University of Notre Dame

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Thermionic Refrigeration

• Jeffrey A. Bean
• EE666 Advanced Semiconductor Devices

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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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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
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

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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
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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
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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, cant 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

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Why the fuss about cooling?

• Power dissipation in electronics is becoming a
  huge issue

Processor Chip Power Density
Intel
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Refrigeration Terms

• Efficiency
• Carnot Efficiency
• Figure of Merit
• Voltage aDT
• a - electrical conductivity
• k - thermal conductivity

http//pubs.acs.org/hotartcl/cenear/000403/7814sci
t1.html
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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
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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

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Thermionic Refrigeration

• Thermionic devices are based on Richardsons
  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.
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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.
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Thermionic Refrigeration Example

• TLTh700K and TRTc500K
• Work functions f0.7eV
• 80 of Carnot efficiency
• Current 1.3W/cm2
• 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

fmC
fmH
V
Mahan, G. D., Thermionic Refrigeration, J.
Appl. Phys, Vol. 76 (7) , pp. 4364, 1994.
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Thermionic Refrigerator Issues

• Lowering the barrier height to provide for room
  temperature cooling
• Metal-Vacuum-Metal thermionic refrigerators only
  operate at high temperatures (gt700K)
• 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)

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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)

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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
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Heterostructure Refrigeration

• Electron mean free path l at
• 300K is assumed to be 0.2mm
• Barrier thickness L must be lt l

fmC
fmH
L
Shakouri, A. and Bowers, J. E., Heterostructure
Integrated Thermionic Refrigeration, 16th Int.
Conf. on Thermoelectrics, pp. 636, 1997
19
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 TH300K
and TC260K as a function of f
Efficiency of a multiple barrier TIR where
TH300K and TC260K 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
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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
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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
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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)

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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
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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 gt100

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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
26
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???