Metal–semiconductor junction

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In

non-rectifying. The rectifying metal–semiconductor junction forms a Schottky barrier, making a device known as a Schottky diode, while the non-rectifying junction is called an ohmic contact.[1] (In contrast, a rectifying semiconductor–semiconductor junction, the most common semiconductor device today, is known as a p–n junction
.)

Metal–semiconductor junctions are crucial to the operation of all semiconductor devices. Usually an ohmic contact is desired, so that electrical charge can be conducted easily between the active region of a transistor and the external circuitry. Occasionally however a

metal–semiconductor field effect transistors
.

The critical parameter: Schottky barrier height

conduction band edge EC and Fermi level
EF.

Whether a given metal-semiconductor junction is an ohmic contact or a Schottky barrier depends on the Schottky barrier height, ΦB, of the junction. For a sufficiently large Schottky barrier height, that is, ΦB is significantly higher than the thermal energy kT, the semiconductor is depleted near the metal and behaves as a Schottky barrier. For lower Schottky barrier heights, the semiconductor is not depleted and instead forms an ohmic contact to the metal.

The Schottky barrier height is defined differently for n-type and p-type semiconductors (being measured from the conduction band edge and valence band edge, respectively). The alignment of the semiconductor's bands near the junction is typically independent of the semiconductor's doping level, so the n-type and p-type Schottky barrier heights are ideally related to each other by:

where Eg is the semiconductor's band gap.

In practice, the Schottky barrier height is not precisely constant across the interface, and varies over the interfacial surface.[2]

Schottky–Mott rule and Fermi level pinning

Schottky–Mott rule: As the materials are brought together, the bands in the silicon bend such that the silicon's work function Φ matches the silver's. The bands retain their bending upon contact. This model predicts silver to have a very low Schottky barrier to n-doped silicon, making an excellent ohmic contact.
Picture showing Fermi level pinning effect from metal-induced gap states: The bands in the silicon already start out bent due to surface states. They are bent again just before contact (to match work functions). Upon contact however, the band bending changes completely, in a way that depends on the chemistry of the Ag-Si bonding.[4]
Band diagrams for models of formation of junction between silver and n-doped silicon.[3] In practice this Schottky barrier is approximately ΦB = 0.8 eV.
See also: Metal-induced gap states

The Schottky–Mott rule of Schottky barrier formation, named for

Nevill Mott, predicts the Schottky barrier height based on the vacuum work function of the metal relative to the vacuum electron affinity (or vacuum ionization energy
) of the semiconductor:

This model is derived based on the thought experiment of bringing together the two materials in vacuum, and is closely related in logic to

semiconductor-semiconductor junctions. Different semiconductors respect the Schottky–Mott rule to varying degrees.[5]

Although the Schottky–Mott model correctly predicted the existence of band bending in the semiconductor, it was found experimentally that it would give grossly incorrect predictions for the height of the Schottky barrier. A phenomenon referred to as "Fermi level pinning" caused some point of the band gap, at which finite DOS exists, to be locked (pinned) to the Fermi level. This made the Schottky barrier height almost completely insensitive to the metal's work function:[5]

where Ebandgap is the size of band gap in the semiconductor.

In fact, empirically, it is found that neither of the above extremes is quite correct. The choice of metal does have some effect, and there appears to be a weak correlation between the metal work function and the barrier height, however the influence of the work function is only a fraction of that predicted by the Schottky-Mott rule.[6]: 143 

It was noted in 1947 by

surface states). These highly dense surface states would be able to absorb a large quantity of charge donated from the metal, effectively shielding the semiconductor from the details of the metal. As a result, the semiconductor's bands would necessarily align to a location relative to the surface states which are in turn pinned to the Fermi level (due to their high density), all without influence from the metal.[3]

The Fermi level pinning effect is strong in many commercially important semiconductors (Si, Ge, GaAs),[5] and thus can be problematic for the design of semiconductor devices. For example, nearly all metals form a significant Schottky barrier to n-type germanium and an ohmic contact to p-type germanium, since the valence band edge is strongly pinned to the metal's Fermi level.[7] The solution to this inflexibility requires additional processing steps such as adding an intermediate insulating layer to unpin the bands. (In the case of germanium, germanium nitride has been used[8])

History

The rectification property of metal–semiconductor contacts was discovered by

Sir Jagadish Chandra Bose
applied for a US patent for a metal-semiconductor diode in 1901. This patent was awarded in 1904.

Wikisource has original text related to this article:

metal–semiconductor field effect transistors.[11] The theory of the field-effect transistor using a metal/semiconductor gate was advanced by William Shockley
in 1939.

The earliest metal–semiconductor diodes in

E. I du Pont de Nemours Company
.

The first theory that predicted the correct direction of rectification of the metal–semiconductor junction was given by

electrons over the metal–semiconductor potential barrier. Thus, the appropriate name for the metal–semiconductor diode should be the Bethe diode, instead of the Schottky diode, since the Schottky theory does not predict the modern metal–semiconductor diode characteristics correctly.[13]

If a metal-semiconductor junction is formed by placing a droplet of mercury, as Braun did, onto a semiconductor, e.g.silicon, to form a Schottky barrier in a Schottky diode electrical setup – electrowetting can be observed, where the droplet spreads out with increasing voltage. Depending on the doping type and density in the semiconductor, the droplet spreading depends on the magnitude and sign of the voltage applied to the mercury droplet.[14] This effect has been termed ‘Schottky electrowetting’, effectively linking electrowetting and semiconductor effects.[15]

The

point-contact diodes and made it possible to build practical Schottky diodes.[20]

See also

References

  1. ISBN 81-203-2398-X
    .
  2. ^ "Inhomogeneous Schottky Barrier".
  3. ^
    doi:10.1103/PhysRev.71.717
    .
  4. doi:10.1103/PhysRevB.64.205310
    .
  5. ^ a b c "Barrier Height Correlations and Systematics".
  6. OCLC 488586029.{{cite book}}: CS1 maint: multiple names: authors list (link
    )
  7. doi:10.1063/1.2789701
    .
  8. doi:10.1063/1.2831918
    .
  9. doi:10.1002/andp.18752291207
  10. doi:10.1103/PhysRevSeriesI.25.31
    .
  11. ^ US 1745175  "Method and apparatus for controlling electric current" first filed in Canada on 22.10.1925.
  12. ^ US 755840, Bose, Jagadis Chunder, "Detector for electrical disturbances", published September 30, 1901, issued March 29, 1904 
  13. ISBN 9810206372
    .
  14. doi:10.1063/1.4818715
  15. doi:10.1039/C4RA04187A
  16. ISBN 9780801886393
    .
  17. U.S. Government Printing Office
    . 1973. p. 1475.
  18. S2CID 51637380
    .
  19. ISBN 9781351093514
    .
  20. ^ a b Siegel, Peter H.; Kerr, Anthony R.; Hwang, Wei (March 1984). NASA Technical Paper 2287: Topics in the Optimization of Millimeter-Wave Mixers (PDF). NASA. pp. 12–13.
  21. ^
    ISBN 9780323150590
    .
  22. ISBN 9781468446555
    .
  23. S2CID 84306885
    .

Further reading
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