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Thermionic emission provides electron
beams.
The electron beam current (density) is given by the Richardson equation: |
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| j = A
· T 2 · exp – |
EA
kT |
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Atheo = 120 A
· cm–2 · K–2 for free electron
gas model
Aexp » (20 - 160) A ·
cm–2 · K–2 |
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EA = work function
» (2 - >6) eV |
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Materials of choice: W,
LaB6 single crystal |
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High field effects (tunneling,
barrier lowering) allow large currents at low T from small (nm)
size emitter |
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There are several thermoelectric
effects for metal junctions; always encountered in non-equilibrium. |
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Seebeck
effect:
Thermovoltage develops if a metal A-metal B junction is at a temperature
different form the "rest", i.e. if there is a temperature
gradeient |
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Essential for measuring (high) temperatures with a
"thermoelement"
Future use for efficient conversion of heat to electricity ??? |
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Peltier
effect:
Electrical current I through a metal - metal (or metal -
semiconductor) junction induces a temperature gradient µ I, i.e. one of the junction may
"cool down". |
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| Used for electrical cooling of (relatively small) devices.
Only big effect if electrical heating (µ
I2) is small. |
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© H. Föll (Electronic Materials - Script)
