Thermionic RefrigerationJeffrey A. BeanEE666 Advanced Semiconductor Devices

Outline Types of refrigerationApplication of each type in electronicsWhy the fuss about cooling? Thermionic refrigeration (TIR) in detailCurrent DevicesImprovementsPossible uses

Types of RefrigerationCompressiveUtilizes a refrigerant fluid and a compressorEfficiency: ~30-50% of Carnot valueThermoelectric Utilizes materials which produce a temperature gradient with potential across deviceEfficiency: ~5-10% of Carnot valueThermionicUtilizes parallel materials separated by a small distance (either vacuum or other material) Efficiency: ~10-30% of Carnot valueShakouri, A. and Bowers, J. E., Heterostructure Integrated Thermionic Refrigeration, 16th Int. Conf. on Thermoelectrics, pp. 636, 1997

Compressive Refrigeration1) 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

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 potentialThermoelectric generation is utilized on deep space missions using a plutonium core as the heat sourcehttp://www.dts-generator.com/main-e.htm

Thermionic Refrigeration (TIR)Investigation into thermionic energy conversion began in the 1950sUtilizes fact that electrons with high thermal energy (greater than the work function) can escape from the metalGeneral idea:A high work function metal cathode in contact with a heat source will emit electrons to a lower work function anodeVacuum Barrier

Impact of Each Type on ElectronicsCompressive Pros: efficient, high cooling power from ambientCons: bulky, expensive, noisy, power consumption, scalingThermoelectricPros: lightweight, small footprintCons: lousy efficiency, low cooling power from ambient, cant be integrated on IC chips, power consumptionThermionicPros: integration on ICs using current technology, low powerCons: only support localized cooling, low cooling power from ambient temperature

Why the fuss about cooling?Power dissipation in electronics is becoming a huge issueIntelProcessor Chip Power Density

Refrigeration Terms

Efficiency:

Carnot Efficiency:

Figure of Merit:

Voltage = aDTa - electrical conductivityk - thermal conductivityhttp://pubs.acs.org/hotartcl/cenear/000403/7814scit1.html

How Thermionic Refrigerators WorkUnder an applied bias, hot electrons flow to the hot side of the junctionRemoving the high energy electrons from the cold side of the junction cools itCharge neutrality is maintained by adding electrons adiabatically through an ohmic contactAmount of heat absorbed in cathode is total current times the average energy of electrons emitted over the barrierStructure under thermal equilibriumStructure under bias

TER vs. TIRThermoelectric RefrigerationElectrons absorb energy from the latticeBased on bulk properties of the semiconductorElectron transport is diffusiveThermionic RefrigerationElectron transport is ballisticSelective emission of hot carriers from cathode to anode yields higher efficiency than TERTunneling of lower energy carriers reduces efficiency

Thermionic RefrigerationThermionic devices are based on Richardsons equationsdescribes 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.

Thermionic Refrigerator Operationfm (eV) vs. Temperature (K)Practical thermionic refrigerators should emit at least 1 A/cm2 from the cathode

For room temperature operation, a work function of ~0.4eV is neededMost metal work functions are in the range of 4-5eVMahan, G. D., Thermionic Refrigeration, J. Appl. Phys, Vol. 76 (7) , pp. 4363, 1994.

Thermionic Refrigeration ExampleTL=Th=700K and TR=Tc=500KWork functions: f=0.7eV

80% of Carnot efficiencyCurrent: 1.3W/cm2Bias Voltage: 0.35V

The total voltage over the barrier is such that the drop across the mean free path is a few kTKnown as the Bethe criterion for thermionic emissionfmHfmCVMahan, G. D., Thermionic Refrigeration, J. Appl. Phys, Vol. 76 (7) , pp. 4364, 1994.

Thermionic Refrigerator IssuesLowering the barrier height to provide for room temperature coolingMetal-Vacuum-Metal thermionic refrigerators only operate at high temperatures (>700K)Anode/Cathode spacingUniformity of electrodesProximity issuesSpace charges in the vacuum regionImpedes the flow of electrons from the anode to the cathode by introducing an extra potential barrierThermal conductivity (in semiconductor devices)

Barrier height problem solved!...kind ofNeed materials with low barrier heightsHeterostructures are perfect for this!Bandgap engineering Layer thickness and composition using epitaxial growth techniques (MBE and MOCVD)Field assisted transport across barrierClose and uniform spacing of anode and cathode is no longer a problemSpace charge can be controlled by modulation doping in the barrier regionAlloys can be used to create desired Schottky barrier heights at contactsDrawback: High thermal conductivity of semiconductors (compared to vacuum)

Heterostructure Cooling PowerEffective mass affects the cooling performance by changing the density of supply electrons and electrons in the barrierThis cooling power reduces at lower temperatures because the Fermi-Dirac distribution of electrons narrows as T decreasesShakouri, A. and Bowers, J. E., Heterostructure Integrated Thermionic Refrigeration, Appl. Phys. Lett. 71 (9), pp. 1234, 1997

Heterostructure RefrigerationElectron mean free path l at 300K is assumed to be 0.2mmBarrier thickness L must be < lfmHfmCLShakouri, A. and Bowers, J. E., Heterostructure Integrated Thermionic Refrigeration, 16th Int. Conf. on Thermoelectrics, pp. 636, 1997

Multilayer (Superlattice) HeterostructuresOverall thermal conductivity reduced to ~10% of the individual materials that compose itEfficiency 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, 1998Efficiency of a single barrier TIR where TH=300K and TC=260K as a function of fEfficiency of a multiple barrier TIR where TH=300K and TC=260K as a function of f

SiGe/Si Microcoolers200 repeated layers of 3nmSi/12nmSi0.75Ge0.25 superlattice (3mm thick)Grown on Si0.8Ge0.2 buffer layer on Si substrateMesa etch to define devicesShakouri, 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

SiGe/Si MicrocoolersOptimum device size: 50x50 ~60x60mm2Author reports maximum cooling of 20-30C and several thousands of W/cm2 cooling power density with optimized SiGe superlattic structuresShakouri, 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

Advantages of Heterostructure TIRCompared to bulk thermoelectric refrigerators1) very small size and standard thin-film fabrication - suitable for monolithic integration on IC chipsPossible to put refrigerator near active devices and cool hot spots directly2) higher cooling power density3) transient response of SiGe/Si superlattice refrigerators is several orders of magnitude faster (105 for these SiGe/Si microrefrigerators)

Further ImprovementReduce 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

More ImprovementsPackaging is also an important aspect of the device optimizationAddition of a package between chip and heat sink adds another thermal barrierUse of Si or Cu packages aided in reducing this thermal resistanceOptimizing length of wire bondsThese improvements have resulted in a maximum cooling increase of >100%

Light EmissionHeat flowing in the reverse direction to the thermionic emission due to lattice heat conduction reduces the temperature difference and destroys efficiencyOpto-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, 16th Int. Conf. on Thermoelectrics, pp. 636, 1997Intersubband Light Emitting CoolerInterband LEC

ConclusionsSmall area, localized cooling, can be implemented with current IC fabrication techniquesWith optimization, current devices could provide:Cooling of 20-30C for ~50x50 mm2 areasSeveral thousands of W/cm2 cooling power density Further exotic structures could increase efficiency furtherQuestions???