EP0959485A1 - Dispositif d'émission d'électrons à cathode froide - Google Patents

Dispositif d'émission d'électrons à cathode froide Download PDF

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Publication number
EP0959485A1
EP0959485A1 EP98870112A EP98870112A EP0959485A1 EP 0959485 A1 EP0959485 A1 EP 0959485A1 EP 98870112 A EP98870112 A EP 98870112A EP 98870112 A EP98870112 A EP 98870112A EP 0959485 A1 EP0959485 A1 EP 0959485A1
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EP
European Patent Office
Prior art keywords
semiconductor layer
type semiconductor
cold cathode
cathode electron
emitting device
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EP98870112A
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German (de)
English (en)
Inventor
Gerrit Verstraete
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Barco NV
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Barco NV
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Publication date
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Priority to EP98870112A priority Critical patent/EP0959485A1/fr
Publication of EP0959485A1 publication Critical patent/EP0959485A1/fr
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
    • H01J1/02—Main electrodes
    • H01J1/30—Cold cathodes, e.g. field-emissive cathode
    • H01J1/308—Semiconductor cathodes, e.g. cathodes with PN junction layers

Definitions

  • the present invention relates to cold cathode electron-emitting devices, in particular to technologies for the construction of cold cathode electron-emitting devices, which can emit electrons into a vacuum.
  • Cold cathode electron-emitting devices are used in applications such as flat panel displays.
  • Spindt Cathodes are based on field emission from tips of cone-shaped structures of a material with a low work function and are used in e.g. FEDs (Field Emission Displays).
  • FEDs Field Emission Displays
  • the anode voltage and control voltage needed for the emission of electrons are both relatively high (respectively above 100 volts and 20 volts).
  • SCE devices are disclosed in EP 0343645, US 5285129 and US 3916227. SCE devices have a low emission efficiency (typically less than 1% of the conducted electrons are injected into the vacuum) and need relatively high source voltages (above 10 volts).
  • Electron-emitting devices based on tunnelling of electrons through a thin insulating layer between two metals (MIM structure) or between a semiconductor and a metal (MIS structure) have been reported by Suzuki in SID 97 Digest and by K. Tahara et al. of Yokoo Lab., Tohoku Univ., Japan.
  • the voltage needed for the referred emission of electrons through tunnelling is relatively low (between 5 and 10 volts), but the efficiency of the emission is also low because the electrons have to travel through a layer of metal wherein the probability of inelastic collisions with free electrons in the metal is high, even if the layer of metal is only a few nanometers thick.
  • Electron-emission from a reverse biased Schottky junction has been disclosed in EP 0331373. Electrons are injected from a p-type semiconductor into a Schottky electrode laying on top by avalanche breakdown. The emission efficiency is low because the electrons must travel through the Schottky electrode.
  • US 2960659 discloses the emission of electrons, created by avalanche breakdown in a PN-junction, or, by injection from the base into the collector of a NPN transistor.
  • the electron emission efficiency is low because the electrons have to travel through a layer of n-type semiconductor material and through a metal layer.
  • US 3098168 discloses an electron-emitting device in which some electrons gain enough energy from an electric field in the bulk of a semiconductor substrate to bridge the electron affinity so that those electrons are injected immediately from the semiconductor into a vacuum. Most of the electrons are collected by a metal electrode structure on top of the semiconductor substrate, which results in a low electron emission efficiency.
  • All of the above mentioned cold cathode electron-emitting devices have a low electron emission efficiency and/or need a relatively high drive voltage, which can result in a high lateral spread of the velocity of the emitted electrons.
  • Some of the cold cathode electron-emitting devices need a strong external electric field to assist the emission of electrons.
  • the cold cathode electron-emitting devices are relatively easy to construct, in particular by means of thin film techniques. It is also an aim of the present invention to provide circuits for driving such cold cathode electron-emitting devices.
  • a cold cathode electron-emitting device which comprises at least a n-type semiconductor layer and a first metal electrode, forming a first Schottky junction.
  • this first Schottky junction is reverse biased, hot electrons are injected from the first metal electrode into the n-type semiconductor layer by tunnelling.
  • the hot electrons have such an energy and the n-type semiconductor layer is of such a material that a part of the hot electrons can flow through the n-type semiconductor layer and penetrate directly from the n-type semiconductor layer into a vacuum.
  • a cold cathode electron-emitting device which, when properly driven, presents a range of operating states, such that at least 5% of the electrons which have tunnelled through the first Schottky junction are emitted into the vacuum, the voltage over the cold cathode electron-emitting device being lower than 10 volts.
  • the material of the n-type semiconductor layer is of the II-VI type, such as BaO or ZnS.
  • circuits are provided for current driving one or more of the hereabove described cold cathode electron-emitting devices.
  • at least one capacitor or resistor, at least one cold cathode electron-emitting device, and at least a voltage source are combined.
  • the at least one capacitor or resistor and the at least one cold cathode electron-emitting device may be mounted on the same substrate.
  • Fig. 1 is shown a cross-sectional view of a first embodiment of a cold cathode electron-emitting device 1 according to the present invention.
  • an anode plate 2 To the cold-cathode electron-emitting device 1 are connected an anode plate 2, a first DC voltage source V D , and a second DC voltage source V A , both presenting a positive and a negative terminal.
  • an insulating substrate 3 On an insulating substrate 3, for instance made of glass, is deposed a first metal electrode 4. Over a part of this first metal electrode 4 and over a part of the insulating substrate 3 is deposed a n-type semiconductor layer 5. The contact between the first metal electrode 4 and the n-type semiconductor layer 5 forms a Schottky junction 6.
  • a second metal electrode 7 is deposed over a part of the n-type semiconductor layer 5 and the insulating substrate 3.
  • the contact 8 between the n-type semiconductor layer 5 and the second metal electrode 7 can be an ohmic contact or a Schottky junction.
  • the second metal electrode 7 can be made of the same or of a different material than the first metal electrode 4.
  • the Schottky junction 6 should be reverse biased for an emission of electrons from the first metal electrode 4 into the n-type semiconductor layer 5 to be possible. Therefore, the second metal electrode 7 is connected to the positive terminal of the first DC voltage source V D . It is an aim of the present invention that as much as possible of the emitted electrons pass through the n-type semiconductor layer 5 and enter into the vacuum being in contact with the n-type semiconductor 5 at a n-type semiconductor layer to vacuum boundary (hereinafter referred to as a boundary) 9.
  • an anode plate 2 is located in the same vacuum and is connected to the positive terminal of the second DC voltage source V A .
  • the voltage of the second DC voltage source V A can be relatively low, e.g. +10 Vdc, as no electric field is needed to provoke or increase the emission of electrons into vacuum.
  • the voltage of the second DC voltage source V A will be several kilovolts in typical applications.
  • V Z The voltage over the Schottky junction 6, further called V Z , is equal to V D minus a voltage drop V F over the contact 8 when this contact is not an ohmic contact but a forward biased Schottky junction.
  • Fig. 2 shows an energy diagram within the first metal electrode 4, the n-type semiconductor layer 5 and the vacuum.
  • E V is the top energy level of the valence band
  • E C is the bottom energy level of the conduction band
  • E g is the band gap
  • E F is the Fermi level
  • E VAC is the minimum energy needed for the electrons to penetrate into the vacuum at boundary 9 (also named the vacuum energy level).
  • the Schottky junction 6 When a positive voltage is applied to the n-type semiconductor layer 5 with respect to the first metal electrode 4, the Schottky junction 6 is reverse biased.
  • the impurity concentration of the n-type semiconductor layer 5 is chosen high enough to restrict the width d of the depletion region to a few tens of nanometers.
  • electrons in the first metal electrode 4 with an energy just below the Fermi level E F have to bridge an energy barrier which is slightly larger than a minimum triangular energy barrier 11 as indicated on Fig. 2.
  • This triangular energy barrier 11 has an energy barrier height ⁇ B and an energy barrier width ⁇ .
  • the bridging of the energy barrier is possible by tunnelling of the electrons 12 through the energy barrier when the energy barrier height ⁇ B is not too high and the energy barrier width ⁇ not too large.
  • the tunnelling current is exponentially related to the energy barrier width ⁇ .
  • a higher concentration of impurities in the n-type semiconductor layer corresponds to a lower Zener voltage V Z over the Schottky junction for a given tunnelling current.
  • the Zener voltage V Z should have such a value that the product q. V Z of the elementary electron charge (q) multiplied by the Zener voltage V Z exceeds the electron affinity ⁇ in the n-type semiconductor layer to vacuum boundary 9.
  • the electrons 12 which are injected from the metal electrode 4 into the n-type semiconductor layer 5 following a path 13 have then a positive residue of energy ⁇ E which permits them to escape into the vacuum at the n-type semiconductor to vacuum boundary 9.
  • the present invention it is a second condition that the above mentioned product q.V Z is lower than the band gap E g , so that the electrons being hot electrons when coming out of the depletion region, do not have enough energy to create electron-hole pairs in the n-type semiconductor layer 5.
  • the hot electrons may, along their path 13 through the n-type semiconductor layer 5, due to scattering by phonons and by impurities and defects inside the n-type semiconductor layer 5, loose a limited amount of kinetic energy, however small as it regards collisions which are highly elastic.
  • first metal electrode 4 In order to meet the above-mentioned first and second conditions, appropriate materials are selected for the first metal electrode 4 and for the n-type semiconductor layer 5.
  • a semiconductor material is selected with a band gap energy E g higher than the electron affinity ⁇ .
  • a metal is selected with a relative low work function ⁇ M so that the energy barrier height ⁇ B is not too high for tunnelling to be possible at relative low voltages and with obtainable impurity concentrations in the n-type semiconductor layer.
  • barium (Ba) is selected as material for the first metal electrode 4
  • barium oxyde (BaO) is selected as material for the n-type semiconductor layer 5.
  • a typical BaO layer thickness is 50 nanometers.
  • a typical size of a cold cathode electron-emitting device according to the invention is 10 micrometers by 10 micrometers.
  • n-type semiconductor layer 5 Other materials are known which can appropriately be used as materials for the first metal electrode 4 and the n-type semiconductor layer 5.
  • ZnS can be used as material for the n-type semiconductor layer 5, by preference with a mono-atomic layer of caesium (Cs) at the boundary 9 with the vacuum 10 in order to decrease the electron affinity ⁇ and to help in this way the hot electrons 12 to pass through the boundary 9 and escape into the vacuum 10.
  • Fig. 3 is shown a part of a cross sectional view of the first embodiment, whereby on the n-type semiconductor layer 5 is deposed a mono-atomic layer 14 of a material with a low work function as for instance Cs, in order to decrease the electron affinity ⁇ of the n-type semiconductor layer 5.
  • Fig. 4 is shown a cross sectional view of a second embodiment which is a cold cathode electron-emitting device 10 similar to the first embodiment, however with a symmetrical structure of a first metal electrode 4, a second metal electrode 15, a n-type semiconductor layer 5 and an insulating substrate 3.
  • the first metal electrode 4 and the second metal electrode 15 are by preference made of the same material, and are positioned under the same n-type semiconductor layer 5.
  • the contact between the first metal electrode 4 and the n-type semiconductor layer 5 is a first Schottky junction 6.
  • the contact between the second metal electrode 15 and the n-type semiconductor layer 5 is a second Schottky junction 16.
  • the materials used for the first and second metal electrodes 4 and 15, and the n-type semiconductor layer 5 of this second embodiment are the same as the ones used for the first metal electrode 4 and the n-type semiconductor layer 5 of the first embodiment.
  • the two Schottky junctions 6 and 16 emit electrons through tunnelling when reverse biased, as explained hereinabove (first embodiment). However, the two Schottky junctions 6 and 16 cannot emit hot electrons at the same time. When tunnelling occurs at one of the two Schottky junctions 6 or 16 which is reverse biased, the other Schottky junction is forward biased.
  • an AC voltage source V S is applied to the cold cathode electron-emitting device 10.
  • a capacitor C can optionally be put in series with the AC voltage source V S in order to obtain current driving as explained furtheron.
  • the voltage over the cold cathode emitting device 10 is substantially constant and is equal to the sum (further called V SS ) of the Zener Voltage V Z over the reverse biased Schottky junction and the voltage V F over the forward biased Schottky junction.
  • V SS V Z +V F .
  • the amplitude of the voltage source V S is chosen higher than V SS for the cold cathode electron-emitting device 10 to be current driven.
  • the cold cathode electron-emitting device 10 is current driven because then, the electron emission is substantially stable and independent on temperature.
  • Current drive is not limited to the use of one single capacitor.
  • One or more capacitors and one or more cold cathode electron-emitting devices can be combined with one or more voltage sources. In such a combination, the cold cathode electron-emitting devices may have different operating voltages over their terminals and may have different emitting areas.
  • the capacitors may have different values.
  • Current drive can also be realised by putting a resistor and a voltage source in series with a cold cathode electron-emitting device.
  • the voltage source can either be a DC or an AC voltage source, and the cold cathode electron-emitting device can either be asymmetrical as shown in Fig. 1 or symmetrical as shown in Fig. 4.
  • one or more resistors can be combined with one or more cold cathode electron-emitting devices, and one or more voltage sources.
  • the contact between the n-type semiconductor layer 5 and the second metal electrode 15 is the second Schottky junction 16; it is not an ohmic contact.
  • Fig. 5 is shown an energy diagram of the second Schottky junction 16 when forward biased. No hot electrons are emitted from such a forward biased Schottky junction.
  • the metal electrode being the second metal electrode 15 has a positive voltage V F with respect to the n-type semiconductor layer 5.
  • the working principle of the second Schottky junction 16 is equal to the working principle of a state-of-the art Schottky diode; some of the majority carriers in the conduction band of the n-type semiconductor layer 5 have enough kinetic energy by thermal agitation to overcome the energy barrier, which is ⁇ B , lowered by a value q.V F because of forward biasing. V F is typically lower than 1 volt.
  • a third embodiment of the present invention is shown in Fig. 6 and is a symmetrical cold cathode electron-emitting device 10 in series with a capacitor C, both mounted on the same substrate 3 and by preference made with the same technology, for instance thin film technology.
  • the capacitor C consists of three layers.
  • the first layer of the capacitor C is a metal electrode which can be one of the two metal electrodes 4 or 15 of the cold cathode electron-emitting device 10.
  • the first layer of the capacitor C is formed by the second metal electrode 15.
  • the first layer of the capacitor C can also be a layer making an ohmic contact with one of the two metal electrodes 4 or 15 of the cold cathode electron-emitting device 10.
  • the second layer of the capacitor C is an insulator layer 18.
  • the third layer of the capacitor C is a metal electrode 19, typically made of aluminium (Al).
  • the metal electrode 19 being the third layer of the capacitor C and a metal connection electrode 20 having an ohmic contact with the first metal electrode 4, both connections by preference being made of the same material, for instance aluminium.
  • a fourth embodiment is shown in Fig. 7. It is a symmetrical cold cathode electron-emitting device 10 of which the n-type semiconductor layer is divided in two parts. In a first part of the semiconductor layer 22, the concentration of donor impurities is high (highly doped). In a second part of the semiconductor layer 23, the concentration of donor impurities is relatively low (weakly doped), for instance 10% of the concentration of donor impurities in the first part of the semiconductor layer 22.
  • the thickness of the parts 22 and 23 of the semiconductor layer is not necessarily equal over the whole cross section of the n-type semiconductor layer. The thickness of both parts 22 and 23 of the semiconductor layer within the active areas 24 from which the electron emission 13 occurs is small (typically a few tens of nanometers).
  • the electric field within the weakly doped second part of the semiconductor layer 23, between an electron-emitting metal electrode 4 or 15 and the highly doped first part of the semiconductor layer 22, is approximately constant when the Schottky junction 6 or 16 between the n-type semiconductor layer and the metal electrode 4 or 15 is reverse biased.
  • the depletion region within the n-type semiconductor layer is then more or less constrained to the weakly doped second part of the semiconductor layer 23.
  • the Zener voltage at which tunnelling happens is mainly determined by the thickness of the second part of the semiconductor layer 23 in the active area 24 and less dependent on the concentration of impurities in the first and second parts 22 and 23 of the semiconductor layer. As a consequence, the Zener voltage, being typically 3 volts, can be better reproduced.
  • the thickness of the weakly doped second part of the semiconductor layer 23 is higher at the edges of the metal electrodes 4 and 7. This causes a decrease in the strength of the electrical field in the depletion region of the n-type semiconductor layer at the edges of the metal electrodes 4 and 7 instead of an increase of the electrical field which could otherwise occur because of geometrical effects as for instance rounding of the metal electrodes 4 or 7 at their edges.
  • a higher field strength at the edges of the metal electrodes 4 or 7 is not wanted because it can constrain the tunnel effect to the edges of the active areas 24 where the conditions for electron emission are not optimal and more difficult to control.
  • connection electrodes 20 of the cold cathode electron-emitting device 10 to a voltage source can be made of for instance aluminium.

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EP98870112A 1998-05-18 1998-05-18 Dispositif d'émission d'électrons à cathode froide Withdrawn EP0959485A1 (fr)

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EP98870112A EP0959485A1 (fr) 1998-05-18 1998-05-18 Dispositif d'émission d'électrons à cathode froide

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EP98870112A EP0959485A1 (fr) 1998-05-18 1998-05-18 Dispositif d'émission d'électrons à cathode froide

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1328002A1 (fr) * 2002-01-09 2003-07-16 Hewlett-Packard Company Dispositif émetteur d'électrons pour applications dans le stockage de données

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2960659A (en) * 1955-09-01 1960-11-15 Bell Telephone Labor Inc Semiconductive electron source
US3098168A (en) * 1958-03-24 1963-07-16 Csf Hot electron cold lattice semiconductor cathode
US3500102A (en) * 1967-05-15 1970-03-10 Us Army Thin electron tube with electron emitters at intersections of crossed conductors
US3808477A (en) * 1971-12-17 1974-04-30 Gen Electric Cold cathode structure
US5233196A (en) * 1990-09-25 1993-08-03 Canon Kabushiki Kaisha Electron beam apparatus and method for driving the same
WO1997039469A1 (fr) * 1996-04-15 1997-10-23 Massachusetts Institute Of Technology Emetteurs d'electrons de grande energie

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2960659A (en) * 1955-09-01 1960-11-15 Bell Telephone Labor Inc Semiconductive electron source
US3098168A (en) * 1958-03-24 1963-07-16 Csf Hot electron cold lattice semiconductor cathode
US3500102A (en) * 1967-05-15 1970-03-10 Us Army Thin electron tube with electron emitters at intersections of crossed conductors
US3808477A (en) * 1971-12-17 1974-04-30 Gen Electric Cold cathode structure
US5233196A (en) * 1990-09-25 1993-08-03 Canon Kabushiki Kaisha Electron beam apparatus and method for driving the same
WO1997039469A1 (fr) * 1996-04-15 1997-10-23 Massachusetts Institute Of Technology Emetteurs d'electrons de grande energie

Non-Patent Citations (3)

* Cited by examiner, † Cited by third party
Title
A.KUDINTSEVA ET AL.: "a cold emitter with semiconductor-metal film structure", RADIO ENGINEERING AND ELECTRONIC PHYSICS, vol. 13, no. 8, 1968, pages 1329 - 1330, XP002083048 *
GEIS M W ET AL: "DIAMOND EMITTERS FABRICATION AND THEORY", May 1996, JOURNAL OF VACUUM SCIENCE AND TECHNOLOGY: PART B, VOL. 14, NR. 3, PAGE(S) 2060 - 2067, XP000621834 *
GEIS M W ET AL: "THEORY AND EXPERIMENTAL RESULTS OF A NEW DIAMOND SURFACE-EMISSION CATHODE", 1997, THE LINCOLN LABORATORY JOURNAL, VOL. 10, NR. 1, PAGE(S) 3 - 18, XP000749210 *

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1328002A1 (fr) * 2002-01-09 2003-07-16 Hewlett-Packard Company Dispositif émetteur d'électrons pour applications dans le stockage de données
US6806630B2 (en) 2002-01-09 2004-10-19 Hewlett-Packard Development Company, L.P. Electron emitter device for data storage applications and method of manufacture

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