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{{Image|Planar Schottky diode.PNG|right|250px|Planar Schottky diode with ''n<sup>+</sup>''-guard rings and tapered oxide.}}
 
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==Schottky diode==
The '''Schottky diode''' is a two-terminal device consisting of conductive ''gate'' (for example, a metal) on top of a semiconductor ''body''. A generic name for this structure is the '''metal-semiconductor diode''' or '''M/S diode'''.<ref name=Sah>
The term "Schottky diode" may be taken erroneously to refer to diffusion as the mechanism of operation as first proposed by Mott, Schottky and Davydov. However, the mechanism in most devices is [[thermionic emission]], as later proposed by Bethe. See {{cite book |title= Fundamentals of solid-state electronics |author=Chih-Tang Sah |url=http://books.google.com/books?id=205wsYbl2fAC&pg=PA474 |pages=p. 474 |chapter=§560: Metal/semiconductor diode |isbn=9810206372 |year=1991 |publisher=World Scientific}}
</ref> For low voltage applications, below 200V, silicon is used, but for higher voltages (up to 3000 V or more) silicon carbide is used to extend the breakdown voltage. These voltages are achievable only when edge breakdown is avoided, which requires special attention to ''edge termination'' designs.<ref name=Baliga>
{{cite book |chapter= §3.2 Schottky diode edge terminations |title=Silicon carbide power devices |author=B. Jayant Baliga |year=2005 |isbn=9812566058 |publisher=World Scientific |url=http://books.google.com/books?id=LNLVwAzhN7EC&pg=PA44 |pages=pp. 44 ''ff''}}
</ref> The figure shows three strategies toward increasing the edge breakdown voltage: an extension of the metal diode contact over a tapered oxide and also an ''n<sup>+</sup>''-guard ring and a floating guard ring. These strategies are sometimes used together, but also are used separately. The substrate contact is made through an ''ohmic contact'' to the ''p''-substrate made using a metal-to-''p<sup>+</sup>'' region on the surface of the diode.
==Applications==
The Schottky diode is used in a large variety of applications, ranging from practical devices for switching, rectification and photo-detection, to test structures for fabrication monitoring and for studies of semiconductor defects and processes.
{{Image|Schottky & pn diode IV curves.PNG|right|250px|Comparison of Schottky and ''pn''-diode current voltage curves.}}
==Operation==
Three different bias cases are examined: zero bias, forward bias, and reverse bias. A simplified one dimensional analysis along a line vertically through the center of the Schottky contact is used throughout. It is imagined that the ''p<sup>+</sup>''-ohmic contact is vertically below the Schottky contact.
As seen in the comparison with the ''pn''-diode shown in the figure, the Schottky diode tends to turn on at lower forward voltages but rises more slowly because of the nonideality factor noted later. The Schottky diode also tends to have a higher reverse leakage current.
{{Image|Schottky barrier height.PNG|right|250px|Schottky barrier formation on ''p''-type semiconductor. Energies are in eV.}}
====Zero bias====
The figure shows (top) a charge-neutral, partly filled metal energy band and a charge-neutral semiconductor valence and conduction band, electrically isolated from each other. The Fermi level in the ''p''-type semiconductor is near its valence band edge, as set by its acceptor impurity doping. (See [[Fermi function#Fermi level|Fermi level]]). The Fermi level in the metal marks the top of the filled electron energy levels in a partly filled band of the metal.
Ordinarily, the Fermi levels of different materials differ. When they are brought into electrical contact, enabling electron transfer between the materials, the work done in removing an electron from one material and placing it in the other is equal to the energy difference in the Fermi levels. Consequently, energy is released upon contact by electron transference from the material with the higher Fermi level to the material with the lower Fermi level. This charge transfer continues until the electrical charge difference means the energy gain from transfer is countered by the electrical work required against the charge difference. At this point the two Fermi levels are brought into coincidence and no further charge transfer occurs. This flat Fermi level situation (bottom panel) corresponds to [[thermal equilibrium]], and no net current flows once equilibrium is reached. 
The occurrence of charge transfer naturally means that the two materials acquire a charge. In the figure, the metal loses electrons and forms an extremely thin positive charge layer near the interface. The semiconductor gains electrons, as indicated by the bending of the valence band edge away from the Fermi level, which increases the valence band occupancy by electrons. Differently stated, the vacancies (holes) in the valence band are reduced in number, and the charge balance in the band-bending region is lost. In this ''depletion layer'' (the holes or ''majority carriers'' are depleted), the immobile negative acceptor dopant ions make this region charge negative, and this charge results in a potential according to [[Maxwell equations#Poisson's equation|Poisson's equation]]. The potential decreases with distance toward the bulk semiconductor, and at some distance (the ''depletion width'') the bulk properties of the semiconductor are regained and the semiconductor bulk is charge neutral.
The resulting potential drop across the semiconductor depletion layer is called the ''Schottky barrier height'', labeled ''&phi;<sub>B</sub>'' in the figure. It is a form of [[contact potential]].
{{Image|Schottky barrier (forward bias).PNG|right|250px|Under forward bias ''V<sub>F</sub>'' the Schottky barrier height is reduced and the Fermi levels are split.}}
====Forward bias====
If a forward bias voltage is applied ''V<sub>F</sub>'', the Fermi level of the bulk metal ''E<sub>Fm</sub>'' (in eV) is raised in energy above the bulk Fermi level in the semiconductor ''E<sub>Fp</sub>'', which lowers the Schottky barrier height to a value ''&phi;<sub>B</sub>−V<sub>F</sub>''. A current of holes now flows from the semiconductor to the metal (or, equivalently, of electrons from the metal to the semiconductor). Notice that the alignment of the metal Fermi level relative to the semiconductor band edges is not changed by the bias: that is fixed by the processes involved in adjusting the Fermi levels to achieve equilibrium at zero applied bias.
A current flows under forward bias. According to the model of [[thermionic emission]], the electron current flowing toward the semiconductor is proportional to the electron density at the metal side of the interface, while the electron current flowing toward the metal is proportional to the electron density at the semiconductor side. At zero bias there is no current, but under forward bias the electron density on the metal side is unaffected, while that on the semiconductor side is reduced by the forward bias. Translating the matter to the terminology of holes, and using a simple [[Fermi_function#Boltzmann_limit|Boltzmann approximation]] to the [[Fermi function]], the hole density on the semiconductor side is increased by a factor:
:<math> p(V_F) = p(0)e^{qV_F/k_BT} \ , </math>
where ''p(0)'' is the zero forward bias value of hole density on the semiconductor side. The hole current from the semiconductor to the metal therefore is increased by this factor. The current, being by convention a flow of positive charge, is therefore positive from the semiconductor to the metal, and the Schottky barrier current in forward bias becomes:
:<math>I(V_F) = I(0)\left( e^{qV_F/k_BT}-1 \right) \ . </math>
Due to some complications of real Schottky barriers, the current is usually represented as:
:<math>I(V_F) = I(0)\left( e^{qV_F/nk_BT}-1 \right) \ , </math>
where the factor ''n'' is usually larger than one and is called the ''ideality factor''.
{{Image|Schottky barrier (reverse bias).PNG|right|250px|Schottky diode under reverse bias ''V<sub>R</sub>''.}}
====Reverse bias====
If a reverse bias ''V<sub>R</sub>'' is applied, the Fermi level of the metal is lowered below that of the semiconductor, and the barrier height for holes is increased on the semiconductor side by the amount of the reverse bias. The hole density is therefore decreased compared to equilibrium (the electron occupancy increases) and the current from the semiconductor falls below the equilibrium value. That means the diode current changes direction compared to the forward biased case.
In reverse bias a leakage current is drawn due to generation of electrons and holes by defects in the depletion layer. If a large enough reverse voltage is applied, breakdown occurs, due to runaway of the generation mechanisms, or more commonly, due to edge breakdown in the high field region near the contact perimeter.


==Notes==
==Notes==
<references/>
<references/>
http://books.google.com/books?id=LNLVwAzhN7EC&pg=PA45&dq=%22Schottky+diode%22&hl=en&ei=iAg6TeO7AYSasAOk67WhAw&sa=X&oi=book_result&ct=result&resnum=9&ved=0CGcQ6AEwCA#v=onepage&q=%22Schottky%20diode%22&f=false
http://books.google.com/books?id=FPlJQ0iO7oQC&pg=PA134&dq="Schottky+diode"&hl=en&ei=iAg6TeO7AYSasAOk67WhAw&sa=X&oi=book_result&ct=result&resnum=8&ved=0CGIQ6AEwBw#v=onepage&q="Schottky diode"&f=false
http://books.google.com/books?id=sh94bLWOTY4C&pg=PA84&dq=%22Schottky+diode%22&hl=en&ei=iAg6TeO7AYSasAOk67WhAw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CDwQ6AEwAA#v=onepage&q=%22Schottky%20diode%22&f=false
http://books.google.com/books?id=GTM2i6ZFpIEC&pg=PA299&dq=%22Schottky+diode%22&hl=en&ei=Afc6TZXzL5G6sQPaoYidAw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CDIQ6AEwADgK#v=onepage&q=%22Schottky%20diode%22&f=false
http://books.google.com/books?id=pRFUZdHb688C&pg=PA245&dq=%22Schottky+diode%22&hl=en&ei=Afc6TZXzL5G6sQPaoYidAw&sa=X&oi=book_result&ct=result&resnum=4&ved=0CEMQ6AEwAzgK#v=onepage&q=%22Schottky%20diode%22&f=false
http://books.google.com/books?id=7WKOfUR-8M4C&pg=PA227&dq=%22Schottky+diode%22&hl=en&ei=tvg6TbqyCIjQsAOTsbTVAw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CDAQ6AEwADgU#v=onepage&q=%22Schottky%20diode%22&f=false
http://books.google.com/books?id=XrSI2C9NlDIC&pg=PA47&dq=%22Schottky+diode%22&hl=en&ei=tvg6TbqyCIjQsAOTsbTVAw&sa=X&oi=book_result&ct=result&resnum=9&ved=0CFwQ6AEwCDgU#v=onepage&q=%22Schottky%20diode%22&f=false
http://books.google.com/books?id=REQkwBF4cVoC&pg=PA599&dq=%22Schottky+diode%22&hl=en&ei=YQM7TedHjsSwA-Tj4fwC&sa=X&oi=book_result&ct=result&resnum=3&ved=0CEAQ6AEwAjge#v=onepage&q=%22Schottky%20diode%22&f=false
http://onlinelibrary.wiley.com/doi/10.1002/1521-4095%2820020605%2914:11%3C789::AID-ADMA789%3E3.0.CO;2-H/pdf
http://books.google.com/books?id=iMSnDxI7JNsC&pg=PA181&dq=%22Schottky+diode%22&hl=en&ei=wkg7TdJ-jKKwA96_3IsD&sa=X&oi=book_result&ct=result&resnum=8&ved=0CFkQ6AEwBzgy#v=onepage&q=%22Schottky%20diode%22&f=false
http://books.google.com/books?id=LNLVwAzhN7EC&pg=PA50&dq=%22guard+ring%22+%22edge+termination%22&hl=en&ei=7ks7TeGTMI-ssAPipv3ZAw&sa=X&oi=book_result&ct=result&resnum=3&ved=0CEYQ6AEwAg#v=onepage&q=%22guard%20ring%22%20%22edge%20termination%22&f=false
[http://books.google.com/books?id=pMxTrOQtIw8C&pg=PA381&dq=edge+termination+breakdown&hl=en&ei=hoA7TauKNpDAsAOHsZS6Aw&sa=X&oi=book_result&ct=result&resnum=3&ved=0CDEQ6AEwAg#v=onepage&q=edge%20termination%20breakdown&f=false Compare pn diode and Schottky diode for speed and breakdown]

Revision as of 00:53, 6 March 2011

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