ms junction

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Ohmic Contact

Rectifying Contact

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This chapter concerns metal-semiconductor junctions.They are also called Schottky contacts to honor WalterSchottky, a pioneer in metal-semiconductorjunctions.IntroductionMetal-semiconductor junction is a type of junction inwhich a metal comes in close contact with asemiconductor material. Similar to a p-n junction, ithas rectifying properties.

Importance of metal-semiconductor junctions is dueto:

1) Ohmic metal-semiconductor junctions (e.g. formetal interconnects contacting a Si device in anintegrated circuit)

2) Rectifying metal-semiconductor junctions =Schottky diodes(e.g. for high-speed rectifier diodes)2


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The structure is verysimple

but also very interesting, important, anduseful


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Many of the properties of p-n junctions can berealized by forming an appropriate metal-semiconductor rectifying contact (Schottkycontact)

Simple to fabricate Switching speed is much higher than that of p-njunction diodes

Metal-Semiconductor junctions are also used

as Ohmic-contact to carry current into and outof the semiconductor device

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Vacuum level, E0 - corresponds to energy of free

electrons. The difference between vacuum level and Fermi-level is called work function . The metal Work function Mis an invariant property of

metal. It is the minimum energy required to free upelectrons from metal. (3.66eV for Mg, 5.15eV for Ni etc.)

The semiconductor work function s, depends on thedoping.

where = (E0EC)|SURFACE is a fundamental property

of the semiconductor called Electron affinity ofSurfaces. (Example: = 4.0 eV, 4.03 eV and 4.07 eVfor Ge, Si and GaAs respectively)

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)( FCs EE


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A few elements gain electrons to form anions.

Electron affinity is the energy changewhen an electron is added:

A(g) + e- ---> A-(g) E.A. = E

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Assumptions - Ideal MS contactsM and S are in intimate contact,on atomic scale.

No oxides or charges at theinterface.No intermixing at the interface.


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M

> S

M < S

(a) and (c) An instantafter contact formation

(b) and (d) underequilibrium conditions


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When negative charges are brought near the metal

surface, positive (image charges) are induced in the

metal.

When this image force is combined with an applied

electric field, the effective work function is somewhat

reduced.

This barrier lowering is called the Schottky effect.

Some facts

MS Junctions

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Four different cases arise

M n SC

M > S

M n SC

M < SM p SC

M > S

M p SC

M < S

M S Junctions

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Soon after the contact formation, electronswill begin to flow from S to M near junction. Creates surface depletion layer, and hence a

built-in electric field (similar to p-n junction). Under equilibrium, net flow of carriers will be

zero, and Fermi-level will be constant. A barrier B forms for electron flow from M to

S. B = M ... ideal MS (n-type) contact. Bis called barrier height. Electrons in semiconductor will encounter anenergy barrier equal to M S while flowingfrom S to M.

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MS Junctions

Metal - n SC ( m > SC )

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A potential barrier formed at the interfacebetween metal and semiconductor.

This barrier hinders the e- flow fromsemiconductor to metal, but not metal tosemiconductor=> asymmetric resistance=>rectifying current

In most cases, the rectifying contacts are

made on n-type semiconductors.

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The periodic potential function is abrupt atthe interface, forming the surface stateswhich allow e- to drop to and resulting in aspace charge region, space charge electric

field, and potential barrier.

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MS Junctions

Metal - p SC ( m < SC )

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n-type p-type

M > Srectifying ohmic

M < S ohmic rectifying


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Ohmic Contactohmic contact is a region on a semiconductor devicethat has been prepared so that the current-voltage (I-V) curve of the device is linear and symmetric. If the I-V characteristic is non-linear and asymmetric, the

contact is not ohmic, but is a blocking or SchottkyContact. Typical ohmic contacts on semiconductors aresputtered or evaporated metal pads that arepatterned using photolithography . Low-resistance,

stable contacts are critical for the performance andreliability of integrated circuit and their preparationand characterization are major efforts in circuitfabrication.

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TheoryThe Fermi level (or strictly speaking, electrochemical

potential) of any two solids in contact must be equalin thermal equilibrium. The difference between theFermi energy and the vacuum level is termed thework function. A contact metal and a semiconductor

can have different work functions, denoted MandSrespectively. If so, when the two materials areplaced in contact, electrons will flow from the onewith the lower work function until the Fermi levelsequilibrate. As a result, the material with the lowerwork function will take on a slight positive chargewhile that with the higher work function will becomeslightly negative.

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Ohmic contacts are metal-to-semiconductor contacts, but in this casethey are not rectifying contacts. At n-type, m < s At p-type, m > s

Ohmic contact is a low-resistance junctionproviding conduction in both directionsbetween the metal and the semiconductor.

Ideally, the current through the ohmic

contact is a linear function of appliedvoltage, and the applied voltage should bevery small.


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Preparation and characterizationof ohmic contactsThe fabrication of ohmic contacts is a much-studied part of materials engineering thatnonetheless remains something of an art. The

reproducible, reliable fabrication of contactsrelies on extreme cleanliness of thesemiconductor surface. Since a native oxiderapidly forms on the surface ofsilicon, forexample, the performance of a contact candepend sensitively on the details of preparation.

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Rectifying contactA rectifying junction can be made when certain

metals are in contact with a semiconductorcrystal. In the case of an n-type germaniumcrystal in contact with tungsten, current isallowed to flow when the tungsten is held at a

positive potential, however, there is negligiblecurrent in the case where tungsten is held at anegative potential. The reverse is true in the caseof p-type semiconductors.These junctions have

some advantages over the standard p-n junction(diode) in that there is a smaller voltage drop,thus better emulating an ideal diode.

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Schottky diodes also allow for fasterswitching times compared to p-n

junctions. The n-type junction (Schottkydiode) can also be used in producing ahydrogen fuel cell. Water and hydrogen

would combine on a palladium layer andgenerate a thermionic current sent into ann-type silicon carbide semiconductor with

a potential greater than the Schottkybarrier.

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The current in a pn-junction is determined bythe diffusion ofminority barriers while thecurrent in a Schottky barrier diode is

determined by thermionic emission ofmajoritycarriers over a potential barrier.

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The fundamental steps in contactfabricationThe fundamental steps in contact fabrication

are semiconductor surface cleaning, contactmetal deposition, patterning and annealing.Surface cleaning may be performed by sputter-

etching, chemical etching, reactive gas etchingor ion milling. For example, the native oxide ofsilicon may be removed with an HF dip, whileGaAs is more typically cleaned by a bromine-

methanol dip. After cleaning, metals aredeposited via sputter deposition, evaporation orchemical vapor deposition.

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A metal on a lightly doped semiconductor canproduce a rectifying contact known Schottky barrier.

Rectifying current: the direction of voltage appliedinfluences the height of barrier.

Although ideal I-V relationship is the same, thecurrent mechanism of Schottky diode is muchdifferent from that of the pn-junction diode.

Metal-semiconductor junction can also form ohmiccontacts.

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References:www.ecse.rpi.edu/~schubert/Course-ECSE.../MT-22-Ch14-1.ppt

http://www.kocw.net/home/common/contents/document/lec/2010/14/02/chapter6-7-11-15.pdfhttp://en.wikipedia.org/wiki/Metal-semiconductor_junctionhttp://en.wikipedia.org/wiki/Schottky_contactwww2.ee.ntu.edu.tw/.../Introduction%20to%20Schottky%20barrier.pptSemiconductor physics and devices, chapter 9, Donald A. Neamen

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http://www.ecse.rpi.edu/~schubert/Course-ECSE.../MT-22-Ch14-1.ppthttp://en.wikipedia.org/wiki/Metal-semiconductor_junctionhttp://en.wikipedia.org/wiki/Metal-semiconductor_junctionhttp://en.wikipedia.org/wiki/Metal-semiconductor_junctionhttp://en.wikipedia.org/wiki/Metal-semiconductor_junctionhttp://www.ecse.rpi.edu/~schubert/Course-ECSE.../MT-22-Ch14-1.ppthttp://www.ecse.rpi.edu/~schubert/Course-ECSE.../MT-22-Ch14-1.ppthttp://www.ecse.rpi.edu/~schubert/Course-ECSE.../MT-22-Ch14-1.ppthttp://www.ecse.rpi.edu/~schubert/Course-ECSE.../MT-22-Ch14-1.ppthttp://www.ecse.rpi.edu/~schubert/Course-ECSE.../MT-22-Ch14-1.ppthttp://www.ecse.rpi.edu/~schubert/Course-ECSE.../MT-22-Ch14-1.ppthttp://www.ecse.rpi.edu/~schubert/Course-ECSE.../MT-22-Ch14-1.ppthttp://www.ecse.rpi.edu/~schubert/Course-ECSE.../MT-22-Ch14-1.ppthttp://www.ecse.rpi.edu/~schubert/Course-ECSE.../MT-22-Ch14-1.ppt

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