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Thermionic Refrigeration

Thermionic Refrigeration. Jeffrey A. Bean EE666 – Advanced Semiconductor Devices. Outline. Types of refrigeration Application of each type in electronics Why the ‘fuss’ about cooling? Thermionic refrigeration (TIR) in detail Current Devices Improvements Possible uses.

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Thermionic Refrigeration

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  1. Thermionic Refrigeration Jeffrey A. Bean EE666 – Advanced Semiconductor Devices University of Notre Dame Department of Electrical Engineering

  2. Outline • Types of refrigeration • Application of each type in electronics • Why the ‘fuss’ about cooling? • Thermionic refrigeration (TIR) in detail • Current Devices • Improvements • Possible uses University of Notre Dame EE666 - Thermionic Refrigeration

  3. Types of Refrigeration • Compressive • Utilizes a refrigerant fluid and a compressor • Efficiency: ~30-50% of Carnot value • Thermoelectric • Utilizes materials which produce a temperature gradient with potential across device • Efficiency: ~5-10% of Carnot value • Thermionic • Utilizes parallel materials separated by a small distance (either vacuum or other material) • Efficiency: ~10-30% of Carnot value Shakouri, A. and Bowers, J. E., Heterostructure Integrated Thermionic Refrigeration, 16th Int. Conf. on Thermoelectrics, pp. 636, 1997 University of Notre Dame EE666 - Thermionic Refrigeration

  4. Compressive Refrigeration • 1) Refrigerant fluid is compressed (high pressure – temperature increases) 2) Fluid flows through an expansion valve into low pressure chamber (phase of refrigerant also changes) 3) Coils absorb heat in the device University of Notre Dame EE666 - Thermionic Refrigeration

  5. Thermoelectric Refrigeration (TER) • A temperature difference between the junctions of two dissimilar metal wires produces a voltage potential (known as the Seebeck Effect) • Peltier cooling forces heat flow from one side to the other by applying an external electric potential • Thermoelectric generation is utilized on deep space missions using a plutonium core as the heat source http://www.dts-generator.com/main-e.htm University of Notre Dame EE666 - Thermionic Refrigeration

  6. fmH fmC Cathode Anode Thermionic Refrigeration (TIR) • Investigation into thermionic energy conversion began in the 1950s • Utilizes fact that electrons with high thermal energy (greater than the work function) can escape from the metal • General idea: • A high work function metal cathode in contact with a heat source will emit electrons to a lower work function anode Vacuum Barrier University of Notre Dame EE666 - Thermionic Refrigeration

  7. Impact of Each Type on Electronics • Compressive • Pros: efficient, high cooling power from ambient • Cons: bulky, expensive, noisy, power consumption, scaling • Thermoelectric • Pros: lightweight, small footprint • Cons: lousy efficiency, low cooling power from ambient, can’t be integrated on IC chips, power consumption • Thermionic • Pros: integration on ICs using current technology, low power • Cons: only support localized cooling, low cooling power from ambient temperature University of Notre Dame EE666 - Thermionic Refrigeration

  8. Why the ‘fuss’ about cooling? • Power dissipation in electronics is becoming a huge issue… Processor Chip Power Density Intel University of Notre Dame EE666 - Thermionic Refrigeration

  9. thermionic emission fmC tunneling Anode fmH E Cathode e- flow fmH fmC Cathode Anode How Thermionic Refrigerators Work • Under an applied bias, ‘hot’ electrons flow to the hot side of the junction • Removing the high energy electrons from the cold side of the junction cools it • Charge neutrality is maintained by adding electrons adiabatically through an ohmic contact • Amount of heat absorbed in cathode is total current times the average energy of electrons emitted over the barrier Structure under thermal equilibrium Structure under bias University of Notre Dame EE666 - Thermionic Refrigeration

  10. TER vs. TIR • Thermoelectric Refrigeration • Electrons absorb energy from the lattice • Based on bulk properties of the semiconductor • Electron transport is diffusive • Thermionic Refrigeration • Electron transport is ballistic • Selective emission of hot carriers from cathode to anode yields higher efficiency than TER • Tunneling of lower energy carriers reduces efficiency University of Notre Dame EE666 - Thermionic Refrigeration

  11. Thermionic Refrigeration • Thermionic devices are based on Richardson’s equations • describes current per unit area emitted by a metal with work function f and temperature T • Cathode barrier height as a function of current Mahan, G. D., “Thermionic Refrigeration”, J. Appl. Phys, Vol. 76 (7) , pp. 4362, 1994. University of Notre Dame EE666 - Thermionic Refrigeration

  12. Thermionic Refrigerator Operation • Practical thermionic refrigerators should emit at least 1 A/cm2 from the cathode • For room temperature operation, a work function of ~0.4eV is needed • Most metal work functions are in the range of 4-5eV • fm (eV) vs. Temperature (K) Mahan, G. D., “Thermionic Refrigeration”, J. Appl. Phys, Vol. 76 (7) , pp. 4363, 1994. University of Notre Dame EE666 - Thermionic Refrigeration

  13. Thermionic Refrigerator Issues • Lowering the barrier height to provide for room temperature cooling • Metal-Vacuum-Metal thermionic refrigerators only operate at high temperatures (>700K) • Anode/Cathode spacing • Uniformity of electrodes • Proximity issues • Space charges in the vacuum region • Impedes the flow of electrons from the anode to the cathode by introducing an extra potential barrier • Thermal conductivity (in semiconductor devices) University of Notre Dame EE666 - Thermionic Refrigeration

  14. Barrier height problem solved!...kind of • Need materials with low barrier heights • Heterostructures are perfect for this! • Bandgap engineering • Layer thickness and composition using epitaxial growth techniques (MBE and MOCVD) • Field assisted transport across barrier • Close and uniform spacing of anode and cathode is no longer a problem • Space charge can be controlled by modulation doping in the barrier region • Alloys can be used to create desired Schottky barrier heights at contacts • Drawback: High thermal conductivity of semiconductors (compared to vacuum) University of Notre Dame EE666 - Thermionic Refrigeration

  15. Heterostructure Cooling Power • Effective mass affects the cooling performance by changing the density of supply electrons and electrons in the barrier • This cooling power reduces at lower temperatures because the Fermi-Dirac distribution of electrons narrows as T decreases Shakouri, A. and Bowers, J. E., Heterostructure Integrated Thermionic Refrigeration, Appl. Phys. Lett. 71 (9), pp. 1234, 1997 University of Notre Dame EE666 - Thermionic Refrigeration

  16. Heterostructure Refrigeration • Electron mean free path l at 300K is assumed to be 0.2mm • Barrier thickness L must be < l fmC fmH L Shakouri, A. and Bowers, J. E., Heterostructure Integrated Thermionic Refrigeration, 16th Int. Conf. on Thermoelectrics, pp. 636, 1997 University of Notre Dame EE666 - Thermionic Refrigeration

  17. Multilayer (Superlattice) Heterostructures • Overall thermal conductivity reduced to ~10% of the individual materials that compose it • Efficiency increases 5-10 times over single barrier structures Efficiency of a single barrier TIR where TH=300K and TC=260K as a function of f Efficiency of a multiple barrier TIR where TH=300K and TC=260K as a function of f Mahan, G. D., J. O. Sofo, and M. Bartkowiak, “Multilayer thermionic refrigerator and generator”, J. Appl. Phys., Vol. 83 No. 9, pp. 4683, 1998 University of Notre Dame EE666 - Thermionic Refrigeration

  18. SiGe/Si Microcoolers • 200 repeated layers of 3nmSi/12nmSi0.75Ge0.25 superlattice (3mm thick) • Grown on Si0.8Ge0.2 buffer layer on Si substrate • Mesa etch to define devices Shakouri, A. and Zhang, Y., On-Chip Solid-State Cooling for ICs Using Thin-Film Microrefrigerators, IEEE Trans. On Comp. and Pack. Tech., Vol. 28 No. 1, pp. 66, 2005 University of Notre Dame EE666 - Thermionic Refrigeration

  19. SiGe/Si Microcoolers • Optimum device size: 50x50 ~60x60mm2 • Author reports maximum cooling of 20-30ºC and several thousands of W/cm2 cooling power density with optimized SiGe superlattic structures Shakouri, A. and Zhang, Y., On-Chip Solid-State Cooling for ICs Using Thin-Film Microrefrigerators, IEEE Trans. On Comp. and Pack. Tech., Vol. 28 No. 1, pp. 67, 2005 University of Notre Dame EE666 - Thermionic Refrigeration

  20. Advantages of Heterostructure TIR • Compared to bulk thermoelectric refrigerators • 1) very small size and standard thin-film fabrication - suitable for monolithic integration on IC chips • Possible to put refrigerator near active devices and cool hot spots directly • 2) higher cooling power density • 3) transient response of SiGe/Si superlattice refrigerators is several orders of magnitude faster (105 for these SiGe/Si microrefrigerators) University of Notre Dame EE666 - Thermionic Refrigeration

  21. Maximum cooling for contact resistance of: 0 Wcm2 10-8Wcm2 10-7Wcm2 10-6Wcm2 Further Improvement • Reduce thermal conductivity (materials) • The current limitation in superlattice coolers is the contact resistance between the metal and cap layer • Ohmic contacts to a thermionic emission device (ballistic transport) will have a non-zero resistance due to joule heating from the large current densities Ulrich, M. D., P. A. Barnes, and C. B. Vining, “Effect of contact resistance in solid-state thermionic emission”, J. Appl. Phys., Vol. 92 No. 1, pp. 245, 2002 University of Notre Dame EE666 - Thermionic Refrigeration

  22. More Improvements • Packaging is also an important aspect of the device optimization • Addition of a package between chip and heat sink adds another thermal barrier • Use of Si or Cu packages aided in reducing this thermal resistance • Optimizing length of wire bonds • These improvements have resulted in a maximum cooling increase of >100% University of Notre Dame EE666 - Thermionic Refrigeration

  23. Light Emission • Heat flowing in the reverse direction to the thermionic emission due to lattice heat conduction reduces the temperature difference and destroys efficiency • Opto-thermionic refrigeration gets the thermionic carriers: e- from n-doped and h+ from p-doped semiconductor from each side could recombine radiatively Intersubband Light Emitting Cooler Interband LEC Shakouri, A. and Bowers, J. E., Heterostructure Integrated Thermionic Refrigeration, 16th Int. Conf. on Thermoelectrics, pp. 636, 1997 University of Notre Dame EE666 - Thermionic Refrigeration

  24. Conclusions • Small area, localized cooling, can be implemented with current IC fabrication techniques • With optimization, current devices could provide: • Cooling of 20-30ºC for ~50x50 mm2 areas • Several thousands of W/cm2 cooling power density • Further exotic structures could increase efficiency further • Questions??? University of Notre Dame EE666 - Thermionic Refrigeration

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