and the protection diode3. The mask28(39) may be made of the same material and by the same method as the mask35shown inFIG.9.

No mask is provided over a region where there will be formed an edge cover40(4) that separates the light-emitting diode2and the protection diode3.

Next, as shown inFIG.13, an insulating material is applied or vapor deposited that will form the edge cover40. The edge cover40may be made of the same insulating material as the partition wall4.

Next, as shown inFIG.14, the mask28(39) is lifted off or peeled, which leaves the edge cover40in the light-emitting diode2and the protection diode3behind.

Finally, as shown inFIG.15, the cathode electrode27(34) is formed across the surface of the product in process shown inFIG.14, which simultaneously completes the manufacture of the light-emitting diode2and the protection diode3adjacent to the light-emitting diode2.

The light-emitting device1in accordance with the present embodiment can be manufactured in this manner. The light-emitting device1can achieve efficient discharging of electric charge because the electrical interface between the same material as at least a portion of the hole transport layer and the electrodes has a low contact resistance. The light-emitting device1also requires a fewer steps to manufacture because the steps of manufacturing the light-emitting diode2and the steps of manufacturing the protection diode3can be performed in parallel.

Location of Protection Diode

FIG.16is a schematic plan view of a structure of the light-emitting device1. Referring toFIG.16, the protection diode3may be located anywhere relative to the light-emitting diode2as these diodes are viewed on the plane of the paper. As another alternative, the protection diode3may be provided surrounding the light-emitting diode2. The light-emitting device1preferably corresponds to a single pixel in a display device100(detailed later) in the present example of the invention. This particular structure facilitates the provision of the protection diode3on a pixel-by-pixel basis.

Example 3

Process of Manufacturing Light-Emitting Device

FIGS.17to23are diagrams representing a process of manufacturing a light-emitting device in accordance with the present example of the invention. The light-emitting device in accordance with the present example of the invention has the same structure as the light-emitting device in accordance with Example 2, except that the edge cover40is located on the top face of the integrated cathode electrode27(34) in the former.

The steps shown inFIGS.17to20are the same as the steps shown inFIGS.8to11, and description thereof is therefore omitted.

Next, as shown inFIG.21, the mask35is remove. The cathode electrode27(34) is then formed across the surface. In this example, the top face of the protection diode3and the top face of the light-emitting diode2have a difference in height that is no more than a few tens of nanometers. The cathode electrode27(34) is therefore naturally formed so as to electrically connect the protection diode3and the light-emitting diode2without allowing for such a height difference that can lead to a disconnection.

Next, as shown inFIGS.22and23, a bank material for the edge cover40is applied. The bank material is subsequently removed from the light output face of the light-emitting diode2by common ashing, which completes the manufacture of the device structure.

An alternative to the present step is to form, on the cathode electrode27(34), a mask that only covers the light output region of the light-emitting diode2, apply a bank material, and remove the mask by lift-off.

Equivalent Circuit of Light-Emitting Device

FIG.24is an equivalent circuit diagram for the light-emitting device1. Referring toFIG.24, the light-emitting device1is equivalent to a circuit in which the light-emitting diode2is connected in parallel with the protection diode3. The power supply is connected to the light-emitting diode2and the protection diode3in such a manner that the light-emitting diode2is operated under forward bias and the protection diode3is operated under reverse bias. In this structure, the protection diode3does not conduct, therefore causing no leak current, when the light-emitting diode2is operated under normal driving conditions.

I-V Characteristics of Light-Emitting Device

FIG.25is a diagram representing the I-V characteristics of the protection diode3in the light-emitting device1. Referring toFIG.25, the protection diode3exhibits the same rectification characteristics as a common diode under forward bias, but has a reverse breakdown voltage that is as low as approximately 3 volts under reverse bias. The reverse breakdown voltage varies depending on the thickness of the p-n junction of the diode. The reverse breakdown voltage tends to decrease with a decrease in the thickness of the depletion layer of the junction. The p-type semiconductor layer and the n-type semiconductor layer need to be doped to a high concentration to allow for a thinned-down depletion layer.

The protection diode3allows a large electric current to flow therethrough once the applied voltage reaches the reverse breakdown voltage without requiring further increases in the voltage as shown in the I-V characteristics diagram. This large electric current discharges excess electric charge.

Example 4

Process of Manufacturing Light-Emitting Device

FIGS.26to30are diagrams representing a process of manufacturing a light-emitting device in accordance with the present example of the invention. The light-emitting device in accordance with the present example of the invention has the same structure as the light-emitting devices in accordance with the previous examples of the invention, except that the edge cover40is provided toward the anode electrode in the former.

The anode electrode21(31) (e.g., Ti) is formed on an array substrate (not shown) by a common electrode forming method such as vapor deposition or sputtering as shown inFIG.26. The anode electrode21(31) is preferably reflective to exploit the light output of the light-emitting diode2, but may be transmissive.

Next, the partition wall4and the edge cover40are formed. Specifically, as shown inFIG.27, the partition wall4, which separates the light-emitting diode2and the protection diode3, and the edge cover40are formed by subjecting a polyimide material applied up to a thickness of 1 to 1.5 μm to such photolithography as to leave the partition wall4and the edge cover40behind.

Next, a hole transport layer is formed, and a region where there will be the protection diode3is covered. Specifically, as shown inFIG.28, a hole transport layer doped with a high concentration of impurity is formed in such a manner as to include the n+semiconductor layer22, the p+semiconductor layer23, and the p-type semiconductor layer24in this order on the anode electrode21(31).

The hole transport layer may be formed by sputtering or if nanoparticles are used, by applying a colloidal solution. Table 3 below shows possible combinations of materials, impurities, and impurity concentrations for the n+semiconductor layer22and the p+semiconductor layer23.

TABLE 3Base Material(Isolated)(Isolated)Energy LevelEnergy LevelN-typeP-typeof Upper Endof Lower EndDopingDopingof Valenceof ConductionAmountAmountMaterialBand [eV]Band [eV]ElementAdded [cm−3]ElementAdded [cm−3]ZnS−5.2−3.2Vacant Al, In,Not less thanN, Li, F, Cl, INot less thanGa, O1E191E+21ZnSe−5.5−2.7Vacant Al, In,Not less thanN, Li, F, Cl, INot less thanGa, O1E191E+21CdS−6.2−3.7Vacant Al, In,Not less thanN, Li, F, Cl, INot less thanGa, O1E191E+21ZnO−7.2−4Vacant Al, In,Not less thanN, Li, F, Cl, INot less thanGa, O1E191E+21GaN−3.2−6.7Si, ONot less thanMg, Zn, BeNot less than1E191E+22AIN−1.3−7.5Si, ONot less thanMg, Zn, BeNot less than1E191E+22

Next, a part of the face of the stack up to the p+semiconductor layer23(to the left of the partition wall4inFIG.28) is covered with a resist or bank material (mask35) by photolithography. This covered region will be the protection diode3.

Next, the light-emitting layer25and the electron transport layer26are formed. Specifically, as shown inFIG.29, the light-emitting layer25and the electron transport layer26are stacked in this order by a common method. The light-emitting layer25is made of quantum dots in the present example of the invention and may be formed by applying a colloidal solution or inkjet printing in this example.

The electron transport layer26may be made of an inorganic or organic material used in known QLEDs or OLEDs such as ZnO, GZO, ZAO, IZO, IGZO, TiO2, WO3, or MoO3. Table 4 below shows the electrical properties of ZnO-based materials.

TABLE 4Egμ [cm2/MaterialCBM [eV]VBM [eV][eV]Ef[eV]n [cm−3]V · sec]ρ [Ω · cm]MaterialIZO−4.4 to −4.8−7.2 to −7.92.8 toUp to CBM101860 to 9010−4IZO3.1GZO−4.2 to −4.6−7.3 to −7.73.1Up to CBM1020to 102116 to 252.8 × 10−4toGZO8 × 10−4AZO−4.1 to −4.4−7.6 to −8.23.5 toUp to CBM1019to 102040 to 6010−4AZO3.8ZnO−4−7.23.2Up to CBM1018to 102030 to 4010−4ZnO

Next, the mask35is removed, and the cathode electrode27is formed. Specifically, as shown inFIG.30, the mask35is removed that covers the region destined to be the protection diode3. The cathode electrode27is formed across the surface of the product in process shown inFIG.29, which simultaneously completes the manufacture of the light-emitting diode2and the protection diode3adjacent to the light-emitting diode2.

Finally, the cathode electrode27of the light-emitting diode2and the protection diode3is connected to pixel wiring, which completes the manufacture of the display device100.

As described here, the display device100includes a plurality of light-emitting devices1. Each light-emitting device1includes a light-emitting diode2and a protection diode3. The light-emitting device1includes: an insular, second anode electrode21(31) that is common to the light-emitting diode2and the protection diode3; and a second cathode electrode27that is common to the plurality of light-emitting devices1. The light-emitting diode2includes: a light-emitting layer25between the second anode electrode21(31) and the second cathode electrode27; and a first hole transport layer22and a second hole transport layer23in this order between the second anode electrode21(31) and the light-emitting layer25. The protection diode3includes a n-type semiconductor layer32and a p-type semiconductor layer33in this order between a first anode electrode21(31) and a first cathode electrode27.

As an example, the protection diode3in accordance with the present embodiment is a Zener diode that is operated by taking advantage of the reverse breakdown thereof. The protection diode3therefore rapidly conducts once the applied voltage reaches the reverse breakdown voltage, which effectively protects the light-emitting diode2from excess voltage. The protection diode3in this condition automatically stops conducting once the applied voltage falls below the reverse breakdown voltage due to decreasing excess charge. This series of protection procedures is non-destructive and does not at all affect the operation of the light-emitting diode2and the protection diode3. Hence, the protection diode3can repeatedly and effectively protect the light-emitting diode2.

Example 5

As an alternative process of manufacturing the light-emitting device1, for example, only either one of the protection diode3and the light-emitting diode2is first independently manufactured, a gap needed to separate the entire light-emitting device1and that diode is then covered with a resist or a metal mask, and the other diode is stacked thereon.

Following the completion of the manufacture of the light-emitting device, the mask covering one of the diodes is removed, and the cathode electrode of the light-emitting device is connected to pixel wiring, which completes the manufacture of the display device100.

This method requires no common portion between the circuit for the protection diode3and the layered structure of the light-emitting diode2and for this reason, allows the protection diode3to be readily and inexpensively added to the known light-emitting diode2that is manufactured by a typical method.

Embodiment 2

FIG.31is a schematic cross-sectional view of a light-emitting device1ain accordance with the present embodiment. The light-emitting device1ain accordance with the present embodiment has the same structure as the light-emitting device1in accordance with Embodiment 1, except that the hole transport layer and the electron transport layer are transposed in the former. A detailed description is given below.

A light-emitting diode2ahas a similar structure including a stack of layers. Each layer can be formed by sputtering a material doped with a desirable impurity or by applying a colloidal solution of such a material prepared in nanoparticle form.

Specifically, as shown inFIG.31, the light-emitting diode2aincludes a cathode electrode27, a first electron transport layer51, a second electron transport layer52, a third electron transport layer53, a light-emitting layer25, a hole transport layer54, and an anode electrode21arranged in this order when viewed from the bottom layer toward the top layer inFIG.31.

A protection diode3aincludes a cathode electrode34(integrated with the cathode electrode27of the light-emitting diode2a), a first electron transport layer61, a second electron transport layer62, and a cathode electrode31(provided separately from the anode electrode21of the light-emitting diode2a) arranged in this order when viewed from the bottom layer toward the top layer inFIG.1.

A display device (not shown) in accordance with the present embodiment includes a plurality of light-emitting devices1ain this manner. Each light-emitting device1aincludes the light-emitting diode2aand the protection diode3a. The light-emitting device1aincludes: an insular, third cathode electrode27(34) that is common to the light-emitting diode2aand the protection diode3a; and a third anode electrode21(31) that is common to the plurality of light-emitting devices1a. The light-emitting diode2aincludes: the light-emitting layer25between the third cathode electrode27and the third anode electrode21; and the first electron transport layer51and the second electron transport layer52in this order between the third cathode electrode27and the light-emitting layer25. The protection diode3aincludes a p-type semiconductor layer61and a n-type semiconductor layer62in this order between a first cathode electrode34and a first anode electrode31.

The electron transport layer and the hole transport layer in accordance with the present embodiment may be made of the same material as the electron transport layer and the hole transport layer in accordance with Embodiment 1 respectively. Detailed description is omitted.

The present embodiment can achieve the same advantages as Embodiment 1.

Layered Metal Oxide Structure of Protection Diode

The protection diodes3and3a, in Embodiments 1 and 2 above, include a first metal oxide layer32and a second metal oxide layer33on the first metal oxide layer32. The first metal oxide layer32may have a different oxygen concentration from the second metal oxide layer33.

Specifically, the first metal oxide layer32is in contact with the first anode electrode31of the protection diodes3and3a. The second metal oxide layer33is in contact with the first cathode electrode34of the protection diodes3and3a. The second metal oxide layer33has a higher oxygen concentration than the first metal oxide layer32.

If the metal oxide has properties like those of the semiconductor, the metal oxide forms a n-type semiconductor layer when the metal oxide is oxygen deficient relative to its stoichiometric composition and a p-type semiconductor layer when the metal oxide is oxygen excessive relative to its stoichiometric composition. Therefore, a Zener diode may be prepared, for example, from metal oxides by connecting the oxygen-excessive, first metal oxide layer to the cathode electrode34of the protection diodes3and3aand the oxygen-deficient, second metal oxide layer to the anode electrode31of the protection diodes3and3arespectively.

In the formation of such a Zener diode, the p-n junction between the first metal oxide layer and the second metal oxide layer can be reduced in thickness by specifying the second metal oxide layer to have a higher oxygen concentration than the first metal oxide layer and the first metal oxide layer to have a lower oxygen concentration than the stoichiometric composition. These structure and specification enable suitable protection of the light-emitting devices1and1afrom excess voltage.

Exemplary Compositions of Semiconductor Layers of Protection Diode

Both the protection diodes3and3ainclude a n-type semiconductor layer and a p-type semiconductor layer in Embodiments 1 and 2 described above. The following will describe exemplary compositions of the n-type semiconductor layer and the p-type semiconductor layer in detail.

Exemplary Composition 1

The protection diodes3and3ainclude the n-type semiconductor layers32and62respectively that may contain a Group III-V compound doped with a Group IV or Group VI impurity.

Exemplary Composition 2

Alternatively, the protection diodes3and3ainclude the n-type semiconductor layers32and62respectively that may contain a Group II-VI compound doped with a Group III impurity.

Exemplary Composition 3

As another alternative, the protection diodes3and3ainclude the n-type semiconductor layers32and62respectively that may contain: a first Group IV compound doped with a Group III impurity; and a second Group IV compound that differs from the first Group IV compound.

Exemplary Composition 4

The protection diodes3and3ainclude the p-type semiconductor layers33and61respectively that may contain a Group III-V compound doped with a Group II impurity.

Exemplary Composition 5

Alternatively, the protection diodes3and3ainclude the p-type semiconductor layers33and61respectively that may contain a Group II-VI compound doped with either any one of Group II, Group V, Group VI, and Group VII impurities or a combination of any of these Group II, V, VI, and VII impurities.

Exemplary Composition 6

As another alternative, the protection diodes3and3ainclude the p-type semiconductor layers33and61respectively that may contain: a third Group IV compound doped with a Group V impurity; and a fourth Group IV compound that differs from the third Group IV compound.

The disclosure is not limited to the description of the embodiments above and may be altered within the scope of the claims. Embodiments based on a proper combination of technical means disclosed in different embodiments are encompassed in the technical scope of the disclosure. Furthermore, new technological features can be created by combining different technical means disclosed in the embodiments.