Thermionic Refrigeration

University of Notre DameDepartment of Electrical Engineering

Thermionic Refrigeration

Jeffrey A. BeanEE666 – Advanced Semiconductor

Devices

University of Notre Dame

EE666 - Thermionic Refrigeration

Outline

Types of refrigerationTypes of refrigeration

Application of each type in electronicsApplication of each type in electronics

Why the ‘fuss’ about cooling? Why the ‘fuss’ about cooling?

Thermionic refrigeration (TIR) in detailThermionic refrigeration (TIR) in detail

Current DevicesCurrent Devices

ImprovementsImprovements

Possible usesPossible uses



Types of RefrigerationCompressiveCompressive

Utilizes a refrigerant fluid and a compressorUtilizes a refrigerant fluid and a compressorEfficiency: ~30-50% of Carnot valueEfficiency: ~30-50% of Carnot value

ThermoelectricThermoelectric Utilizes materials which produce a Utilizes materials which produce a temperature gradient with potential across temperature gradient with potential across devicedevice

Efficiency: ~5-10% of Carnot valueEfficiency: ~5-10% of Carnot value

ThermionicThermionicUtilizes parallel materials separated by a Utilizes parallel materials separated by a small distance (either vacuum or other small distance (either vacuum or other material)material)

Efficiency: ~10-30% of Carnot valueEfficiency: ~10-30% of Carnot valueShakouri, A. and Bowers, J. E., Heterostructure Integrated Thermionic Refrigeration, 16th Int. Conf. on Thermoelectrics, pp. 636, 1997



Compressive Refrigeration

1) Refrigerant fluid is compressed (high 1) Refrigerant fluid is compressed (high pressure – temperature increases) pressure – temperature increases)

2) Fluid flows through an 2) Fluid flows through an expansion valve into low expansion valve into low pressure chamber (phase of pressure chamber (phase of refrigerant also changes)refrigerant also changes)

3) Coils absorb heat in the 3) Coils absorb heat in the devicedevice



Thermoelectric Refrigeration (TER)

A temperature difference between A temperature difference between the junctions of two dissimilar metal the junctions of two dissimilar metal wires produces a voltage potential wires produces a voltage potential (known as the Seebeck Effect) (known as the Seebeck Effect)Peltier cooling forces heat Peltier cooling forces heat flow from one side to the flow from one side to the other by applying an other by applying an external electric potential external electric potential

Thermoelectric generation Thermoelectric generation is utilized on deep space is utilized on deep space missions using a plutonium missions using a plutonium core as the heat source core as the heat source

http://www.dts-generator.com/main-e.htm



Thermionic Refrigeration (TIR)

Investigation into thermionic energy Investigation into thermionic energy conversion began in the 1950sconversion began in the 1950s

Utilizes fact that electrons with high Utilizes fact that electrons with high thermal energy (greater than the work thermal energy (greater than the work function) can escape from the metalfunction) can escape from the metal

General idea:General idea:A high work function A high work function metal cathode in contact metal cathode in contact with a heat source will with a heat source will emit electrons to a lower emit electrons to a lower work function anodework function anode

mH mC

Cathode AnodeVacuum

Barrier



Impact of Each Type on Electronics

Compressive Compressive Pros: efficient, high cooling power from ambientPros: efficient, high cooling power from ambient

Cons: bulky, expensive, noisy, power consumption, Cons: bulky, expensive, noisy, power consumption, scalingscaling

ThermoelectricThermoelectricPros: lightweight, small footprintPros: lightweight, small footprint

Cons: lousy efficiency, low cooling power from ambient, Cons: lousy efficiency, low cooling power from ambient, can’t be integrated on IC chips, power consumptioncan’t be integrated on IC chips, power consumption

ThermionicThermionicPros: integration on ICs using current technology, low Pros: integration on ICs using current technology, low powerpower

Cons: only support localized cooling, low cooling power Cons: only support localized cooling, low cooling power from ambient temperaturefrom ambient temperature



Why the ‘fuss’ about cooling?

Power dissipation in electronics Power dissipation in electronics is becoming a huge issue…is becoming a huge issue…

Intel

Processor Chip Power Density



Under an applied bias, ‘hot’ Under an applied bias, ‘hot’ electrons flow to the hot electrons flow to the hot side of the junctionside of the junctionRemoving the high energy Removing the high energy electrons from the cold side electrons from the cold side of the junction cools itof the junction cools itCharge neutrality is Charge neutrality is maintained by adding maintained by adding electrons adiabatically electrons adiabatically through an ohmic contactthrough an ohmic contactAmount of heat absorbed in Amount of heat absorbed in cathode is total current cathode is total current times the average energy of times the average energy of electrons emitted over the electrons emitted over the barrierbarrier

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



TER vs. TIR

Thermoelectric RefrigerationThermoelectric RefrigerationElectrons absorb energy from the latticeElectrons absorb energy from the lattice

Based on bulk properties of the semiconductorBased on bulk properties of the semiconductor

Electron transport is diffusiveElectron transport is diffusive

Thermionic RefrigerationThermionic RefrigerationElectron transport is ballisticElectron transport is ballistic

Selective emission of hot carriers from cathode Selective emission of hot carriers from cathode to anode yields higher efficiency than TERto anode yields higher efficiency than TER

Tunneling of lower energy carriers reduces efficiencyTunneling of lower energy carriers reduces efficiency



Thermionic RefrigerationThermionic devices are based on Thermionic devices are based on Richardson’s equationsRichardson’s equations

describes current per unit area emitted by a describes current per unit area emitted by a metal with work function metal with work function and temperature T and temperature T

Cathode barrier height as a function of currentCathode barrier height as a function of current

Mahan, G. D., “Thermionic Refrigeration”, J. Appl. Phys, Vol. 76 (7) , pp. 4362, 1994.



Practical thermionic refrigerators should emit at Practical thermionic refrigerators should emit at least least 1 A/cm1 A/cm22 from the cathode from the cathode

For room temperature operation, a work function of For room temperature operation, a work function of ~0.4eV is needed~0.4eV is needed

Most metal work functions are in the range of 4-5eVMost 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.

mm (eV) vs. Temperature (K) (eV) vs. Temperature (K)



Thermionic Refrigerator Issues

Lowering the barrier height to provide for room Lowering the barrier height to provide for room temperature coolingtemperature cooling

Metal-Vacuum-Metal thermionic refrigerators only Metal-Vacuum-Metal thermionic refrigerators only operate at high temperatures (>700K)operate at high temperatures (>700K)

Anode/Cathode spacingAnode/Cathode spacingUniformity of electrodesUniformity of electrodes

Proximity issuesProximity issues

Space charges in the vacuum regionSpace charges in the vacuum regionImpedes the flow of electrons from the anode to the Impedes the flow of electrons from the anode to the

cathode by introducing an extra potential cathode by introducing an extra potential barrierbarrier

Thermal conductivity (in semiconductor Thermal conductivity (in semiconductor devices)devices)



Barrier height problem solved!...kind of

Need materials with low barrier heightsNeed materials with low barrier heightsHeterostructures are perfect for this!Heterostructures are perfect for this!

Bandgap engineering Bandgap engineering Layer thickness and composition using epitaxial Layer thickness and composition using epitaxial growth techniques (MBE and MOCVD)growth techniques (MBE and MOCVD)Field assisted transport across barrierField assisted transport across barrier

Close and uniform spacing of anode and Close and uniform spacing of anode and cathode is no longer a problemcathode is no longer a problemSpace charge can be controlled by modulation Space charge can be controlled by modulation doping in the barrier regiondoping in the barrier regionAlloys can be used to create desired Schottky Alloys can be used to create desired Schottky barrier heights at contactsbarrier heights at contacts

Drawback: High thermal conductivity of Drawback: High thermal conductivity of semiconductors (compared to vacuum)semiconductors (compared to vacuum)



Heterostructure Cooling Power

Effective mass affects Effective mass affects the cooling performance the cooling performance by changing the density by changing the density of supply electrons and of supply electrons and electrons in the barrierelectrons in the barrier

This cooling power This cooling power reduces at lower reduces at lower temperatures because temperatures because the Fermi-Dirac the Fermi-Dirac distribution of electrons distribution of electrons narrows as T decreasesnarrows as T decreases

Shakouri, A. and Bowers, J. E., Heterostructure Integrated Thermionic Refrigeration, Appl. Phys. Lett. 71 (9), pp. 1234, 1997



Heterostructure Refrigeration

Electron mean free path Electron mean free path at at

300K is assumed to be 0.2300K is assumed to be 0.2mm

Barrier thickness L must be < Barrier thickness L must be <

mH

mC

L

Shakouri, A. and Bowers, J. E., Heterostructure Integrated Thermionic Refrigeration, 16th Int. Conf. on Thermoelectrics, pp. 636, 1997



Multilayer (Superlattice) Heterostructures

Overall thermal conductivity reduced to ~10% of the Overall thermal conductivity reduced to ~10% of the individual materials that compose itindividual materials that compose it

Efficiency increases 5-10 times over single barrier structuresEfficiency 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 TH=300K and TC=260K as a function of

Efficiency of a multiple barrier TIR where TH=300K and TC=260K as a function of



SiGe/Si Microcoolers

200 repeated layers of 3nmSi/12nmSi200 repeated layers of 3nmSi/12nmSi0.750.75GeGe0.250.25 superlattice (3superlattice (3m thick)m thick)

Grown on SiGrown on Si0.80.8GeGe0.20.2 buffer layer on Si substrate buffer layer on Si substrate

Mesa etch to define devicesMesa 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 Microcoolers

Optimum device size: 50x50 ~60x60Optimum device size: 50x50 ~60x60mm22

Author reports maximum cooling of 20-30ºC and Author reports maximum cooling of 20-30ºC and several thousands of W/cmseveral thousands of W/cm22 cooling power density cooling power density with optimized SiGe superlattic structureswith 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



Advantages of Heterostructure TIR

Compared to bulk thermoelectric Compared to bulk thermoelectric refrigeratorsrefrigerators

1) very small size and standard thin-film 1) very small size and standard thin-film fabrication - suitable for monolithic fabrication - suitable for monolithic integration on IC chipsintegration on IC chips

Possible to put refrigerator near active devices Possible to put refrigerator near active devices and cool hot spots directlyand cool hot spots directly

2) higher cooling power density2) higher cooling power density

3) transient response of SiGe/Si superlattice 3) transient response of SiGe/Si superlattice refrigerators is several orders of magnitude refrigerators is several orders of magnitude faster (10faster (1055 for these SiGe/Si for these SiGe/Si microrefrigerators)microrefrigerators)



Further ImprovementReduce thermal Reduce thermal conductivity (materials)conductivity (materials)The current limitation The current limitation in superlattice coolers in superlattice coolers is the contact is the contact resistance between the resistance between the metal and cap layer metal and cap layer

Ohmic contacts to a Ohmic contacts to a thermionic emission thermionic emission device (ballistic device (ballistic transport) will have a transport) will have a non-zero resistance due non-zero resistance due to joule heating from the to joule heating from the large current densitieslarge current densities

Maximum cooling for contact resistance of:

0 cm2 10-8 cm2 10-7 cm2 10-6 cm2

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 Improvements

Packaging is also an important Packaging is also an important aspect of the device optimizationaspect of the device optimization

Addition of a package between chip and Addition of a package between chip and heat sink adds another thermal barrierheat sink adds another thermal barrier

Use of Si or Cu packages aided in reducing Use of Si or Cu packages aided in reducing this thermal resistancethis thermal resistance

Optimizing length of wire bondsOptimizing length of wire bonds

These improvements have resulted These improvements have resulted in a maximum cooling increase of in a maximum cooling increase of >100%>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, 1997

Intersubband Light Emitting Cooler

Interband LEC



ConclusionsSmall area, localized cooling, can be Small area, localized cooling, can be implemented with current IC implemented with current IC fabrication techniquesfabrication techniquesWith optimization, current devices With optimization, current devices could provide:could provide:

Cooling of 20-30ºC for ~50x50 Cooling of 20-30ºC for ~50x50 mm22 areas areasSeveral thousands of W/cmSeveral thousands of W/cm22 cooling cooling power density power density

Further exotic structures could Further exotic structures could increase efficiency furtherincrease efficiency furtherQuestions???Questions???

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Thermionic Refrigeration