Metal-semiconductor Junctions -...
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1 Metal-semiconductor Junctions INEL 5209 - Solid State Electronics April 23, 2012
Transcript of Metal-semiconductor Junctions -...
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Metal-semiconductor Junctions
INEL 5209 - Solid State Electronics
April 23, 2012
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• Used to construct Schottky diodes.
Figure 1: Schottky diode.
• Energy-band diagrams are drawn so that the vacuum levelmatch.
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• Electrons in solid have negative energy with respect to that ofan electron in vacuum.
• The energy needed to remove an electron from the metal is thework function, qφm, is the difference between the vacuumenergy and the Fermi level.
Figure 2: Separated metal and semiconductor.
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• The Fermi level of an n-type semiconductor is higher that themetal’s. Conduction band electrons are at a higher energy inthe semiconductor than in the metal.
• When the metal-semiconductor is formed, electrons from thesemiconductor flow into the metal leaving behind ions andforming a depletion region.
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Figure 3: N-type Si and metal in contact.
• Thermal equilibrium is reached when the Fermi levels areequal at the interface.
• A potential barrier equal to the difference between the Fermi
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levels is formed.qV0 = qφm − qφs
• The barrier in the metal is
qφB = qφm − qξs
where ξs is the electron affinity (difference in energy betweenbottom of the conduction band and vacuum) of thesemiconductor.
• The reverse bias current in a Schottky diode is
IS = IS0e− qφB
kT
where IS0 is a constant that depend on temperature. Thisquantity plays the same role than the saturation current in thejunction diode.
• The forward-bias current in the Schottky diode can be
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modeled with the same equation used for the junction diode:
ID = ISeVDkT
• the build-in voltage V0 is smaller for Schottky diodes than forjunction diodes.
Figure 4: Reverse bias.
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Figure 5: Forward bias.
• In Schottky diodes all carriers are electrons. Thus there is nocharge storage and consequently Schottky diodes are fasterthan junction diodes. The cost is a larger IS .
• In the case of heavily-doped n-type semiconductor, a very
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narrow energy barrier is formed. This barrier is not highenough to prevent electrons from crossing. Also the depletionregion is very narrow and tunneling takes place. Since thebarrier is not effective to stop electrons from crossing acrossthe interface, an Ohmic contact is formed.
Figure 6: Ohmic contact.
