US 3872489 A
An electron emitter comprising a body of a wide band gap material using double injection space-charge limited current. By using double injection of carriers to establish space-charge limited currents in high resistivity p-type semiconductors, the number of minority carriers can be increased considerably without raising the Fermi level above mid band gap. By using such double injection space-charge limited current a sufficient amount of large energy minority carriers are placed in the conduction band in a p-type semiconductor. A monoatomic layer of cesium and oxygen is positioned on the positively biased contact. This places a negative electron affinity surface layer on the device. The electrons in the conduction band then have enough energy to impel electrons into a vacuum.
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Description (OCR text may contain errors)
United States Patent 1 Hagenlocher l l ELECTRON EMISSION FROM A COLD CATHODE  Inventor: Arno K. Hagenlocher, Framingham,
 Assignee: GTE Laboratories Incorporated,
 Filed: Feb. 22, 1973  Appl. No.: 335,273
OTHER PUBLICATIONS D. Geppert, A Proposed P-N Junction Cathode, Proc. lEEE, Vol. 54 1, Jan. 1966, p. 61. B. Williams et 211., Electron Emission from a Cold 3 Cathode GaAs P-N Junction, App. Phys. Let, Vol.
14 7, Apr. 1969, p. 214-216.
Primary Examiner-Andrew J. James Assistant Examiner-Joseph E. Clawson, Jr.
Attorney, Agent, or'Firm-lWing M. Kriegsman; Leslie J. Hart  ABSTRACT I An electron emitter comprising a body of a wide band gap material using double injection space-charge limited current. By using double injection of carriers to establish space-charge limited currents in high resistivity p-type semiconductors, the number of minority carriers can be increased considerably without raising the Fermi level above mid band gap. By using such double injection space-charge limited current a sufficient amount of large energy minority carriers are placed in the conduction band in a p-type semiconductor. A monoatomic layer .of cesium and oxygen is positioned on the positively biased contact. This places a negative electron affinity surface layer on the device. The electrons in the conduction band then have enough energy to impel electrons into a vacuum.
7 Claims, 2 Drawing Figures ELECTRON EMISSION FROM A COLD CATIIODE BACKGROUND OF THE INVENTION This invention relates to electron emitters or cathodes, and more particularly, to an electron emitter which comprises a body of semiconductor material.
Electron emission into vacuum from a semiconductor surface with negative electron affinity has been reported several times in recent years. The search for a cold cathode as a replacement for the thermionic emitter in vacuum tubes is a long standing one. An efficient cold cathode would reduce power dissipation and increase the life of tubes, eliminate the time lag for filament to warm up, and generate an electron beam with narrower velocity distribution with correspondingly improved resolution.
Cold cathode electrode emission into a vacuum has been achieved from cesiated surfaces for forward biased P-N junctions of silicon, gallium arsenide and other IIIIV compounds. The principle of operation in the present invention for a cold cathode is to get sufficient minority carriers into the conduction band in a p-type semiconductor, then .place a negative electron affinity surface layer on the device. The electrons in the conduction bands have then enough energy to escape into vacuum. Up to now negative electron affinity was only obtained by a monoatomic layer of cesium and oxygen. Since the work function of a monomolecular layer of cesium and oxygen is about 1.4 eV, the Fermi level has to be more than 1 eV below the conduction band to make the higher energy electrons escape. Unfortunately, silicon is just at the limit to fulfill this condition. The negative electron affinity will increase with increasing bandgap. This would lead to a preference of wide bandgap material.
SUMMARY OF THE INVENTION An electron emitter is provided which comprises: a wide bandgap type semiconductor body which utilizes double injection space-charge limited current to emit electrons into a vacuum. In the present invention, no P-N junction is used; rather, the holes are injected by double injection space-charge limited current. By using such a technique, wide bandgap material can be used and gallium phosphide (GaP) appears to be the best candidate for this purpose. The wide bandgap material maybe activated by an electro-positive monoatomic layer of cesium and oxygen.
Activation of a silicon P-N junction for electron emission is very critical and its efficiency marginal. The
activation process has to be performed at extreme high I vacuum and it appears that it is impossible to integrate this process into tube production where heat treatment and outgassing are required. However, by using double injection of carriers to establish the space-charge limited currents in high resistivity P-type conductors, as in the present invention, the number of minority carriers can be increased considerably without raising the Fermi level above the middle of the bandgap. Gallium phosphide has a bandgap of 2.2 eV. With double injected spaee-charge-limited currents the Fermi level can reach the middle of the bandgap. The electrons enter through the area of negative electron affinity with at least 1.1 eV above the Fermi level giving the electrons with 0.3 eV kinetic energy a probability to escape into vacuum. Since the relation between electron and holes shifts to more holes than electrons close to the eesium layer, the Fermi level will actually be lower.-The voltages which have to be applied to reach spacecharge limited currents are large enough to make them hot electrons with high kinetic energy, therefore largely increasing their escape probability. In this fact lies the big advantage over forward biased junctions where only small voltages, about one volt or less, can be applied in order to prevent heating of the device and only few electrons have enough energy to escape.
The features of the present invention which are believed to be novel are set forth with particularity in the attendant claims. The invention, together with further objects and advantages thereof, may best be understood by referenee to the following description taken in connection with the drawings. In the several figures, like reference numerals identify like elements.
DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of the electron emitter in accordance with the present invention; and
FIG. 2 is a cross-sectional view of the active portion of the emitter shown in FIG. I with corresponding energy band and injected carrier concentration diagrams.
DETAILED DESCRIPTION An electron emitting semiconductor l0 utilizing double injection space-charge limited current, as shown in FIG. 1, comprises a thin layer or substrate 11 of wide bandgap p-type semiconductor material such as gallium phosphide (GaP). Ohmic electrical contact to substrate 11 is provided by means of an electrically conductive plate 12 such as a thin metal electrode. The electrode 12 also may be connected to a heat sink (not shown) to keep the device below 60C during operation since higher temperatures may destroy the activation layer 20. The second ohmic electrical contact to the substrate 11 is provided by means of electrode layer 13. Electrode 13 has an aperture 15 for emitting the electrons produced by the wide bandgap material 11.
Electrode l3 acts as a drain contact for the electrons produced within the wide bandgap material. A thin activation of electron affinity layer 20 preferably comprising a monoatomic layer of cesium and oxygen on the exposed surface of the wide bandgap material layer 1 l in aperture 15. The overabundance of electrons produced by the double injection space-charge in the wide bandgap material 11 may then be emitted from the exposed surface of the monoatomic cesium oxide layer 20.
The electrons emitted from aperture 15 are created by a double injection space-charge limited current introduced across the wide bandgap material 11. This current may be developed by a DC voltage source such as a battery 20 in series with a resistor 21 which are connected in series to electrodes 12 and 13. This voltage places a forward bias on the wide bandgap p-type material and introduces the holes by double injection space-charge limited current. By using double injection of carriers to establish space-charge limited currents in high resistivity p-type semiconductors, the number of minority carriers can be increased considerably without raising the Fermi level. This creates an abundance of electrons which propagate to the positively charged drain contacts 13 and are emitted through aperture 15. These electrons may be attracted or focused by any positive potential which is well known in vacuum tube technology.
A partial cross-sectional view of the active portion of the electron emitter shown in FIG. 1 appears in FIG. 2, with a corresponding energy band diagram being vertically aligned therewith. As shown, the active portion of the emitter comprises a P-ty-pe gallium phosphide layer 11 and the cesium surface layer 20. The total length, L, of the active region of the gallium phosphide layer 11 may typically be on the order of about 2 mils. The thickness, d, of the cesium layer may be on the order of l to 10 angstroms.
In FIG. 2, both the energy band diagram for the active portion of the emitter l0 and carrier concentrations are vertically aligned with the active portion of the emitter 10. As indicated in FIG. 2, the energy gap is the distance between the valence and conduction bands with an intrinsic" line drawn midway-between the valence and conduction bands. As is well known, portions of the semiconductor layer in which the Fermi level lies below the intrinsic line exhibit P-type conductivity or portions of the, layer in which the Fermi level is above the intrinsic line exhibit N-type conductivity. The Fermi level varies through the material due to the double injection spacecharge current present in the P- type material.
The electro-positive cesium layer 20 pins the bottom of the conduction band to the Fermi level at the emitting surface of the P-type layer 11, as illustrated in FIG. 2. The pinning of the bottom of the conduction band at the Fermi level at the emitting surface necessitates a sharp bending of the valence and conduction bands in the immediate vicinity of the emitting surface. The residual energy imparted to the electrons in the active portion of the emitter (due to the height of the conduction band above vacuum energy level) is sufficient to overcome the work function or energy requiredfor the electrons to be emitted. The electro-positive cesium layer 20 introduces electrons into the adjacent portion of the galliumphosphide layer 11, so that a thin N-type inversion region exists at the emitting surface.
The carrier concentrations shown in arbitrary units are also vertically aligned with the active portion of the emitter 10. This diagram shows the amount of electrons injected on one side of the gallium phosphide layer as well as the holes injected on the opposite side of the gallium phosphide layer. As shown, these carrier concentrations decrease as they traverse the gallium phosphide layer 11; this decrease in carrier concentration is due to the loss mechanisms encountered during the traversal of the P-type layer 11, such as recombination.
The operation of the space-charged semiconductor electron emitter is controlled by the double injection of carriers. Space-charged limited currents (SCLC) in a semiconductor current transport occurs according to the transport equations. In additional carriers are injected into the bulk minority carrier, this carrier will recombine within the lifetime 1. Majority carriers will disappear even faster within the dielectric relaxation time:
- where e is the dielectric constant, 6,, is the dielectric constant in vacuum and a the conductivity. If in the case of majority carriers the field strength is large enough to move the carriers faster through the distance L, from contact to contact than the dielectricrelaxation time, an excess of majority carriers will build up,
such that the transit time 17 is less than or equal to the relaxation time:
rrmuzir S TI'II.
L/ E S n/ where p. is the mobility and E is the electric field strength. The threshold voltage, V,,,, is therefore for this phenomenon:
with n being the thermal carrier density and e being the charge of an electron. Above this voltage a space charge of majority carriers exists and the current .I is limited by the space charge:
Since the current depends now from the square of the applied voltage, the space-charge limited current is larger than the ohmic current. It is important that no warm up effect by ohmic current exists since otherwise It in equation (4) would increase exponentially and no space-charge limited currents could be observed. In order to keep the threshold low enough a small electrode distance (V L and a material with low thermal carrier density (wide bandgap) has to be used. In this case the field strength has to be relatively large since it has to overcome the repulsive action of the space charge. If both types of carriers are injected from the electrodes (electrons from one electrode, holes from the other) the space charges partially compensate each other and smaller electrical fields can be applied. We have now double injected space-charge limited currents. Now a space charge can be already built up over a part of the bulk when 7 is still 1 and the current is then J: e lelp'h T vz/La By increasing the applied voltage, eventually the condition lrunxi! TI'PI.
will be realized. The current voltage relation is now It can easily be shown that now the current is considerably larger than is the case of (5) at the same voltage. Formula (7) is a theoretical value and practical experience has shown that the current density is usually two orders of magnitude lower than equation (7). This is for most materials still large above equation (5). Furthermore, when achieving double injection spacecharge limited currents, a small electrode distance L (J m L). large mobilities for electron and holes and a large lifetime 1- of the minority carriers is required.
Since a large current density is required, mobilities and lifetimes have to be large and the sample lengths small. The most useful material appears to be gallium phosphide, which has mobilities of lOO for electrons and 300 for holes. A lifetime of close to seconds or longer and a carrier concentration 10 to i0 per cubic centimeters is required. To achieve the proper carrier concentration small amounts of zinc or cadmium impurities may be added to the gallium phosphide. When a sample of gallium phosphide is thinned down to 2 mils, the theoretical current at 80 volts would be around 10 amps per centimeter and experiments have shown a current of only 10 amps per centimeter which is adequate. With an emission efficiency of only l% into vacuum after cesiation lA/cm would be emitted. An active area of 4 mils would emit about 100 micro A. If the contact area is l millimeter in diameter, the power dissipated in the sample would be about l watt. Samples operated under double injection conditions are very rugged and can easily stand the temperature required for the cleaning process previous to cesiation. To prepare a wafer of gallium phosphide to a thickness of 2 mils, standard etching techniques may be utilized. By photolithographic methods, a mesa pattern may be etched out in the region to be used for emitting the electrons.
The various features and advantages of the invention are thought to be clear from the foregoing description. Various other features and advantages not specifically enumerated will undoubtedly occur to those versed in the art, as likewise will many variations and modifications of the preferred embodiment illustrated, all of which may be achieved without departing from the spirit and scope of the invention as defined by the following claims.
What is claimed is:
l. A cold cathode electron emitter using double injection space-charge limited current, comprising:
a single semiconductor body, said single body consisting of a homogeneous P-type wide bandgap material;
first and second conductor plates mounted on opposite sides of said single semiconductor body, said second plate being formed with an aperture defining an electron emitting surface of said single semiconductor body;
a thin layer of work function reducing material coated on said electron emitting surface;
bias voltage means connected across said plates, said voltage at said second plate being more positive than said voltage at said first plate; and
said single semiconductor body having a thickness measured between said plates, a lifetime and ,a thermal carrier concentration and said bias means having a magnitude such that said single semiconductor body and said means produce double injection space-charge limited current within said single semiconductor body in which electrons are injected from said first plate and holes are injected from said second plate to excite electrons into the conduction band of said single semiconductor body.
2. The emitter defined in claim 1, wherein said work function reducing material comprises a material selected, from the group consisting of cesium and oxygen.
3. The emitter defined in claim 1 wherein said single semiconductor body is gallium phosphide.
4. The emitter defined in claim 1 wherein said bias voltage means includes biasing said single semiconductor body at about volts.
5. The emitter defined in claim 1 wherein said single semiconductor body has a lifetime greater than about 10 seconds.
6. The emitter defined in claim 1 wherein said single semiconductor body has a sample thickness less than about 2 mils.
7. The emitter defined in claim 1 wherein said single semiconductor body has a thermal carrier concentration of between l0 to l0 atoms per cubic centimeter.