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

2. University of Notre Dame EE666 - Thermionic Refrigeration 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

3. University of Notre Dame EE666 - Thermionic Refrigeration 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, 16 th Int. Conf. on Thermoelectrics, pp. 636, 1997

4. University of Notre Dame EE666 - Thermionic Refrigeration 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

5. University of Notre Dame EE666 - Thermionic Refrigeration 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

6. University of Notre Dame EE666 - Thermionic Refrigeration 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  mH  mC Cathode Anode Vacuum Barrier

7. University of Notre Dame EE666 - Thermionic Refrigeration 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

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

9. University of Notre Dame EE666 - Thermionic Refrigeration Refrigeration Terms Efficiency: Carnot Efficiency: Figure of Merit: Voltage =  T  electrical conductivity  thermal conductivity http://pubs.acs.org/hotartcl/cenear/000403/7814scit1.html

10. University of Notre Dame EE666 - Thermionic Refrigeration 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 How Thermionic Refrigerators Work  mH  mC Cathode Anode Structure under thermal equilibrium Structure under bias  mH  mC Cathode Anode E e - flow tunneling thermionic emission

11. University of Notre Dame EE666 - Thermionic Refrigeration 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

12. University of Notre Dame EE666 - Thermionic Refrigeration Thermionic Refrigeration Thermionic devices are based on Richardson’s equations describes current per unit area emitted by a metal with work function  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.

13. University of Notre Dame EE666 - Thermionic Refrigeration Practical thermionic refrigerators should emit at least 1 A/cm 2 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 Thermionic Refrigerator Operation Mahan, G. D., “Thermionic Refrigeration”, J. Appl. Phys, Vol. 76 (7), pp. 4363, 1994.  m (eV) vs. Temperature (K)

14. University of Notre Dame EE666 - Thermionic Refrigeration T L =T h =700K and T R =T c =500K Work functions:  =0.7eV 80% of Carnot efficiency Current: 1.3W/cm 2 Bias Voltage: 0.35V The total voltage over the barrier is such that the drop across the mean free path is a few kT Known as the Bethe criterion for thermionic emission Thermionic Refrigeration Example  mH  mC V Mahan, G. D., “Thermionic Refrigeration”, J. Appl. Phys, Vol. 76 (7), pp. 4364, 1994.

15. University of Notre Dame EE666 - Thermionic Refrigeration 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)

16. University of Notre Dame EE666 - Thermionic Refrigeration 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)

17. University of Notre Dame EE666 - Thermionic Refrigeration 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

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

19. University of Notre Dame EE666 - Thermionic Refrigeration 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 Mahan, G. D., J. O. Sofo, and M. Bartkowiak, “Multilayer thermionic refrigerator and generator”, J. Appl. Phys., Vol. 83 No. 9, pp. 4683, 1998 Efficiency of a single barrier TIR where T H =300K and T C =260K as a function of  Efficiency of a multiple barrier TIR where T H =300K and T C =260K as a function of 

20. University of Notre Dame EE666 - Thermionic Refrigeration SiGe/Si Microcoolers 200 repeated layers of 3nmSi/12nmSi 0.75 Ge 0.25 superlattice (3  m thick) Grown on Si 0.8 Ge 0.2 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

21. University of Notre Dame EE666 - Thermionic Refrigeration SiGe/Si Microcoolers Optimum device size: 50x50 ~60x60  m 2 Author reports maximum cooling of 20-30ºC and several thousands of W/cm 2 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

22. University of Notre Dame EE666 - Thermionic Refrigeration 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 (10 5 for these SiGe/Si microrefrigerators)

23. University of Notre Dame EE666 - Thermionic Refrigeration 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 Maximum cooling for contact resistance of: 0  cm2 10 -8  cm 2 10 -7  cm 2 10 -6  cm 2 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

24. University of Notre Dame EE666 - Thermionic Refrigeration 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%

25. University of Notre Dame EE666 - Thermionic Refrigeration 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 Shakouri, A. and Bowers, J. E., Heterostructure Integrated Thermionic Refrigeration, 16 th Int. Conf. on Thermoelectrics, pp. 636, 1997 Intersubband Light Emitting Cooler Interband LEC

26. University of Notre Dame EE666 - Thermionic Refrigeration 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  m 2 areas Several thousands of W/cm 2 cooling power density Further exotic structures could increase efficiency further Questions???