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In [[solid-state physics]], a '''metal–semiconductor (M–S) junction''' is a type of [[Junction (semiconductor)|junction]] in which a [[metal]] comes in close contact with a [[semiconductor]] material.  It is the oldest practical [[semiconductor device]]. M–S junctions can either be [[rectifying]] or [[nonrectifying junction|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]].<ref>''Semiconductor Devices: Modelling and Technology'', Nandita Dasgupta, Amitava Dasgupta.(2004) ISBN 81-203-2398-X.</ref> (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 [[Schottky barrier]] is useful, as in [[Schottky diode]]s, [[Schottky transistor]]s, and [[metal–semiconductor field effect transistor]]s.
 
== The critical parameter: Schottky barrier height ==
 
[[File:Schottky barrier zero bias.svg|thumb|[[Band diagram]] for metal-semiconductor junction at zero bias (equilibrium). Shown is the graphical definition of the '''Schottky barrier height''', Φ<sub>B</sub>, for an ''n''-type semiconductor as the difference between the interfacial [[conduction band]] edge ''E''<sub>C</sub> and [[Fermi level]] ''E''<sub>F</sub>.]]
 
Whether a given metal-semiconductor junction is an ohmic contact, or Schottky barrier, depends on the Schottky barrier height, Φ<sub>B</sub>, of the junction.
For a sufficiently large Schottky barrier height, where Φ<sub>B</sub> is significantly higher than the thermal energy ''kT'', the semiconductor is [[depletion region|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:
:<math>\Phi_{\rm B}^{(n)} + \Phi_{\rm B}^{(p)} = E_{\rm g}</math>
where ''E''<sub>g</sub> 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.<ref>http://academic.brooklyn.cuny.edu/physics/tung/Schottky/inhomo.htm</ref>
 
== Schottky–Mott rule and Fermi level pinning ==
 
{{multiple image
|image1  = Metal semiconductor junction Schottky approximation.svg
|caption1 = '''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.
|image2  = Metal semiconductor junction Bardeen picture.svg
|caption2 = 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.<ref>{{cite doi|10.1103/PhysRevB.64.205310}}</ref>
|footer=[[Band diagram]]s for models of formation of junction between silver and ''n''-doped silicon.<ref name="bardeen">{{cite doi|10.1103/PhysRev.71.717}}</ref> In practice this Schottky barrier is approximately Φ<sub>B</sub>&nbsp;=&nbsp;0.8&nbsp;eV.
}}
 
{{see also|Metal-induced gap states}}
 
The [[Walter H. Schottky|Schottky]]–[[Nevill Mott|Mott]] model of Schottky barrier formation 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:
:<math>\Phi_{\rm B}^{(n)} \approx \Phi_{\rm metal} - \chi_{\rm semi}</math>
This model is derived based on the thought experiment of bringing together the two materials in vacuum, and is closely related in logic to [[Anderson's rule]] for [[semiconductor-semiconductor junction]]s. Different semiconductors respect the Schottky–Mott rule to varying degrees.<ref name="tungsys"/>
 
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 the center of the band gap to be locked (pinned) to the Fermi level. This made the Schottky barrier height almost completely insensitive to the metal's work function:<ref name="tungsys">http://academic.brooklyn.cuny.edu/physics/tung/Schottky/systematics.htm</ref>
:<math>\Phi_{\rm B} \approx \frac{1}{2} E_{\rm bandgap}</math>
where ''E''<sub>bandgap</sub> is the size of [[band gap]] in the semiconductor.
 
It was noted in 1947 by [[John Bardeen]] that the Fermi level pinning phenomenon would naturally arise if there were chargeable states in the semiconductor right at the interface, with energies inside the semiconductor's gap. These would either be induced during the direct chemical bonding of the metal and semiconductor ([[metal-induced gap states]]) or be already present in the semiconductor–vacuum surface ([[surface state]]s). 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.<ref name="bardeen"/>
 
The Fermi level pinning effect is strong in many commercially important semiconductors (Si, Ge, GaAs),<ref name="tungsys"/> and thus can be very frustrating 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.<ref>{{cite doi|10.1063/1.2789701}}</ref> 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<ref>{{cite doi|10.1063/1.2831918}}</ref>)
 
== History ==
 
The rectification property of metal–semiconductor contacts was discovered by [[Ferdinand Braun]] in 1874 using [[mercury (element)|mercury]] metal contacted with [[copper]] and [[iron sulfide]] semiconductors.<ref>{{Citation |last=Braun |first=F. |authorlink=Ferdinand Braun |year=1874 |url=http://books.google.com/books?id=YBJbAAAAYAAJ&pg=PA556#v=onepage&q&f=false |title=Ueber die Stromleitung durch Schwefelmetalle |trans_chapter=On current conduction through metal sulfides |language=German |journal=Annalen der Physik und Chemie |volume=153 |issue=4 |pages=556–563}}</ref>
 
[[Greenleaf Whittier Pickard|G.W. Pickard]] received a [[patent]] in 1906 on a point-contact rectifier using [[silicon]]. In 1907, [[George W. Pierce]] published a paper in [[Physical Review]] showing rectification properties of [[diode]]s made by [[sputtering]] many metals on many semiconductors.<ref>{{cite doi|10.1103/PhysRevSeriesI.25.31}}</ref> The use of the metal–semiconductor diode rectifier was proposed by [[Julius Edgar Lilienfeld|Lilienfeld]] in 1926 in the first of his three transistor patents as the gate of the [[metal–semiconductor field effect transistor]]s.<ref>{{patent|US|1745175|"Method and apparatus for controlling electric current" first filed in Canada on 22.10.1925.}}</ref>
The correct theory of the field-effect transistor using a metal/semiconductor gate was advanced by [[William Shockley]] in 1939.
 
The earliest metal–semiconductor diodes in [[electronics]] application occurred around 1900, when the [[Cat's-whisker detector|cat's whisker rectifiers]] were used in [[Receiver (radio)|receivers]].<ref>{{Citation |inventor-first=Jagadis Chunder |inventor-last=Bose |inventorlink=Jagadish Chandra Bose |country-code=US |patent-number=755840 |title=Detector for electrical disturbances |publication-date=September 30, 1901 |issue-date=March 29, 1904}}</ref> They consisted of pointed tungsten wire (in the shape of a cat's whisker) whose tip or point was pressed against the surface of a [[galena]] (lead sulfide) crystal. The first large area rectifier appeared around 1926 which consisted of a [[copper(I) oxide]] semiconductor thermally grown on a copper [[Substrate (materials science)|substrate]]. Subsequently, [[selenium]] films were [[evaporation (deposition)|evaporated]] onto large metal substrates to form the rectifying diodes. These [[selenium rectifiers]] were used (and are still used) to convert alternating current to direct current in electrical power applications. During 1925–1940, diodes consisting of a pointed [[tungsten]] metal wire in contact with a [[silicon]] crystal base, were fabricated in laboratories to detect [[microwaves]] in the [[UHF]] range. A World War II program to manufacture high-purity silicon as the crystal base for the point-contact rectifier was suggested by [[Frederick Seitz]] in 1942 and successfully undertaken by the Experimental Station of the [[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 [[Nevill Francis Mott|Nevill Mott]] in 1939. He found the solution for both the [[diffusion]] and [[drift (telecommunication)|drift]] currents of the [[majority carriers]] through the semiconductor surface space charge layer which has been known since about 1948 as the Mott barrier. [[Walter H. Schottky]] and Spenke extended Mott's theory by including a donor [[ion]] whose [[density]] is spatially constant through the semiconductor surface layer. This changed the constant [[electric field]] assumed by Mott to a linearly decaying electric field. This semiconductor space-charge layer under the metal is known as the [[Schottky barrier]]. A similar theory was also proposed by [[Davydov]] in 1939. Although it gives the correct direction of rectification, it has also been proven that the Mott theory and its Schottky-Davydov extension  gives the wrong current limiting mechanism and wrong current-voltage formulae in silicon metal/semiconductor diode rectifiers. The correct theory was developed by [[Hans Bethe]] and reported by him in a [[M.I.T.]] Radiation Laboratory Report dated November 23, 1942. In Bethe's theory, the current is limited by [[thermionic emission]] of [[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.<ref>{{cite isbn|9810206372}}</ref>
 
== References ==
 
<References/>
 
{{DEFAULTSORT:Metal-semiconductor junction}}
[[Category:Semiconductor structures]]

Revision as of 15:19, 11 January 2014

In solid-state physics, a metal–semiconductor (M–S) junction is a type of junction in which a metal comes in close contact with a semiconductor material. It is the oldest practical semiconductor device. M–S junctions can either be rectifying or 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 Schottky barrier is useful, as in Schottky diodes, Schottky transistors, and metal–semiconductor field effect transistors.

The critical parameter: Schottky barrier height

Band diagram for metal-semiconductor junction at zero bias (equilibrium). Shown is the graphical definition of the Schottky barrier height, ΦB, for an n-type semiconductor as the difference between the interfacial conduction band edge EC and Fermi level EF.

Whether a given metal-semiconductor junction is an ohmic contact, or Schottky barrier, depends on the Schottky barrier height, ΦB, of the junction. For a sufficiently large Schottky barrier height, where 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

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The Schottky–Mott model of Schottky barrier formation 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 Anderson's rule for semiconductor-semiconductor junctions. Different semiconductors respect the Schottky–Mott rule to varying degrees.[3]

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 the center of the band gap to be locked (pinned) to the Fermi level. This made the Schottky barrier height almost completely insensitive to the metal's work function:[3]

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

It was noted in 1947 by John Bardeen that the Fermi level pinning phenomenon would naturally arise if there were chargeable states in the semiconductor right at the interface, with energies inside the semiconductor's gap. These would either be induced during the direct chemical bonding of the metal and semiconductor (metal-induced gap states) or be already present in the semiconductor–vacuum surface (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.[4]

The Fermi level pinning effect is strong in many commercially important semiconductors (Si, Ge, GaAs),[3] and thus can be very frustrating 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.[5] 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[6])

History

The rectification property of metal–semiconductor contacts was discovered by Ferdinand Braun in 1874 using mercury metal contacted with copper and iron sulfide semiconductors.[7]

G.W. Pickard received a patent in 1906 on a point-contact rectifier using silicon. In 1907, George W. Pierce published a paper in Physical Review showing rectification properties of diodes made by sputtering many metals on many semiconductors.[8] The use of the metal–semiconductor diode rectifier was proposed by Lilienfeld in 1926 in the first of his three transistor patents as the gate of the metal–semiconductor field effect transistors.[9] The correct theory of the field-effect transistor using a metal/semiconductor gate was advanced by William Shockley in 1939.

The earliest metal–semiconductor diodes in electronics application occurred around 1900, when the cat's whisker rectifiers were used in receivers.[10] They consisted of pointed tungsten wire (in the shape of a cat's whisker) whose tip or point was pressed against the surface of a galena (lead sulfide) crystal. The first large area rectifier appeared around 1926 which consisted of a copper(I) oxide semiconductor thermally grown on a copper substrate. Subsequently, selenium films were evaporated onto large metal substrates to form the rectifying diodes. These selenium rectifiers were used (and are still used) to convert alternating current to direct current in electrical power applications. During 1925–1940, diodes consisting of a pointed tungsten metal wire in contact with a silicon crystal base, were fabricated in laboratories to detect microwaves in the UHF range. A World War II program to manufacture high-purity silicon as the crystal base for the point-contact rectifier was suggested by Frederick Seitz in 1942 and successfully undertaken by the Experimental Station of the 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 Nevill Mott in 1939. He found the solution for both the diffusion and drift currents of the majority carriers through the semiconductor surface space charge layer which has been known since about 1948 as the Mott barrier. Walter H. Schottky and Spenke extended Mott's theory by including a donor ion whose density is spatially constant through the semiconductor surface layer. This changed the constant electric field assumed by Mott to a linearly decaying electric field. This semiconductor space-charge layer under the metal is known as the Schottky barrier. A similar theory was also proposed by Davydov in 1939. Although it gives the correct direction of rectification, it has also been proven that the Mott theory and its Schottky-Davydov extension gives the wrong current limiting mechanism and wrong current-voltage formulae in silicon metal/semiconductor diode rectifiers. The correct theory was developed by Hans Bethe and reported by him in a M.I.T. Radiation Laboratory Report dated November 23, 1942. In Bethe's theory, the current is limited by thermionic emission of 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.[11]

References

  1. ↑ Semiconductor Devices: Modelling and Technology, Nandita Dasgupta, Amitava Dasgupta.(2004) ISBN 81-203-2398-X.
  2. ↑ http://academic.brooklyn.cuny.edu/physics/tung/Schottky/inhomo.htm
  3. ↑ 3.0 3.1 3.2 http://academic.brooklyn.cuny.edu/physics/tung/Schottky/systematics.htm
  4. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named bardeen
  5. ↑ Template:Cite doi
  6. ↑ Template:Cite doi
  7. ↑ Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.

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    In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.

    Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region

    Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.

    15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.

    To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010
  8. ↑ Template:Cite doi
  9. ↑ Template:Patent
  10. ↑ Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.

    Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.

    In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.

    Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region

    Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.

    15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.

    To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010
  11. ↑ Template:Cite isbn
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