IL35039A - Electroluminescent device - Google Patents

Description

s-.v BLECTBOUJMINESCENT DEVICE ■■I j' Intel Corporation The present invention relates to electroluminescent 1 I devices and more particularly, to solid state devices which emit significant quantities of light at low power level and at room temperature. The invention further relates to processes of generating light in such devices and to processes of manufacturing the devices.

Solid state electroluminescent devices have many potential applications. For example, these devices can be embodied in electro nic circuits to provide an indication of the logic or function of the circuitry. Furthermore, currently a substantial effort to develop' devices which could be incorporated in an array to provide a digital display of complex images is in progress. This array could serve as an output interf ce- etween a computer and a human being. Gaseous lig t**emitting devices that are available are operated at high voltages and require special and costly transistors and circuits to power them. The cost per numeral at the present state of the art renders the construction and operation of an instru ment expensive* j! Much of the recent effort has been directed to generation of light in solid state semiconductor-type of devices. Electrons \ and holes are generated within the device and recombine to release a quantum of energy in the form of a photon having a wavelength within the visible range. The eye is most efficient when viewing blue or green light having a wavelength of less than about 5600 Angstroms. Present devices emitting light at these wavelengths can be operated only at very low temperatures.

However, the devices or materials that have reasonable efficiencies at room temperature generally emit radiation of wave* lengths longer than 6500 Angstroms. The efficiency of the "hurr.an eye is only about 10% when viewing light having a wavelength of - ' Angstroms, the efficiency of the eye is very low.

If devices emitting light at room temperature in th< in which the eye is between 50 and 100% efficient, could be processed so as to be compatible with standard transistor voltages and integrated circuit parameters, operating efficiency and rllia-bility of the display would be improved. Furthermore, the ability I t to emit light in this region would permit color variation of the ! light in the visible region from red to yellow to green to blue to provide multicolor displays. An enormous quantity of information can be conveyed by means of color.

Therefore, an object of this invention is to produce at room temperature and at a low power level light over the range ι of-the visible region at which the eye is most efficient* 1 A further object of the invention1 is to provide new and improved electroluminescent devices which generate blue, and green light at room temperatures.

Another object of the invention is the generation of light at low power levels in crystalline wide band gap semiconductor materials .

A still further object of the invention is to provide hole injection at low voltage and low power levels into a wide band gap, high luminescent efficiency. semiconductors such as zinc sulfide.

These and other objects and many attendant advantages of the invention will become readily apparent as the description n proceeds .

In accordance with the invention an electroluminescent device having high luminescent e##**Hahfty m tem er tu e 1 comprises a body of a low resistivity, erystalline semiconductor of a first conductivity type having a wide band gap. The crystalline body is of high purity but contains a controlled doping of deep luminescent recombination centers having an ionization energy high out of the center at room temperature is small. for injecting minority carr ers into the body. On applicatio of a low forward bias voltage of below 10 volts and preferably below 5 volts to the first and second regions , both types of carriers are injected into the body and recombined at the centers to release energy in the form of radiated visible light at wavelengths at which the human eye is very efficient. In fact, devices have been operated at applied voltages below the band gap of zinc sulfide. ' The novel features of the invention are set forth with i" . particularity in the appended claims. The invention will best ibe understood from the following description when read in conjunctiOn with the accompanying drawings .

Figure 1 is a schematic energy diagram of a typical wide band gap luminescent material; Figure 2 is a schematic energy diagram illustrating the energy barriers of metal-zinc sulfide interfaces; Figure 3 is a schematic view of a zinc sulfide luminescent device according to the invention; Figure is a further energy diagram of the hole injecting contact region and metal electrode interface before applying bias; Figure 5 is another energy diagram of the interface of Figure t shown under forward bias ; Figure 6 is a set of current-voltage curves for a device and »et i«ai«e sulfide interface barrier according to the invention; and Figure 7 is a set of capacitance-voltage curves for a device and metal-zinc sulfide interface according to the invention.

Photons having energies of approximately 1.8 eV are required for visible radiation in the red, photons having energies of about 2.3 are re uired for reen emission and of about 2.8 eV for blue light output and the maximum attainable photon energy of the light emitted is directly dependent on the band gap width of the compound forming the body of the device.

Referring now to Figure 1 the energy diagram of a typical wide band gap, electroluminescent semiconductor is illustrated. The i band gap is measured as the difference between the energy Ec of an electron 10 in the conduction band 12 and the energy ∑v of a hole 14 in the Valence band 16. In a wide band gap material, direct recombination of the two free carrier is highly unlikely as there are many other mechanisms which non-radiatively dissipate the energy of free carriers.

No appreciable number of states exist in the region tween the conduction and valence bands; this region being known as the forbidden gap. However defect or impurity states known as centers can be introduced into the forbidden gap by deliberately treating or doping the semi-conductor. Centers may at a shallow or deep energetic level.

Luminescent centers are capable of capturing and trapping a free carrier until an oppositely charged carrier arrives , re qm-bines with the trapped carrier in a manner favoring a visible I radiative transition. Radiative processes are rendered more probable by introducing the luminescent centers at a deep level. Then the trapped electron or hole is not as likely to be thermally excited out of the center 20 at room temperature, without radiative recombination. The energy of the center should be greater than about 0.S eV from the nearest band edge and is preferably no more than about 1.5 eV from the band §§i§i The center 20 is usually a hole trap and he preferred transition is usually considered to involve trapping of a hole 1¼ in a luminescent recombination center 20 near the valence band edge, the subsequent entry of an electron 10 from the conduction bination of the carriers and emission of a quantum of radiation having energy, hy essentially equal to the band gap energy less the trap energy. Therefore, the band gap of the compound forming the body of the device must be at least about 2.3 eV in order to provide transitions sufficiently energetic to emit visible radiation and is preferably at least 3.0 eV to produce radiation over the wavelength range where the eye is the most efficient.

Compounds formed from a Group II metal and a Group VI anion having minimum band gaps in this range are suitable materials for use in the invention. Zinc sulfide is the most preferred material since it has a band gap of about 3.6 eV permitting attain-ment of a wide variety of color emissions. Color variation may be affected by introducing deeper or shallower recombination centers to affect the energy of the transition or by forming a body of mixed semi-conductor materials* For example, cadmium sulfide has a band gap of 2.U eV. By combining cadmium sulfide and zinc sulfide, a mixed crystal can be formed having an average band gap of abjout 3.0 eV.

High luminescent efficiency zinc sulfide is formed ss t discussed by introducing a localized level capable of radiative recombination of free charge carriers. Metal impurities such asj the r- · ^- ls , copper, silver or gold have been recognized to'' lurainescently activate zinc sulfide when present alone or preferably in combination with coactivators such as halogens, e.g., chloJine, . bromine or iodine, or a Group III metal, for example, gallium,; indium nd aluminum* ' There is also evidence that activator or co-activator impurities can form complex or compensated states with each other or may form a localized level by self-compensation with a lattice defect such as a sulfur or a zinc vacancy. This mech > is believed responsible for the self-activated blue emission observed leading to the self-activated blue emission as well as making it difficult to produce highly conducting n-type material.

As another example of compensation, silver though usually considered an acceptor, has been identified as a complex donor center consisting of the acceptor associated with a doubly ionized sulfLr anion vacancy and responsible for the orange silver emission. i The low-resistivity material comprises a high-purity crystalline wide band gap material doped with sufficient levels of donor or acceptor atoms to provide a resistivity in a range compat¬ The above discussion has dealt with the internal bulk parameters and mechanisms which favor light emission by the recombination of free charge carriers. However, the major difficulty in development of room temperature solid state electroluminescent devices in wide band gap materials such as zinc sulfide which are capable of emitting light in the green or blue regions of the at low bias voltage into the bulk of the crystal.

Experimentally, visible radiation has been generated in zinc sulfide by reverse bias impact ionization of a solid to which electrodes _¾re attached and by applying an alternating voltage to a cell consisting of a thin layer and a dielectric binder between two planar electrodes . The lat!ter efficient emission of long wavelength light in the red region has been obtained. However, for the wide band gap materials, this approach is neither thermodynamically nor electrically compatible with the properties of the material as will be described.

It is very difficult to render the wide band gap materials &@fh ii»ah Formation of a conductin layer of opposite type of conductivity in a wide band gap material such as n-type zinc sulfide electrodes usually formed of metal which are applied to the surfaces of the body of material. An energy barrier is created at the njetal ¾emi-conductor interface inhibiting the flow of carriers. Even if a p-n junction could be formed, it probably would not be recognized since the device would behave essentially as an insulator.

In fac*, the necessity of forming low voltage contacts is the cause of a major part of the difficulty in forming sucdes3ful electroluminescent devices in wide. band gap materials.

At the moment of forming an interface between a metal and a semiconductor, without bias, a barrier, Φ, develops and the Fermi level of the metal is at a distance from the band edge such that neither electrons nor holes can be introduced into the semiconductor. This barrier is analogous to the work function between a vacuum and a metal interface. The barrier energies exhibited by zinc sulfide - metal interfaces have been found to be a function of, the electronegativity of the metal employed. The barrier energies i of vario^^etal contacts on n-type zinc sulfide were investigated and reported by Aven and Mead at pp. 8-10 of Vol. 7, No. 1-of Ί|1· I W Referring now to Figure 2, it is seen that interfaces between electropositive metals electroded to zinc sulfide exhibit ' i a barrier of about 1 to l.S eV from the conduction band edge and the most electro? egative metals still exhibit a barrier of about l.S eV from the valence band edge. With such high energy barrier energies, thermionic current is very small and the applied voltage would drop across the metal-semiconductor interface and would be much larger than the voltage appearing across the active region of the device.

Of the two carriers necessary for luminescent action, introduction of electrons into a body of zinc sulfide the body of zinc sulfide underlying an electropositive metal contact electrode above about 10·"' cm""3 and preferably above 10^3 cm""3 the| contact exhibits ohmic characteristics and electrons are introduced i, into the device under low voltage forward bias. However, as d s- j cussed, it is very difficult to render zinc sulfide material highly conducting p-type. Therefore, it is not possible to narrow the width of the depletion layer underlying a hole injection contact | ί electrode by introducing a very high population of holes. Furthermore, a metal does not exist with a sufficient electronegativity to lower the Fermi level to near the valence band edge. Neither of these techniques are utilized for hole injection in accordance with the present invention.

Hole injection is accomplished herein by specially treating the region of the zinc sulfide body underlying the contact electrode. This region is treated to introduce at least, lO17 cm"3 of states that generate under a low forward bias voltage a net negative charge in a very thin layer. The layer is preferably no o more than 1,000 A in thickness and the net negative charge density is preferably at least 101δ cm"3.

The hole injection process is enhanced enormously by pro-viding an intermediate hole trapping state in the thin layer having a deep energy level below the Fermi level of the contact electrode. If the energy of the state or center is approximately halfway between the metal Fermi level and the valence band, then the holes u can be injected directly into this state or into the valence band of the zinc sulfide. If the luminescent centers are hole traps^ having this energy state they may function as the intermediate ' state or a separate hole trap may be introduced at this level.

For example, the complexed aluminum-anion vacancy recombination center may be utilized to enhance hole injection or an additional Referring now to Figure 3, an electroluminescent devipe is fabricated from an n-type, low-rosiotivity zinc sulfide crystal of high luminescent efficiency. The crystal has a central eg on 100 of low resistivity. An electron injecting, ohmic contact region ί 102 is formed on one region of the surface of the crystal and a i hole injection layer 104 is formed as a very thin layer on another region of the surface of the crystal. A contact electrode dot 106 of a very electronegative metal is formed on the region 104 ana a contact electrode dot 108 of a relatively electropositive metal is formed on the' region 102. if The electrode 106 is formed of a metal having an electronegativity as measured on the Pauling scale of no less than 1.8 eV such as gold, platinum, palladium, silver or copper. The electrode 108 is formed of a relatively electropositive metal having an elect-ronegativity of less than 1.8 as measured on the Pauling scale such as indium, aluminum or magnesium.

Conductors 110 are connected to the electrodes 106 and 108 and to a potential source 112 through a switch 1¾4. When the switch 114 is closed, a positive bias is applied to layer 104 and a negative bias is applied to region 102. Electrons are injected into region 102 and thence flow to region 100 and holes flow into layer 104. The electrons enter layer 104 from region 100 and are trapped © build up a net negative space charge of at least 1017 3 * Gm. within he thin layer which raises the hole intermediate state to a level near the metal Fermi level, for example within 0.2S eV of the Fermi level and preferably above the metal Fermi level. Electrons combine with the injected holes at the recombina-tion centers in sufficient numbers to emit radiation visible under ordinary illumination at room temperature. , f The hole injecting process according to the invention or intermediate hole trapping state toward the metal Fermi level. A 1.5 eV voltage difference between the valence band and Fermi level requires a charge of at least lO^2 e/cm2 or a field of SjlO5 to 106 volts per centimeter. If the layer thickness is below X micron the voltage drop is acceptable, However, the positive voltage on electrode 106 tends to pull the electrons entering the layer 104 from the bulk of the body into the electrode 106. A barrier must be created to retard the electron flow through layer 104 so that the electrons will not leak past the layer 104 and enter the electrode 106. This is accomplished by providing a high density of electron traps thajt retard lowering the barrier to electron flow. A density of electron traps above about 1017 cm"3 is sufficient to terminate the field in a distance such that the holes that enter the layer 104 radiative-ly recombine with electrons. The electron trapping states ar| preferably donor states which have a high capture cross-section for electrons and a small capture cross-section for holes to eliminate competition for the injected holes.

Referring now to Figure 4, the energy diagram for a! ! i zinc sulfide electroluminescent device according to the invention ! ---is illustrated. The diagram is for the zero-bias condition. A contact electrode 200 such as gold exhibits a barrier Φ of about 2.0 eV and provides a difference of about 1.6 eV from the valence band 202 to the metal Fermi level 206. A surface region of the device has been treated to form a layer having a thickness below 1000 A containing a hole trapping state e.g. silver at a level 204 intermediate the metal Fermi level 206 and the valence band 17 —3 202. The layer further contains at least 10 cm " empty states that Hit UR4*P ι*9βν&ΰβ £'fH*« M W® itt the* Idye*1: Referring now to Figure 5, under forward bias, the net negative charge in the layer raises the level 204 of the inter Holes 208 are injected into the intermediate state level 2 O or directly into the valence band 202. Stated in another sense, electrons leave the valence band 202 or the states 20U directly and enter the metal of the contact electrode. The net negative charge in the layer also raises the conduction band 210 to maintain a barrier 211 having a height and a width of 2x. The energy j barrier W prevents the electrons 214 from flowing out of the layer. The electrons 2m from the conduction band combine with the holes 208 at luminescent centers to emit a photon, p. The radiative transition is at an energy sufficient to radiate visible light at room temperature. I The invention will now become better understood by reference to the following examples. It is to be understood that thJ examples are presented only for purposes of illustration and ha numerous substitutions, alterations and modifications may readily be made by those skilled in the art without departing from the scope of the purity crystal zinc sulfide doped with a sufficient level of donor atoms to provide a resistivity in a range compatible with the desired 1 t voltage operating characteristics, that is, the resistivity should be about 1 to 100 ohm-cra so that at applied voltages of below 15 volts and at current levels of 1 to 100 milliamps there is a very small voltage drop across the body of material. For zinc sulfide, a suitable donor material is a group III metal, such as aluminum, which is substitutional^ introduced at a level of about 100 ppm into the crystal material and -treated to provide a free carrier 17 ' concentration of approximately 10 electrons per cubic centimeter. The body may be in the form of a grown crystal or a film deposited as an acceptor and binds the extra electron present on the aluminum atom. Instead of the electron enterin the conduction band, i : is trapped in the center. The presence of zinc vacancies is further evidenced by very bright emission of blue light when the crystal is irradiated with ultra'-violet photons. Therefore, the first prcess-ing s\tep in the construction of the device is related to lowering I-the resistivity of the crystal body according to the following | procedure.

EXAMPLE I ί ' A 50 mil thick slice of Eagle Pitcher Crystal No. D686, a zinc sulfide crystal having an aluminum doping level of 100 parts per million corresponding to about 10^9 aluminum atoms per cubic centimeter, was treated at high temperature in an environment containing zinc atoms. The environment may be either a zinc con- i taining liquid or a zinc containing vapor. It is preferred to carry out the treatment in liquid because the large body of liquid zinc can also sequester impurities from the crystal as it introduces e*et§§ sifts β¾Θίη§ % mM~ H%% m wien* the QlioG of oryotal was treated in a body of liquid zinc at QQ0°C for 20 minutes the resistivity decreased from 105 ohm-cm to less than 1 ohm-cm and the net concentration of donors was raised to about 1017 cm"3. However, it is apparent that zinc vacancies Remain since the carrier concentration is not as high as the aluminum doping level 19 cm 3. ·■ of 10 ELECTRON INJECTING ELECTRODE The ohmic electron injecting contact is formed by intro-ducing a net donor density of at least 10 18 cm—3 , preferably of at aluminum atom or other donor atom which contributes an electron to the crystal body. The processing should be conducted without the simultaneous introduction of vacancies or acceptor impurities.

The processing may be by various techniques such as the procedure^j^orted by Avcn and Mead in Volume 7, No. 1 Applied Physics Letters. This technique relies on the combination of vef^ powerful chemical getter agents and a chemically etched zinc sulfide According to the latter procedure an lectro injecting contact is formed on the surface of the zinc sulfide} body by applying to the surface a group II metal or alloy thereof in the presence of a source of a donor precursor and heating the region to above the melting temperature of the metal or alloy. The donor precursor is preferably a group Illb metal such as aluminum galliumor indium or a halogen such as CI, Br, and I. The donor precursor must be present in the surface region in a density of at least 10^ cm"^ before treatment or maybe substitutional^ introduced into the rich surface region during the treatment by being I present on the surface alloyed with the group II metal. A typical o w EXAMPLE II A slice of low resistivity n-type zinc sulfide crystal was mechanically cleaved frontj the material prepared in accordance with Example I. A surface region of the slice was mechanicall etched in HC1 at 50°C for 5 minutes. The etched surface was then scrubbed with an indium-mercury amalgam to set the surface. A preformed slug of a slightly cadmium-rich indium-cadmium alloy was pressed onto the surface and the slice was heated on a platinum strip heater for 1 minute at 350-H50°C. The heating was conducted in an argon atmosphere. The slice was cooled to room temperature. The contact resistance of the electrode was measured and was found to exhibit a resistance of about 1 ohm-cm^. The slug was in firm metalurgical contact with the surface. , ,7 _ HOLE INJECTING CONTACT LAYER j ' A thin semi-insulating hole injecting layer is p o id'^ in one procedure by introducing a very high density of a Group !l metal into the layer for a short distance in a manner forming aj very high density of states that generate a net negative charge ' of at least 10^ cm""3 under forward bias to raise the level of [the hole injection states and to retard the lowering of the barrierj to electron flow. The Group I metal such as silver can also form an acceptor which is an efficient hole trap havin an energy of about 0,3 aV from the valence band and is therefore in an excellent position to aid hole injection. I Replacement of zinc atoms in the lattice with large densities of silver requires special processing to render the surface very zinc-poor so that the silver atoms may be introduced into the lattice in a very high density and to a very short depth. One processing technique according to the invention effects substitut-ion of silver into the zinc lattice by use of a diffusion technique.

The surface of the zinc sulfide bod is coated with a silver or copper or mixtures thereof such as silver, or copper metal or compounds thereof, suitably silver or copper sulfide.

The slice of crystal is then placed on a platinum strip heater and heated rapidly to a temperature of about 650°C to 950°C suitably for less than a minute and is then cooled rapidly. ∑jxcess coating material is removed mechanically or with appropriate et-chants . - Ji Rapid cooling of the slice permits immediate transfixing of the material in a state in which the surface relaxes to retain the substitutional^ introduced silver atoms at a density of ΙΟ^·9 cm"3 in a very thin surface layer. The silver is introduced in a manner to form a high-resistivity » semi-insulating, thin layer which contains the states creating the net negative charge at low applied bias. The hole injecting contact processing is completed by evaporating a dot of a highly electronegative contact metal such as gold onto the layer. A typical example follows.

EXAMPLE III Ca) A film of silver, approximately lOoX thick wasi evaporated onto a cleaved surface region of a low resistivity slice of zinc sulfide having a high luminescent efficiency prepared in accordance with Example I. The slice was placed in an argon / atmosphere containing a small amount of sulfur vapor arid heated at abouT ^00oC on a platinum strip heater for 10 seconds and allowed to cool* Six mil diameter gold dots were evaporated citto w i flpli e nh# mi* βθϋ$&βι· arnm electron injecting contact was then applied to the back surface of the slice according to the procedure of Example II. (b) A film of silver, approximately 1000 A thick was evaporated onto a oleaved surface of a low resistivity, slice of zinc sulfide prepared in accordance with Example I. The coated f cool. The silver remaining on the surface was removed by etching the surface with concentrated nitric acid. The processing was completed according to the procedure described in Example III (a). (c) A thin layer of silver sulfide about 100A thick was applied to a cleaved surface of a low resistivity slice of zinc sulfide prepared in accordance with Example I. The coated slice was then placed in an inert atmosphere and heated at about 700°C on a platinum strip heater 'for ten seconds and allowed to cool.

The processing was completed according to the procedure described in Example Ill(a).

The device of Example III (a) when drawing 10 miliamps of current at 2.5 volts under forward bias radiated clear blue light which was clearly visible under normal room illumination. j( Referring now to Figure 6, the current-voltage characteristic for the device of Example III is illustrated in comparisdh" j to the characteristics of the gold barrier alone. The gold barrier curve shows a linear increase of current with voltage from about 1.2 to 1.6 volts. The curve of the device of Example III illustrates a first region in which the current increases with bias up to a point at about 2 volts at which the onset of hole injections occurs and the slope of the curve increases dramatically.

The slope of the curve follows the relationship: 1 - 120 AT2 e -w/JcT 2 where: A is the area in cm of the active device T is the temperature; and W is the height of the barrier to electron flow.

The active region of the device of Example III has a diameter of about 6 mils and the measured barrier is therefore about O.'.S eV.

Referring now to Figure 7, the voltage-capacitance characteristics of the device of Example III (a) are again comp red n-type conductivity zinc sulfide. The width of the depletion layer I represented by the parabolic shaped barrier W is 2x. The value of this width is determined from: —12 ε is the permitivity of ZnS (about 10 ) where C is the capacitance in microfarads, yfj and A is the active area of the device.

From Figure 7 the width of the layer at an applied voltage o of 2 volts is about 22 OA.

The net negative charge density, N, is then determined from the relationship: „ ! ' at Φ= V a about 2 volts 1.6 x IP"19 x N x 10"12 - u , n ,R 2 x 10"12 ~~ is therefore approximately 2.S x 10 8/cm~3 over a layer o thickness of about 200A.

The efficiency of the device can be further iaproved ' y increasing the number or electron capture cross section of the \ electron trapping states in the thin region under the hole injecting contact dot to increase the height of the barrier. An electron trapping state that can be introduced to maintain the barrier to electron flow is silver paired with a sulfur vacancy. This state is a deep donor and when empty it is positive but when full is charge neutral. Therefore, it has little affinity for holes when full or empty.

The material may be further treated to introduce into the hole injecting layer a luminescent center such as copper having an energy level different from that of the intermediate state. A typical example follows: the six mil gold dot was applied to the layer, a layer of copper sulfide about 10OA thick was deposited on top of the silver doped layer and the crystal body heated on the platinum strip heater in an inert atmosphere at about 650°C for a few seconds. The gold dots were then applied. When the device was connected to a battery it radiated bright blue-green light at about the same power level as reported for the device of Example III. (b) The thickness of the copper sulfide layer was in- o creased to 1000A and the procedure of Example IV (a) repeated with the additional step of removing the copper sulfide remaining on the surface with an NH^OH etch before applying the gold dots. The device radiated bright blue-green light at about the same power level as reported in Example IIKa). o (c) A 100OA thick layer of copper sulfide was directly j applied to a cleaved surface of a slice of crystal treated in accordance with Example I. The coated slice was placed on a platinum strip heater and heated in an inert atmosphere at about 500°C 1 \ for a few seconds. The copper sulfide remaining on the surface was removed with NH^OH. The device radiated bright green light at about the same power level as reported in Example IIKa).

It is to be understood that the foregoing relates only to preferred embodiments of the invention and that numerous substitutions, modifications and alterations are all permissible without departing from the spirit and scope of the invention as defined in the following claims.

Claims (9)

35039/2 ' Claims

1. A direct current forward biased* low voltage electroluminescent diode device comprising: a body of n-type semiconductor material having a resistivity below 100 ohm cm, a band gap of at least 3·0 eV, and at least 17 -3 10 cm donor atoms throughout said body; ohmic electron injecting contact means formed on one of said two opposite surfaces; hole injecting contact means formed on. the other of said two opposite surfaces; and a built-in electron barrier layer in the region of said body immediately underlying the region of said hole injecting . contact means, said barrier layer having a thickness of less than 1,000 17 -3 angstroms, containing a concentration of at least 10 cm donor atoms, and a similar order of a concentration of acceptor atoms.

2. A diode device as recited in Claim 1 wherein said body of n-type semiconductor material is n-type zinc sulfide material.

3. A diode device as recited in Claim 2 wherein metal atoms are present in said barrier layer.

4. A diode device as recited in Claim 3 wherein said metal atoms are silver metal atoms.

5. A diode device as recited in Claim 2, wherein the donor atoms in said n-type zinc sulfide body are aluminum atoms.

6. A direct current forward biased, low voltage electroluminescent, diode device comprising a body of n-type semiconductor material having two opposite surface, and having a resistivity less than 35039/2 100 ohm cm, a band gap of at least 3.0 eV, and a free 15 carrier concentration of at least 10 electrons per cubic centimeter, an electron injecting contact means formed on one of said two opposite surfaces for injecting electrons into said body, a hole injecting contact means formed on the other of said two opposite surfaces for injecting holes into said body, and a built-in electron trapping barrier layer-means formed in the material of said body immediately adjacent said hole injecting contact means for reducing electron flow into said hole injecting contact means, said electron trapping layer including metal atoms and having a thickness of less than 1000 angstroms.

7. A diode device as recited in Claim 6 wherein said n-type semiconductor material is zinc sulfide and said metal atoms in said electron trapping layer are silver metal atoms.

8. A diode device as recited in Claim 6 wherein said electron trapping layer means contains a concentration of at least 17 —3 10 cm donor atoms and a concentration of acceptor atoms on the same order.

9. A diode device as recited in Claim 6 wherein said electron injecting contact means includes a metal having an electronegativity as measured on the Pauling Scale of less than 21 - Θ-- 35039/2 1.8 eV, and the hole injecting contact means includes a metal having an electronegativity on the Pauling Scale of no less than 1.8 eV. A direct current forward biased, low voltage electroluminescent diode device, substantially as hereinbefore described with reference to the accompanying drawings. For the Applicants i>r. Yitzhak Hess