Heterostructure thermionic coolers

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OTHER PUBLICATIONS L. W. Whitlow, et al., “Superlative applications to thermo L.D. Hicks, et al., “Effect of quantum-well structures on the electricity’, Journal of Applied Physics, Nov. 1, 1995, vol. thermoelectric figure of merit”, Physical Review B (con 78, No. 9, 5460-5466.

densed matter), vol. 47, No. 19, May 15, 1993, pp. G. D. Mahan, et al., “Thermoelectric devices using Semi 12727-12731. conductor quantum wells”, Journal of Applied Physics, Vol. L. D. Hicks, et al., “Thermoelectric figure of merit of a 76, No. 3, Aug. 1, 1994, 1899–1901. one-dimensional conductor”, Physical Review B (con densed matter), vol. 47, No. 24, Jun. 15, 1993, 16631-4. G. D. Mahan, et al., “Thermionic refrigeration”, Journal of L. D. Hicks, et al., “Use of quantum-Well Superlattices to Applied Physics, vol. 76, No. 7, Oct. 1, 1994, 4362–6. obtain a high figure of merit from nonconventional thermo Sofo, J.O., “Thermoelectric figure of merit of Superlattices”, electric materials”, Applied Physics Letters, vol. 63, No. 23, Applied Physics Letters, vol. 65, No. 21, Nov. 21, 1994, Dec. 6, 1993, 3230–2. 2690-2.

L. D. Hicks, et al., Experimental study of the effect of quantum-Well Structures on the thermoelectric figure of D.M. Rowe, et al., “Multiple Potential Barriers as a Possible merit, Physical Review B (condensed matter), vol. 53, No. Mechanism to Increase the Seebeck Coefficient and Elec 16, R10493-6. trical Power Factor', Thirteenth International Conference on J. M. Houston, “Theoretical efficiency of the thermionic Thermoelectrics, Kansas City, Mo, USA, Aug. 30, -Sep. 1, energy converter”, Journal of Applied Physics, vol. 30, No. 1994). AIP Conference Proceedings, 1995 (No. 316) pp. 4, Apr. 1959, pp. 481-487. 339-342.

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HETEROSTRUCTURE THERMONIC carriers from cathode to anode (and Suppressing the reverse COOLERS current) will create a cold junction at cathode and a hot junction at anode. This device will function at room tem

STATEMENT AS TO RIGHTS TO INVENTIONS perature and below. Using the same device in contact with MADE UNDER FEDERALLY SPONSORED 5 a hot and a cold bath will create a thermionic generator RESEARCH AND DEVELOPMENT which also works at room temperature and below. This invention was made with Government support under electronic One object of the present invention is to provide better Contract No. F49620-96-1-0349, awarded by the Air Force. of the invention cooler fabrication techniques. It is a further object to reduce electronic cooler fabrication costs.

The Government has certain rights in this invention. It is a further object of the invention to make more efficient BACKGROUND OF THE INVENTION electronic coolers which reach lower temperatures. 1. Field of the Invention

These and various other advantages and features of nov elty which characterize the invention are pointed out with

This invention relates in general to electronic devices, and particularity in the claims annexed hereto and form a part more specifically to the first use of Semiconductor materials 15 hereof. However, for a better understanding of the invention, to fabricate thermionic coolers and generators. its advantages, and the objects obtained by its use, reference 2. Description of Related Art should be made to the drawings which form a further part The use of electronics to transport heat to and away from hereof, and to accompanying descriptive matter, in which certain areas has expanded in recent years due to increased there are illustrated and described Specific examples of an packing densities and hostile environments. For cooling apparatus in accordance with the invention. applications, thermoelectric coolers (TE Coolers) have been BRIEF DESCRIPTION OF THE DRAWINGS used to cool areas both in electronic and non-electronic applications. TE coolers are typically a p-type doped region Referring now to the drawings in which like reference alternatively connected to an n-type doped region, which 25 numbers represent corresponding parts throughout: creates cooling effects at one metal-doped region junction FIG. 1 shows the overall cooling power for thermionic and heating effects at the other metal-doped region junction, devices and thermoelectric devices, depending on the direction of the current through the device.

However, TE coolers are limited in their overall perfor the Peltiershows

FIG.2 the ratio of the thermionic cooling term over cooling term as a function of current;

mance by the bulk properties of the materials used in the TE cooler. More efficient cooling is needed in many applica present invention; diagrams of a first embodiment of the

tions. Reliability of assemblies of many elements is often not sufficient for many high reliability designs. The cost of TE FIGS. 4A-4E are graphs of the conduction bandedge of coolers has not plummeted at the same rate as other elec devices made using the present invention; tronic devices Such as transistor circuits, lasers and 35 FIG. 5 shows an alternative structure for the device 10; detectors, because TE cooler elements are not fabricated FIG. 6 shows a cascaded device using Stages with differ using high Volume planar integrated circuit technology. ent bandedge discontinuities, and

Further, TE coolers that can generate a large cooling effect FIG. 7 shows a combination of n-doped and p-doped tend to be large devices, typically 1 cmx1 cm or larger and devices as described in the present invention. thus, are not acceptable in Small electronic devices. 40

It can be seen then that there is a need for better electronic DETAILED DESCRIPTION OF THE coolers. It can also be seen then that there is a need for better INVENTION electronic cooler fabrication techniques. It can also be seen that there is a need for low cost electronic coolers. It can also Overview be seen that there is a need for more Space efficient electronic 45 The present invention uses thermionic emission in Semi coolers. It can also be seen that there is a need for more energy efficient electronic coolers. It can also be seen that high powerheterostructures conductor for heat pumping and cooling of electronic and optoelectronic devices. These there is a need for more reliable electronic coolers. It can also be seen that there is a need for electronic coolers that integrated micro-coolers can improve the efficiency and reach lower temperatures. 50 lifetime of electrical and optoelectronic components. The thermionic coolers could also be used as an additional means

SUMMARY OF THE INVENTION for tuning temperature Sensitive devices. The coolers that are commercially available are typically

The present invention discloses a powerful and highly thermoelectric (TE) coolers, based on the Peltier effect at the productive Semiconductor thermionic cooler. 55 junction of two dissimilar materials. TE coolers use mate The present invention minimizes the above-described rials bulk properties, Such as the Seebeck coefficient, elec problems by using bandgap engineering and modulation trical and thermal conductivity, and are mostly based on doping to fabricate Small thermionic coolers that operate at Bismuth Telluride (Bi-Te) for room temperature applica room temperature. By using proper materials and tions. The basis of the heterostructure thermionic (HTI) geometries, efficient and Space conserving thermionic cooler 60 cooler described here is to use bandstructure engineering to elements which can reach lower temperatures are fabricated increase the cooling power and efficiency. in a cost-effective manner. Recent proposals to use quantum wells, quantum wires, The principles of the present invention comprise growing and Superlattice Structures to increase the TE cooler figure of two Semiconductor layers. The Second layer has a variable merit can be divided into two categories. The first category conduction bandedge as a function of distance (for the case 65 changes the density of electronic States of the cooler mate of electron transport) which has its maximum bigger than rials to make it more "peaked’ and also more asymmetric the first layer. Selective thermionic emission of high energy with respect to the Fermi energy. This will increase the

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electrical power factor, So, and thus the TE cooler figure of p, e.g., electrons are moving from the p-branch to a metallic merit Z=So/B, where S is the Seebeck coefficient or contact between the branches and then to the n-branch, the thermopower, O is the electrical conductivity, and B is the heat is absorbed at the junctions p-metal and metal-n. thermal conductivity. Electrons in the p-branch have an average transport energy The Second category uses perpendicular transport of elec Smaller than the Fermi energy, and the ones in the n-branch trons in Superlattices in a way that modifies the mobility of have an average transport energy larger than the Fermi low energy electrons with respect to high energy electrons. energy. Metals are considered to have their average transport This asymmetry also increases the electrical power factor. energy equal to their Fermi energy. Both methods are expected to only give moderate improve In a perfect ohmic conduction from the p-branch to metal ments when various non-ideal effects, Such as the role of to the n-branch, electrons should absorb energy in the form barriers and finite level widths, are included in the final of heat to increase their average energy. The same argument models and devices, as shown in papers written by Mahan can be applied to the contacts at the outside ends of the (Appl. Phys. Lett. 65 (21) p. 2690, 1994) and Rowe (13th branches where the heat is generated. The heat absorption or International Conference on Thermoelectrics, Kansas City, 15 generation occurs at distances very close to the contacts, on Mo., 1994, p. 339). the order of electron average Velocity times its thermaliza The present invention uses thermionic emission in het tion time constant. This heat absorption or generation, called erostructures. Thermionic emission is beyond the linear the Peltier effect, is a reversible thermodynamic phenomena Boltzmann transport equation that is typically used in the depending on the direction of the current flow. analysis of TE devices, as shown in “Solid State Physics.” Thermionic Coolers Ashcroft and Mermin, Sanders College, 1976. Thermionic emission from metallic plates into a vacuum or gas filled To create a heterostructure thermionic (HTI) cooler, one diode is a key technology for the conversion of heat into uses precise control of layer thickness and composition, electricity at high temperatures (>1000°K). If metals with achieved by molecular beam epitaxy (MBE), metal-organic very low work functions were available and could be placed 25 chemical vapor deposition (MOCVD), or other growth at Small distances apart, the same principle would make a techniques, in conjunction with bandgap engineering to thermionic refrigerator at room temperatures. allow for the design of Specific conduction or Valence band The present invention uses Semiconductor heterostruc profiles within a device. The use of a typically higher tures to tailor the thermionic emission by using bandedge bandgap material between two lower bandgap materials, the discontinuity between various compounds. The accurate two lower bandgap materials comprising the cathode and the epitaxial growth of thin and uniform layers in conjunction anode, will produce a barrier for electrons or holes as they with modulation doping can eliminate the problem of Space travel from cathode to anode. Thermionic emission of car charge which limits the operation of vacuum thermionic riers over this barrier Selectively removes high energy car diodes at low temperatures. riers. The Strong electron-electron interaction at the cathode 35 will tend to restore quasi-Fermi distribution by absorbing

Thermoelectric Cooler Background heat from the lattice. Electrons that reach the anode will lose their energy by generating heat. By choosing the appropriate

Electron conduction in a Solid is affected by the tempera band-edge ture and the temperature gradient. This interaction between are typicallydiscontinuities

at the cathode and anode, which the reverse current is Suppressed the “electrical' current, e.g., the amount of charge trans and cooling is achieved at room temperatures. ported by electrons I, and the “thermal” current, e.g., the 40 amount of heat transported by electrons Q, has been used for Depending on the growth constraints and lattice mismatch various applications Such as thermoelectric cooling (I to Q) between materials, the barrier composition can be graded or thermoelectric generation (Q to I) and thermal (bolometric) modulated to produce internal fields and to enhance electron detectors (AT to V). transport properties. In the case of a vacuum diode, the 45 problem of Space charge, which is the presence of charged

The periodicity of crystalline Solids allows a description of electron movement in a complicated Voltage potential of will electrons in the Space between the cathode and the anode, many atoms, using Some parameterS Such as bandgap, create an extra potential barrier for the current going from effective mass, etc. In a point-particle picture, localized temperature the cathode to the anode, further limits the low Scattering events can be assumed with acoustic and optical 50 heterostructure cooling or power generation applications. Using phonons, along with various impurities, and coherent Scat ing of cathode thermionic coolers, close and uniform Spac and anode is less of a problem, and is tering events can be neglected. The electron motion can be controlled by accurate adequately described and modeled using the electronic dis Furthermore, doping the crystal barrier growth technologies.

material (modulation tribution function and the Boltzmann Transport Equation doping) can be used to create internal fields, modify the (BTE). 55 electron flow, and control Space charge effects. The materials used for TE cooling application are usually To see the inherent advantage of heterostructure thermi described by the linearized Boltzmann equation and Small onic cooling (HTI) over thermoelectric cooling, we consider perturbation of the electronic distribution function by exter a piece of BiTe Semiconductor with ohmic metallic con nal fields and temperatures. In contrast, the heterostructure tacts at both thermionic device is based on a large perturbation of the 60 coolers), andsides the (which is typically used in commercial TE same piece with two Schottky barrier electronic distribution function.

contacts at the two ends, with barrier heights optimized for

Thermoelectric Cooler Modeling cooling at room temperature. This example will illustrate the Selectivity of thermionic emission for transporting high

A Single element TE cooler is composed of two branches, energy electrons, with respect to bulk Selectivity which is one branch of n-doped and one branch of p-doped material. 65 governed by the Seebeck coefficient. The best material for The two branches are connected electrically in Series and TE cooling is not necessarily the best for HTI cooling, so thermally in parallel. When the current is flowing from n to further improvements are possible.

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The problems associated with Space charge effects and the other materials, Such as Silicon (Si), indium phosphide (InP), interplay between carrier diffusion and thermionic emission lead telluride (PbTe), gallium nitride (GaN), Gallium phos is ignored in the example below. All these effects are phide (Gap), indium arsenide (InAs), germanium (Ge), controllable using modulation doping and bandgap engi mercury cadmium Selenide (HgCdSe), indium gallium neering in the barrier layer. The example is given to show the arsenide (InGaAs), indium arsenide (InAs), indium anti inherent advantage of HTI over the Peltier effect based on monide (InSb), indium gallium arsenide antimonide bulk properties. (InGaAs, Sb) mercury cadmium telluride (HgCd. The cooling capacity per unit area of a conventional TE a Te), aluminum gallium nitride (AlGaN), indium gallium cooling element of 1 um length, made of a piece of BiTes nitride (In GaN), indium arsenide phosphide (InASP), Semiconductor between two metallic contact of tempera indium gallium arsenide phosphide (InGaAS, P), tures 300 and 301 K, is: indium gallium aluminum arsenide (InGaAl-AS), lead tin telluride (PbSn-Te), aluminum arsenide (AIAS), alu

S=200 u VIK, o=1000S2 cm", B=1.6 WimK minum antimonide (AISb), Zinc Selenide (ZnSe), Zinc tel luride (ZnTe), boron nitride (BN), gallium phosphide (GaP),

Q(W/cm)=0.06-1-5x1OP-160; 15 gallium antimonide (GaSb), gallium aluminum arsenide If we use the same element between two Schottky con (GaAll-AS), gallium arsenide phosphide (GaASP), gal tacts for thermionic cooling, with appropriate barrier heights lium indium phosphide (Gan-P), gallium indium anti at the cathode and (p at the anode, we will get: monide (GanSb), bismuth telluride (Bi-Te), and bis muth Selenide (Bi-Sea) or other ternary or quaternary kRT materials, where the Subscripts X, y, 1-X, and 1-y denote the

. I - 5x 108. I? - 160, relative amounts of the atomic Species in each ternary or quartenary material and range from Zero to one, inclusive.

kBT emke’T Further, layer 12 may be doped n-type or p-type, or can be sig () (volts) = -- lin 22:3 - ln(I) = 0.02514 - ln(I) a metal layer.

25 FIG. 3B shows device 10 being constructed by adding layer 14 on top of layer 12. Layer 14 is a barrier layer for (p' should be high enough to Suppress the reverse current device 10, and for layer 12 consisting of GaAs, layer 14 is from anode to cathode. FIG. 1 shows the overall cooling typically a graded AlGaAS layer. Layer 14 creates a slight power for thermionic devices and thermoelectric devices. A internal electric field throughout the thickness of layer 14 to Superior performance for HTI over TE cooling for all eliminate the Space charge problem around the cathode, currents is evident. FIG. 2 shows the ratio of the thermionic which, in this case, is layer 12. Layer 14 is typically grown cooling term (p(I)+2kT/e):I over the Peltier cooling term by MBE or MOCVD techniques, but can be grown in other (ST) I as a function of current. ways. If layer 14 is n-type, layer 14 has a conduction By using the thermionic effect, instead of the thermoelec bandedge that increases as a function of the distance from tric effect, two immediate advantages are evident. The TE 35 the layer 12, e.g., the value of the conduction bandedge at a cooler materials are restricted to those materials that have first distance from the layer 12 is more than the value of the high electrical conductivity and thermopower, and low ther conduction bandedge at a Second, greater distance from the mal conductivity. These materials then only produce cooling layer 12. The layer 14 conduction bandedge increases for or heating at the junction between two materials, with layerS 14 that are doped n-type, corresponding p-type doped different Seebeck coefficients. 40 layerS 14 will have a Valence bandedge that is decreasing. Heterostructure thermionic cooling, on the other hand, The conduction bandedge of the layer 14 can be monotoni does not have a requirement for high thermopower materi cally increasing, Stepped, or piecewise linear, or any other als. Bandedge discontinuities, also known as Schottky shape, So long as at Some point in the layer 14 the conduction barriers, between the anode and cathode will perform the bandedge has a level higher than the conduction bandedge of cooling or heating as needed. One has to find the barrier 45 the layer 12. Layer 14 can also be a Strained layer, to material that has high electrical conductivity and low ther increase the bandedge offset between layer 12 and layer 14. mal conductivity. The cooling and/or heating effect pro For layerS 12 that are SiGe, layer 14 can be a Silicon duced by thermionic devices, depending on the direction of germanium (SiGe), layer, where y>X. For layers 12 that the current through the device, will be called a non are InGaASP-1, layer 14 is typically In, Ga. isothermal effect, as it is a change in temperature in either 50 x2AS2P1-2, where X2<X1 or y2<y 1. For layers 12 that are direction from a device that has no current flow. PbTe, layer 14 is typically Pb, Eu Te. For layer 12 that is The Second advantage is that thermionic structures are HgCdTe., layer 14 is typically HgCdTe., where leSS space-consuming than TE coolers. Although both ther X-X. For layer 12 that is HgCdSe, layer 14 is mionic and TE coolers typically use a Series connection HgCdSe, where X-X. Layer 14 can also be made of between cooler elements to achieve lower temperatures, 55 other materials, Such as Silicon oxide, aluminum oxide, thermionic coolers can be fabricated in a vertical manner, Vacuum, air, indium gallium arsenide antimonide, indium Such that each cooler is Stacked on top of the other, whereas, gallium aluminum nitride, bismuth telluride, bismuth for practical connectivity reasons, TE coolers need to be Selenide, boron nitride, Zinc telluride, Zinc Selenide, lead tin fabricated in a planar manner to achieve reasonably high telluride, aluminum antimonide, lead telluride, other Voltage and reasonably low currents. The planar TE coolers 60 insulators, other gases, or other gradations or modulations of can then be Stacked for increased cooling. materials for a given layer 12. The structure of FIG. 3B is an HTI device. The HTI

Detailed Drawings device 10 of FIG. 3B can be used as a thermal imaging

FIGS. 3A-3E are diagrams of a first embodiment of the System, wherein the device 10 is a single pixel or multiple present invention. 65 pixels of a thermal imaging System. FIG. 3A shows device 10 consisting initially of layer 12. FIG. 3C shows device 10 in another format, where anode Layer 12 is typically gallium arsenide (GaAs), but can be layer 16 is added to device 10 on top of layer 14. Anode

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layer 16 is typically made of the same material as layer 12, "hot electrons' because they are of Sufficient energy to carry but can be made of other materials. Because of the gradation heat away from the cold junction at transition 24. or modulation of layer 14, the difference in conduction There will also be electrons in layer 12 that do not have bandedge between layer 16 and layer 14 is larger than the enough energy to get Over the barrier at transition 24. These difference in conduction bandedge between layer 12 and electrons are called “cold electrons' because they are not of layer 14. This difference in conduction bandedge between Sufficient energy to carry heat away from the cold junction. the three layerS 12-16 creates, under an applied Voltage, a AS the hot electrons get Over transition 24, the hot large forward current from layer 12 to layer 16 and a small electrons encounter the conduction bandedge of layer 14. reverse current from layer 16 to layer 12. Within layer 14, Under bias voltage V 34, the level 26 of the conduction the gradation or modulation creates a Small electric field that bandedge of layer 14 becomes tilted in the other direction, assists electron movement away from layer 12 and through e.g., it is slightly "downhill,” and thus aids the hot electrons layer 14. in their travel away from the layer 12 and through the layer FIG. 3D shows a series of layers 14-20 on layer 12 to 14.

form device 10. This series connection creates two devices 10 back to back, and thus, multiple forward and reverse 15 electrons Once the hot electrons get to transition 30, the hot See a large energy drop because of the large barriers are created in Series. This Series connection places difference in conduction bandedge between layer 14 and the layers 14-20 in thermal series, which allows a larger layer 16. The electrons lose their energy to the lattice, temperature difference between the layer 12 and the final heating the lattice up, and heat up the junction between layer layer 20 of the device 10. Layers 14-20 can be successive barrier layers, alternating barrier layerS and Semiconductor 14 and layer 16, the hot junction of the device 10. layers, or any combination of barrier layers and Semicon make In the device 10, reversing the direction of the current will ductor layers. a heterostructure thermionic heater. Note that the same FIG. 3E is a diagram showing the selective removal of contact 10, device in contact with two heat baths (a hot bath in with layer 16 and a cold bath in contact with layer layer 14 in the device 10. After growing layer 16 on layer 14, 25 12) will produce electricity. This thermionic generator certain portions 14A of layer 14 can be removed by photo should operate at temperatures much lower than required in lithography and dry or wet Selective etching techniques. This Vacuum thermionic diode generators.

creates a vacuum Space in layer 14, and provides even lower thermal conductivity between layer 12 and layer 16. This under FIG. 4C is a graph of several devices 10 placed in series Structure allows for a precise cathode-anode Separation in an shown bias conditions. The device 10 has a structure as in FIG. 3D.

extremely Small Space, namely the thickness of layer 14.

Device 10 is a monolithic version of the original metal The barrier layer can be graded So do is Small or Zero, as Vacuum diodes. Layer 12 could be a negative electron shown in FIG. 4D. These barrier layers can also be stacked affinity material such as Aluminum Nitride (AIN), or coated as shown in FIG. 4E. The barrier layer does not have to be with an electronegative material Such as Cesium. linear; FIG. 4E shows that the barrier layers can have a

FIGS. 4A-4E are graphs of the conduction bandedge of curved bandedge. The bandedge of the barrier layer can take an n-type device 10, or the Valence bandedge of a p-type any shape.

device 10 made using the present invention. For a p-type By operating the devices 10 in a cascade arrangement, device 10, the increasing energy is in the downward direc which is a Series thermal arrangement, higher temperature tion. 40 differences can be achieved. The first Stage of the cascade

FIG. 4A is a graph of the conduction bandedge of the (layers 36,38, and 40) provides a low temperature heat sink device 10. Level 22 is the conduction bandedge of layer 12. for the second stage (layers 40, 42, and 44) which in turn Transition 24 is the change in conduction bandedge at the provides a temperature Sink at an even lower temperature for boundary between layer 12 and layer 14. This amount, the third stage (layers 44, 46, and 48). By adding more labeled d, is the difference in conduction bandedge at the 45 Stages, more temperature heat SinkScan be added, and larger cold side of the device 10. Level 26 is the conduction temperature differences can be achieved. It is necessary that bandedge acroSS layer 14. This layer is increasing as the the cooling capacity of the higher temperature stages (layers distance from boundary 24 increases. Distance 28 is the 36,38, and 40) be greater than those which operate at lower thickness of layer 14. Distance 28 is typically 0.01 to 1 temperatures (layers 40, 42, and 44 and layers 44, 46, and micron. 50

Transition 30 is the change in conduction bandedge at the For example, the first stage (layer 36, 38, and 40) should boundary between layer 14 and layer 16. This difference, have a cooling power equal to the Sum of the cooling labeled d, is the difference in conduction bandedge at the capacities of all of the other Stages in the device, in this case, hot side of device 10. If level 26 increases across the the Second Stage (layers 40, 42, and 44), and the third Stage distance of layer 14, the transition 30 will have a larger 55 (layers 44, 46, and 48). This can be achieved by constructing difference than transition 24. Level 32 is the conduction the layers in a pyramid structure, shown in FIG. 5. bandedge acroSS layer 16. d can be Small, or even Zero, if The structure of FIG. 5 can be fabricated by using the maximum bandedge is at the beginning of layer 14, as Selective wet etching, or reactive ion etching, or other shown in FIG. 4D. techniques. Current flows through device 10 by having FIG. 4B shows the conduction bandedge graph of FIG. 4A electrons injected at contact 50 and emerging from contact

when device 10 is being biased. A bias voltage V 34 is 52.

applied acroSS device 10, with the positive Voltage applied The current can be adjusted within each Stage by injecting at layer 16 and the negative Voltage applied at layer 12. The current into or withdrawing current from contacts 54 and 56 level 22 of the conduction bandedge of layer 12 will thus be on the Side of the pyramid.

increased by bias voltage 34. Certain electrons in layer 12 65 The Series connection places the devices 10 in the Stack in will have enough energy to be able to get Over transition 24 thermal Series, which is easily done during the construction and continue on through layer 14. These electrons are called of the device 10.

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The same device 10 can be p-doped, resulting in a device precise form disclosed. Many modifications and variations 10 that utilizes hole thermionic emission to perform the are possible in light of the above teaching. It is intended that cooling task. The p-doped device 10 uses the bandedge the scope of the invention be limited not with this detailed discontinuity in the Valence band to modify non-isothermal description, but rather by the claims appended hereto. current transport in the device 10, Similar to the conduction What is claimed is:

band discontinuity in the n-doped electron thermionic device 1. A thermionic cooling device, comprising: 10 previously described. a first Semiconductor layer with a first bandedge; Devices 10 that are optimized for electron current cooling a first barrier layer attached to the first Semiconductor are not necessarily optimized for hole current cooling. Thus, layer, wherein the first barrier layer has a higher band materials that have larger Valence bandedge discontinuities edge than the bandedge of the first Semiconductor layer and different effective masses may provide better cooling and Selectively allows charge carriers to travel from the performance under hole thermionic cooling than electron first Semiconductor layer via thermionic emission to thermionic cooling. Surmount the bandedge of the first barrier layer; and FIG. 4D shows the bandedge of a device 10 with a barrier a heat Sink, coupled to the first barrier layer, wherein the layer as the final layer of the device 10. The layer 58 is the 15 charge carriers that travel from the first Semiconductor initial layer, and under bias voltage 60, the barrier layer layer to Surmount the bandedge of the first barrier layer bandedge 62 will extend all the way down to level 64, which carry heat away from the first Semiconductor layer to is at one end of the voltage potential for the device 10. The the heat Sink.

bandedge 62 can take any shape. 2. The thermionic cooling device of claim 1, further FIG. 4E shows the bandedge of a device 10 that has comprising the first a Second Semiconductor layer attached between barrier layer and the heat sink.

Several devices 10 cascaded together. Layer 66 is coupled to 3. The thermionic cooling device of claim 2, further barrier layer 68. Barrier layer 68 is shown to have a curved comprising at least one additional barrier layer attached bandedge, but barrier layer 68 can have a bandedge of any between the Second Semiconductor layer and the heat Sink.

shape. Barrier layer 68 is directly coupled to another barrier 25 4. The thermionic layer 70, again shown with a curved bandedge. This cascade comprising at least a Second cooling device of claim 3, further connection continues, with barrier layer 70 coupled directly between the additional barrierSemiconductor layer attached layer and the heat sink.

to barrier layer 72, which is further coupled to barrier layer 5. The thermionic cooling device of claim 2, further 74, ending at layer 76. Layer 76 is coupled to the bias comprising at least one additional pair of alternating barrier voltage 78.

layerS and Semiconductor

FIG. 6 shows a cascaded device using Stages with differ ond Semiconductor layer and layers attached between the Sec ent bandedge discontinuities. The difference in bandedge the heat Sink. from level 80 to level 82 is not the same as the difference in 6. The thermionic cooling device of claim 1, further comprising at least one additional bandedge between level 84 and level 86. The difference in between the first barrier layer and the heat sink. barrier layer attached bandedge from level 84 to level 86 is not the same as the 35 7. The thermionic cooling device of claim 1, wherein a difference in bandedge between level 88 and level 90. The thickness of the first barrier layer is between 0.01 and 1 difference in bandedge from level 88 to level 90 is not the micron.

same as the difference in bandedge between level 92 and 8. The thermionic cooling device of claim 1, wherein the level 94. Thus, the cascade Structure can accommodate first Semiconductor layer is Selected from a group compris different cooling capacities. 40 ing gallium arsenide, indium phosphide, Silicon, Silicon FIG. 7 shows a combination of n-doped and p-doped germanium, lead telluride, indium gallium arsenide (In Ga devices as described in the present invention. The combi xAS), indium arsenide (InAs), indium antimonide (InSb), nation of n-doped and p-doped devices 10 can be used in indium gallium arsenide antimonide (InGaAs, Sb), electrical Series, Similar to conventional thermoelectric cool mercury cadmium telluride (HgCdTe), mercury cad ers. The cascade structure of FIG. 7 will increase the cooling 45 mium Selenide (HgCdSe), gallium nitride (GaN), alumi area of the device 10 without increasing the input current num gallium nitride (AlGaN), indium gallium nitride through the device 10. Electrons are injected at contact 50 (In GaN), indium arsenide phosphide (InASP), indium and emitted from contact 52. Electrons travel through each gallium arsenide phosphide (InGaASP), indium gal layer and contacts 56 in a Serpentine fashion, as in thermo lium aluminum arsenide (InGaAl, Sb), lead tin telluride electric coolers. 50 (PbSn-Te), aluminum arsenide (AIAS), aluminum anti General Considerations monide (AISb), zinc selenide (ZnSe), zinc telluride (ZnTe), boron nitride (BN), germanium (Ge), gallium phosphide

The use of thermionic coolers provides a method for (Gap), gallium antimonide (GaSb), gallium aluminum ars cooling electronics that is currently unavailable. The use of both electron and hole thermionic emission to cool electron 55 enide

(GaAl-AS), gallium arsenide phosphide (GaAs, P.

gallium indium phosphide (Gan-P), gallium indium ics will allow for Small thermionic coolers that can be antimonide (GanSb), bismuth telluride (Bi-Te), and fabricated as part of integrated circuits to allow those bismuth Selenide (Bi-Sea), where the Subscripts x, y, 1-X, circuits to be cooled while in use in various applications. and 1-y denote the relative amounts of the atomic Species in In the description of the preferred embodiment, reference each ternary or quartenary materials and range from Zero to is made to the accompanying drawings which form a part 60 one, inclusive.

hereof, and in which is shown by way of illustration the 9. The thermionic cooling device of claim 1, wherein the Specific embodiment in which the invention may be prac first barrier layer is Selected from a group comprising ticed. It is to be understood that other embodiments may be aluminum gallium arsenide, indium gallium arsenide utilized as Structural changes may be made without depart phosphide, Silicon germanium, lead europeum telluride, ing from the Scope of the present invention. 65 Silicon oxide, aluminum oxide, Vacuum, mercury cadmium The description of the preferred embodiment is not telluride, mercury cadmium Selenide, indium gallium arS intended to be exhaustive or to limit the invention to the enide antimonide, indium gallium aluminum nitride, bis

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muth telluride, bismuth Selenide, boron nitride, Zinc 22. The thermionic power generation device of claim 15, telluride, Zinc Selenide, lead tin telluride, aluminum wherein the first Semiconductor layer is Selected from a antimonide, lead telluride, and air. group comprising gallium arsenide, indium phosphide, 10. The thermionic cooling device of claim 2, wherein the Silicon, Silicon germanium, lead telluride, indium gallium Second Semiconductor layer is the same material as the first arsenide (InGaAs), indium arsenide (InAs), indium anti Semiconductor layer. monide (InSb), indium gallium arsenide antimonide 11. The thermionic cooling device of claim 1, wherein the (InGaAs, Sb), mercury cadmium telluride (HgCd. bandedge of the first barrier layer is piecewise linear. a Te), mercury cadmium Selenide (HgCdSe), gallium 12. The thermionic cooling device of claim 1, wherein the nitride (GaN), aluminum gallium nitride (AlGaN), first barrier layer is selectively removed. 1O indium gallium nitride (In GaN), indium arsenide phos 13. The thermionic cooling device of claim 1, wherein the phide (InASP), indium gallium arsenide phosphide first Semiconductor layer is n-type, and the bandedge is a (InGaAS, P), indium gallium aluminum arsenide conduction bandedge. (InGaAll-Sb), lead tin telluride (PbSn-Te), aluminum 14. The thermionic cooling device of claim 1, wherein the arsenide (AlAS), aluminum antimonide (AlSb), Zinc first Semiconductor layer is p-type, and the bandedge is a 15 selenide (ZnSe), zinc telluride (ZnTe), boron nitride (BN), Valence bandedge. germanium (Ge), gallium phosphide (GaP), gallium anti 15. A thermionic power generation device, comprising: monide (GaSb), gallium aluminum arsenide (GaAl-AS), a heat Source; gallium arsenide phosphide (GaAsP), gallium indium a heat Sink, phosphide (GanP), gallium indium antimonide (Gan aSb), bismuth telluride (BiTes), and bismuth selenide a first Semiconductor layer with a first bandedge; and (Bi-Sea), where the Subscripts x, y, 1-X, and 1-y denote the a first barrier layer attached to the first Semiconductor relative amounts of the atomic Species in each ternary or layer, wherein the first barrier layer has a higher band quartenary materials and range from Zero to one, inclusive. edge than the bandedge of the first Semiconductor layer 23. The thermionic power generation device of claim 15, and Selectively allows charge carriers to travel from the 25 wherein the first barrier layer is Selected from a group first Semiconductor layer via thermionic emission to comprising aluminum gallium arsenide, indium gallium Surmount the bandedge of the first barrier layer, arsenide phosphide, Silicon germanium, lead europeum wherein the charge carriers that travel from the first telluride, Silicon oxide, aluminum oxide, Vacuum, mercury Semiconductor layer to Surmount the bandedge of the cadmium telluride, mercury cadmium Selenide, indium gal first barrier layer are generated when the first Semicon lium arsenide antimonide, indium gallium aluminum nitride, ductor layer is exposed to the heat Source and the first bismuth telluride, bismuth Selenide, boron nitride, zinc barrier layer is exposed to the heat Sink. telluride, Zinc Selenide, lead tin telluride, aluminum 16. The thermionic power generation device of claim 15, antimonide, lead telluride, and air.

further comprising a Second Semiconductor layer attached 24. The thermionic power generation device of claim 15, between the first barrier layer and the heat sink, wherein the 35 wherein the Second Semiconductor layer is the same material first barrier layer is exposed to the heat Sink through the as the first Semiconductor layer.

Second Semiconductor layer. 25. The thermionic power generation device of claim 15, 17. The thermionic power generation device of claim 16, wherein the bandedge of the first barrier layer is piecewise further comprising at least one additional barrier layer linear.

attached between the Second Semiconductor layer and the 40 26. The thermionic power generation device of claim 15, heat Sink. wherein the first barrier layer is selectively removed. 18. The thermionic power generation device of claim 17, 27. The thermionic power generation device of claim 15, further comprising at least one additional Semiconductor wherein the first Semiconductor layer is n-type, and the layer attached between the additional barrier layer and the bandedge is a conduction bandedge.

heat Sink. 45 28. The thermionic power generation device of claim 15, 19. The thermionic power generation device of claim 16, wherein the first Semiconductor layer is p-type, and the further comprising at least one additional pair of alternating bandedge is a Valence bandedge.

barrier layers and Semiconductor layerS attached between 29. The thermionic power generation device of claim 15, the Second Semiconductor layer and the heat Sink. wherein the thermionic power generation device is a pixel of 20. The thermionic power generation device of claim 15, 50 an imaging System.

further comprising at least one additional barrier layer 30. The thermionic power generation device of claim 15, attached between the first barrier layer and the heat sink. wherein the thermionic power generation device forms more 21. The thermionic power generation device of claim 15, than one pixel of an imaging System.

wherein a thickness of the first barrier layer is between 0.01 and 1 micron. k k k k k

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UNITED STATES PATENT AND TRADEMARK OFFICE

CERTIFICATE OF CORRECTION

PATENT NO. : 5,955,772 Page 1 of

INVENTOR(S) : Ali Shakouri and John E. Bowers it is certified that error appears in the above-identified patent and that said Letters Patent is hereby corrected as shown below:

Column 5

Line 18, "O'B" should read -- (be -:

Line 18, "de" should read -- d'B-;

Column 6

Line 47, begin a new paragraph at "For layers 12.... Signed and Sealed this

Twenty-third Day of April, 2002

JAMESE. ROGAN

Attesting Officer Director of the United States Patent and Trademark Office