E E 339 - CN15-metal semiconductor junctions

E E 339 - CN15-metal semiconductor junctions -...

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1 Metal-Semiconductor Junctions Metal Semiconductor Junctions 1 “Ideal” Rectifying/Schottky contacts and ohmic contacts Schottky barrier contacts/diodes: equilibrium conditions Outline current flow under bias non-ideal/real diodes capacitance and switching Ohmic contacts ideal real and via tunneling Summary 2
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2 There are two basic types of metal- semicnductor junctions: Rectifying ” junctions “diodes” that I V rectifying Junctions type: “rectifying” vs. “ohmic” behave much but not entirely like a p-n junction. Ohmic contacts that provide low resistance coupling between semiconductors and metal for current flow in either direction . I V ohmic 3 Critical parameters for determining type are: q χ = ( E vacuum –E c )| bulk semiconductor = semiconductor “ electron affinity = material dependent only vacuum (energy) level Φ s ( F )| =semiconductor“work function material & doping dependent Φ m = ( )| bulk metal = metal “work function material dependent only As well as the semiconductor band gap (Note that is fixed with respect to the metal band structure and there are allowed (eigen) energy states available at , so there is no need to define an electron affinity.) Figure 5—40 (a), modified band diagrams for the metal and the semiconductor before joining 4
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3 When the metal and the semiconductor are brought into contact, for an “ideal ” contact , the relative band alignments of metal adequately represented by q Φ m and the semiconductor q Φ –q χ must be maintained at the interface. Figure 5—40 (b), modified equilibrium band diagram for the junction. contact potential In addition, Φ s χ ( = E c –E F ) must be maintained in the semiconductor quasi neutral regions sufficientlyfarfromthe junction q Φ s –q χ depletion sufficiently far from the junction. In equilibrium , E must be position independent, requiring a band bending of Φ Φ = ±qV o = ±q · (contact potential) in the semiconductor toward the metal surface. region contact potential defined as positive 5 This is actually the same case as for a simple pn junction in a homogeneous material (e.g., pure Si) where the equilibrium band bending from right to left required to maintain a constant Fermi level is Aside: Comparison to a p-n junction Φ left Φ right = ( Φ χ ) ( Φ χ ) = ( c,left F, l e f t ) ( c,right F, r i g h t ) = ±qV vacuum level (l b j tt Φ left Φ Φ χ E F E c,p,o E v,p,o E x E v,n,o E c,n,o o o n c o p c F o n c F o p c qV E E E E E E , , , , , , , , ) ( ) ( F o n c E E , , F o p c E E , , (also subject to “band” bending) Φ χ 6
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4 If the band-bending produces a significant (TBD) barrierto transport of majoritycarriers from the semiconductor to the metal a “ rectifying contact ,” Schottky barrier ” or “ Schottky barrier diode ” is formed (different names for same thing).
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This note was uploaded on 11/07/2011 for the course E E 339 taught by Professor Leonardregister during the Spring '11 term at University of Texas at Austin.

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E E 339 - CN15-metal semiconductor junctions -...

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