US 20020033188A1 (19) United States (12) Patent Application Publication (10) Pub. N0.2 US 2002/0033188 A1 Shakouri et al. (43) Pub. Date: Mar. 21, 2002 (54) HETEROSTRUCTURE THERMIONIC Publication Classi?cation COOLERS (76) Inventors: Ali Shakouri, Santa Barbara, CA (US); (51) Int. Cl.7 ........................ .. H01L 35/34; H01L 21/00; John E. Bowers, Santa Barbara, CA H01L 37/00; H01L 35/30 (US) (52) us. Cl. ................. .. 136/201; 136/205; 252/623 T; 257/467; 438/22 Correspondence Address: Attention: George H. Gates Gates & Cooper LLP (57) ABSTRACT Howard Hughes Center 2221:5133“: lélgvgogifsstzgglte 1050 A heterostructure thermionic cooler and a method for mak ’ ing thermionic coolers, employing a barrier layer of varying (21) APPL N0. 09 976,173 conduction bandedge for n-type material, or varying valence bandedge for p-type material, that is placed betWeen tWo (22) Filed; ()CL 12, 2001 layers of material. The barrier layer has a high enough barrier for the cold side to only alloW “hot” electrons, or Related US, Application Data electrons of high enough energy, across the barrier. The barrier layer is constructed to have an internal electric ?eld (60) Continuation of application No. 09/548,011, ?led on such that the electrons that make it over the initial barrier are Apr. 12, 2000, noW Pat. No. 6,323,414, Which is a assisted in travel to the anode. Once electrons drop to the continuation of application No. 09/280,284, ?led on energy level of the anode, they lose energy to the lattice, thus Mar. 29, 1999, noW Pat. No. 6,060,331, Which is a heating the lattice at the anode. The barrier height of the division of application No. 08/767,935, ?led on Dec. barrier layer is high enough to prevent the electrons from 17, 1996, noW Pat. No. 5,955,772. traveling in the reverse direction. Patent Application Publication Mar. 21, 2002 Sheet 1 0f 8 US 2002/0033188 A1 Cr,» . Patent Application Publication Mar. 21, 2002 Sheet 2 0f 8 US 2002/0033188 A1 thermlomcllhermoeleclrlccapacltyCooling M. l Patent Application Publication Mar. 21, 2002 Sheet 3 0f 8 US 2002/0033188 A1 HqB/i » a 54.3.3 Fl? F1456 H439 Patent Application Publication Mar. 21, 2002 Sheet 4 0f 8 US 2002/0033188 A1 I'M \ I?“ »-/+ Patent Application Publication Mar. 21, 2002 Sheet 6 0f 8 US 2002/0033188 A1 Patent Application Publication Mar. 21, 2002 Sheet 7 0f 8 US 2002/0033188 A1 .50 M 92,] '15 ""~\\\\\\\\\\\\\§§?”% “\\\\\\\ /55L 56 ~ 44, Patent Application Publication Mar. 21, 2002 Sheet 8 0f 8 US 2002/0033188 A1 51 //D (54 ( 5Q \50 US 2002/0033188 A1 Mar. 21, 2002 HETEROSTRUCTURE THERMIONIC COOLERS the case of electron transport) Which has its maXimum bigger than the ?rst layer. Selective thermionic emission of [0001] This invention Was made With Government support high energy carriers from cathode to anode (and suppressing under Contract No. F49620-96-1-0349, awarded by the Air Force. The Government has certain rights in this invention. the reverse current) Will create a cold junction at cathode and a hot junction at anode. This device Will function at room temperature and beloW. Using the same device in contact BACKGROUND OF THE INVENTION With a hot and a cold bath Will create a thermionic generator [0002] 1. Field of the Invention. Which also Works at room temperature and beloW. [0003] This invention relates in general to electronic [0011] One object of the present invention is to provide devices, and more speci?cally to the ?rst use of semicon better electronic cooler fabrication techniques. It is a further ductor materials to fabricate thermionic coolers and genera object of the invention to reduce electronic cooler fabrica tors. tion costs. It is a further object of the invention to make more ef?cient electronic coolers Which reach loWer temperatures. [0004] 2. Description of Related Art. [0012] These and various other advantages and features of [0005] The use of electronics to transport heat to and aWay novelty Which characteriZe the invention are pointed out from certain areas has expanded in recent years due to With particularity in the claims annexed hereto and form a increased packing densities and hostile environments. For part hereof. HoWever, for a better understanding of the cooling applications, thermoelectric coolers (TE Coolers) invention, its advantages, and the objects obtained by its use, have been used to cool areas both in electronic and non reference should be made to the draWings Which form a electronic applications. TE coolers are typically a p-type further part hereof, and to accompanying descriptive matter, doped region alternatively connected to an n-type doped in Which there are illustrated and described speci?c region, Which creates cooling effects at one metal-doped eXamples of an apparatus in accordance With the invention. region junction and heating effects at the other metal-doped region junction, depending on the direction of the current BRIEF DESCRIPTION OF THE DRAWINGS through the device. [0013] Referring noW to the draWings in Which like ref [0006] HoWever, TE coolers are limited in their overall erence numbers represent corresponding parts throughout: performance by the bulk properties of the materials used in the TE cooler. More ef?cient cooling is needed in many [0014] FIG. 1 shoWs the overall cooling poWer for ther applications. Reliability of assemblies of many elements is mionic devices and thermoelectric devices; often not suf?cient for many high reliability designs. The [0015] FIG. 2 shoWs the ratio of the thermionic cooling cost of TE coolers has not plummeted at the same rate as term over the Peltier cooling term as a function of current; other electronic devices such as transistor circuits, lasers and detectors, because TE cooler elements are not fabricated [0016] FIGS. 3A-3E are diagrams of a ?rst embodiment of using high volume planar integrated circuit technology. the present invention; Further, TE coolers that can generate a large cooling effect [0017] FIGS. 4A-4E are graphs of the conduction band tend to be large devices, typically 1 cm ><1 cm or larger and edge of devices made using the present invention; thus, are not acceptable in small electronic devices. [0018] FIG. 5 shoWs an alternative structure for the device [0007] It can be seen then that there is a need for better 10; electronic coolers. It can also be seen then that there is a need for better electronic cooler fabrication techniques. It [0019] FIG. 6 shoWs a cascaded device using stages With can also be seen that there is a need for loW cost electronic different bandedge discontinuities; and coolers. It can also be seen that there is a need for more space [0020] FIG. 7 shoWs a combination of n-doped and ef?cient electronic coolers. It can also be seen that there is p-doped devices as described in the present invention. a need for more energy ef?cient electronic coolers. It can also be seen that there is a need for more reliable electronic DETAILED DESCRIPTION OF THE coolers. It can also be seen that there is a need for electronic INVENTION coolers that reach loWer temperatures. [0021] OvervieW SUMMARY OF THE INVENTION [0022] The present invention uses thermionic emission in semiconductor heterostructures for heat pumping and cool [0008] The present invention discloses a poWerful and highly productive semiconductor thermionic cooler. ing of high poWer electronic and optoelectronic devices. These integrated micro-coolers can improve the ef?ciency [0009] The present invention minimiZes the above-de and lifetime of electrical and optoelectronic components. scribed problems by using bandgap engineering and modu The thermionic coolers could also be used as an additional lation doping to fabricate small thermionic coolers that means for tuning temperature sensitive devices. operate at room temperature. By using proper materials and geometries, ef?cient and space conserving thermionic cooler [0023] The coolers that are commercially available are typically thermoelectric (TE) coolers, based on the Peltier elements Which can reach loWer temperatures are fabricated effect at the junction of tWo dissimilar materials. TE coolers in a cost-effective manner. use materials bulk properties, such as the Seebeck coef? [0010] The principles of the present invention comprise cient, electrical and thermal conductivity, and are mostly groWing tWo semiconductor layers. The second layer has a based on Bismuth Telluride (Bi2Te3) for room temperature variable conduction bandedge as a function of distance (for applications. The basis of the heterostructure thermionic US 2002/0033188 A1 Mar. 21, 2002 (HTI) cooler described here is to use bandstructure engi [0031] The materials used for TE cooling application are neering to increase the cooling poWer and efficiency. usually described by the lineariZed BoltZmann equation and small perturbation of the electronic distribution function by [0024] Recent proposals to use quantum Wells, quantum external ?elds and temperatures. In contrast, the heterostruc Wires, and superlattice structures to increase the TE cooler ture thermionic device is based on a large perturbation of the ?gure. of merit can be divided into tWo categories. The ?rst electronic distribution function. category changes the density of electronic states of the cooler materials to make it more “peaked” and also more [0032] Thermoelectric Cooler Modeling asymmetric With respect to the Fermi energy. This Will [0033] A single element TE cooler is composed of tWo increase the electrical poWer factor, S20, and thus the TE branches, one branch of n-doped and one branch of p-doped cooler ?gure of merit Z=s2o/[3, Where S is the Seebeck material. coef?cient or thermopoWer, o is the electrical conductivity, and [3 is the thermal conductivity. [0034] The tWo branches are connected electrically in series and thermally in parallel.