![Schottky Diode Section](https://upload.wikimedia.org/wikipedia/commons/9/9c/Schottky_Diode_Section.JPG)
A Schottky barrier, named after
Walter H. Schottky
Walter Hans Schottky (23 July 1886 – 4 March 1976) was a German physicist who played a major early role in developing the theory of electron and ion emission phenomena, invented the screen-grid vacuum tube in 1915 while working at Siemens ...
, is a
potential energy
In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors.
Common types of potential energy include the gravitational potentia ...
barrier for electrons formed at a
metal–semiconductor junction
In solid-state physics, a metal–semiconductor (M–S) junction is a type of electrical 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 ...
. Schottky barriers have
rectifying
A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an inver ...
characteristics, suitable for use as a
diode
A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other.
A diode ...
. One of the primary characteristics of a Schottky barrier is the Schottky barrier height, denoted by Φ
B (see figure). The value of Φ
B depends on the combination of metal and semiconductor.
Not all metal–semiconductor junctions form a rectifying Schottky barrier; a metal–semiconductor junction that conducts current in both directions without rectification, perhaps due to its Schottky barrier being too low, is called an
ohmic contact
An ohmic contact is a non-rectifying electrical junction: a junction between two conductors that has a linear current–voltage (I–V) curve as with Ohm's law. Low-resistance ohmic contacts are used to allow charge to flow easily in both direct ...
.
Physics of formation
When a metal is put in direct contact with a semiconductor, a so called Schottky barrier can be formed, leading to a rectifying behavior of the electrical contact. This happens both when the semiconductor is
n-type and its
work function
In solid-state physics, the work function (sometimes spelt workfunction) is the minimum thermodynamic work (i.e., energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. Here "immediately" ...
is smaller than the work function of the metal, and when the semiconductor is
p-type and the opposite relation between work functions holds.
At the basis of the description of the Schottky barrier formation through the
band diagram
In solid-state physics of semiconductors, a band diagram is a diagram plotting various key electron energy levels (Fermi level and nearby energy band edges) as a function of some spatial dimension, which is often denoted ''x''. These diagrams ...
formalism, there are three main assumptions:
# The contact between the metal and the semiconductor must be intimate and without the presence of any other material layer (such as an oxide).
# No interdiffusion of the metal and the semiconductor is taken into account.
# There are no impurities at the interface between the two materials.
To a first approximation, the barrier between a metal and a semiconductor is predicted by the
Schottky–Mott rule to be proportional to the difference of the metal-vacuum
work function
In solid-state physics, the work function (sometimes spelt workfunction) is the minimum thermodynamic work (i.e., energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. Here "immediately" ...
and the semiconductor-vacuum
electron affinity
The electron affinity (''E''ea) of an atom or molecule is defined as the amount of energy released when an electron attaches to a neutral atom or molecule in the gaseous state to form an anion.
::X(g) + e− → X−(g) + energy
Note that this is ...
. For an isolated metal, the
work function
In solid-state physics, the work function (sometimes spelt workfunction) is the minimum thermodynamic work (i.e., energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. Here "immediately" ...
is defined as the difference between its
vacuum energy
Vacuum energy is an underlying background energy that exists in space throughout the entire Universe. The vacuum energy is a special case of zero-point energy that relates to the quantum vacuum.
The effects of vacuum energy can be experiment ...
(i.e. the minimum energy that an electron must possess to completely free itself from the material) and the
Fermi energy
The Fermi energy is a concept in quantum mechanics usually referring to the energy difference between the highest and lowest occupied single-particle states in a quantum system of non-interacting fermions at absolute zero temperature.
In a Fermi ga ...
, and it is an invariant property of the specified metal:
On the other hand, the work function of a semiconductor is defined as:
Where
is the
electron affinity
The electron affinity (''E''ea) of an atom or molecule is defined as the amount of energy released when an electron attaches to a neutral atom or molecule in the gaseous state to form an anion.
::X(g) + e− → X−(g) + energy
Note that this is ...
(i.e. the difference between the
vacuum energy
Vacuum energy is an underlying background energy that exists in space throughout the entire Universe. The vacuum energy is a special case of zero-point energy that relates to the quantum vacuum.
The effects of vacuum energy can be experiment ...
and the energy level of the
conduction band). It is valuable to describe the work function of the semiconductor in terms of its electron affinity since this last one is an invariant fundamental property of the semiconductor, while the difference between the conduction band and the Fermi energy depends on the
doping.
![Msemictoghandsep](https://upload.wikimedia.org/wikipedia/commons/1/15/Msemictoghandsep.png)
When the two isolated materials are put into intimate contact, the equalization of the Fermi levels brings to the movement of charge from one material to the other, depending on the values of the work functions. This leads to the creation of an energy barrier, since at the interface between the materials some charge get collected. For electrons, the barrier height
can be easily calculated as the difference between the metal work function and the electron affinity of the semiconductor:
While the barrier height for holes is equal to the difference between the energy gap of the semiconductor and the energy barrier for electrons:
In reality, what can happen is that charged interface states can pin the Fermi level at a certain energy value no matter the work function values, influencing the barrier height for both carriers. This is due to the fact that the chemical termination of the semiconductor crystal against a metal creates electron states within its
band gap
In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in ...
. The nature of these
metal-induced gap states In bulk semiconductor band structure calculations, it is assumed that the crystal lattice (which features a periodic potential due to the atomic structure) of the material is infinite. When the finite size of a crystal is taken into account, the wa ...
and their occupation by electrons tends to pin the center of the band gap to the Fermi level, an effect known as
Fermi level pinning
Enrico Fermi (; 29 September 1901 – 28 November 1954) was an Italian (later naturalized American) physicist and the creator of the world's first nuclear reactor, the Chicago Pile-1. He has been called the "architect of the nuclear age" and t ...
. Thus the heights of the Schottky barriers in metal–semiconductor contacts often show little dependence on the value of the semiconductor or metal work functions, in strong contrast to the Schottky–Mott rule. Different semiconductors exhibit this
Fermi level pinning
Enrico Fermi (; 29 September 1901 – 28 November 1954) was an Italian (later naturalized American) physicist and the creator of the world's first nuclear reactor, the Chicago Pile-1. He has been called the "architect of the nuclear age" and t ...
to different degrees, but a technological consequence is that ohmic contacts are usually difficult to form in important semiconductors such as
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
and
gallium arsenide
Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a Zincblende (crystal structure), zinc blende crystal structure.
Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monoli ...
. Non-ohmic contacts present a parasitic resistance to current flow that consumes energy and lowers device performance.
Rectifying properties
In a rectifying Schottky barrier, the barrier is high enough that there is a
depletion region
In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobile ...
in the semiconductor, near the interface.
This gives the barrier a high resistance when small voltage biases are applied to it. Under large voltage bias, the
electric current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...
flowing through the barrier is essentially governed by the laws of
thermionic emission
Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the mate ...
, combined with the fact that the Schottky barrier is fixed relative to the metal's Fermi level.
* Under forward bias, there are many thermally excited electrons in the semiconductor that are able to pass over the barrier. The passage of these electrons over the barrier (without any electrons coming back) corresponds to a current in the opposite direction. The current rises very rapidly with bias, however at high biases the series resistance of the semiconductor can start to limit the current.
* Under reverse bias, there is a small leakage current as some thermally excited electrons in the metal have enough energy to surmount the barrier. To first approximation this current should be constant (as in the
Shockley diode equation
The ''Shockley diode equation'' or the ''diode law'', named after transistor co-inventor William Shockley of Bell Telephone Laboratories, gives the I–V (current-voltage) characteristic of an idealized diode in either forward or reverse bias (appl ...
); however, current rises gradually with reverse bias due to a weak barrier lowering (similar to the vacuum
Schottky effect
The Schottky effect or field enhanced thermionic emission is a phenomenon in condensed matter physics named after Walter H. Schottky. In electron emission devices, especially electron guns, the thermionic electron emitter will be biased negative ...
). At very high biases, the depletion region breaks down.
''Note: the discussion above is for a Schottky barrier to an ''n''-type semiconductor; similar considerations apply for a ''p''-type semiconductor.''
The current-voltage relationship is qualitatively the same as with a
p-n junction, however the physical process is somewhat different.
Conduction values
The
thermionic emission
Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the mate ...
can be formulated as following:
While the
tunneling current density can be expressed, for a triangular shaped barrier (considering
WKB approximation
In mathematical physics, the WKB approximation or WKB method is a method for finding approximate solutions to linear differential equations with spatially varying coefficients. It is typically used for a semiclassical calculation in quantum mecha ...
) as:
From both formulae it is clear that the current contributions are related to the barrier height for both electrons and holes. If a symmetric current profile for both n and p carriers is then needed, the barrier height must be ideally identical for electrons and holes.
Minority carrier injection
For very high Schottky barriers where Φ
B is a significant fraction of the
band gap
In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in ...
of the semiconductor, the forward bias current may instead be carried "underneath" the Schottky barrier, as minority carriers in the semiconductor.
An example of this is seen in the
Point-contact transistor
The point-contact transistor was the first type of transistor to be successfully demonstrated. It was developed by research scientists John Bardeen and Walter Brattain at Bell Laboratories in December 1947. They worked in a group led by physicis ...
.
Devices
A
Schottky diode
The Schottky diode (named after the German physicist Walter H. Schottky), also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltag ...
is a single metal–semiconductor junction, used for its rectifying properties. Schottky diodes are often the most suitable kind of diode when a low forward
voltage drop
Voltage drop is the decrease of electrical potential along the path of a current flowing in an electrical circuit. Voltage drops in the internal resistance of the source, across conductors, across contacts, and across connectors are undesirab ...
is desired, such as in a high efficiency DC
power supply
A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As a r ...
. Also, because of their majority-carrier conduction mechanism, Schottky diodes can achieve greater switching speeds than p–n junction diodes, making them appropriate to rectify high-frequency signals.
Introducing a second semiconductor/metal interface and a gate stack overlapping both junctions, one can obtain a Schottky barrier field effect transistor (SB-FET). The gate steers the carrier injection inside the channel modulating the band bending at the interface, and thus the resistance of the Schottky barriers. Generally the most significantly resistive path for the current is represented by the Schottky barriers, and so the channel itself does not contribute significantly to the conduction when the transistor is turned on. This kind of device has an ambipolar behavior since when a positive voltage is applied to both junctions, their
band diagram
In solid-state physics of semiconductors, a band diagram is a diagram plotting various key electron energy levels (Fermi level and nearby energy band edges) as a function of some spatial dimension, which is often denoted ''x''. These diagrams ...
is bent downwards enabling an electron current from source to drain (the presence of a
voltage is always implied) due to direct
tunneling. In the opposite case of a negative voltage applied to both junctions the band diagram is bent upwards and holes can be injected and flow from the drain to the source. Setting the gate voltage to 0 V suppresses the tunneling current and enables only a lower current due to
thermionic
Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the mater ...
events. One of the main limitations of such a device is strongly related to the presence of this current that makes it difficult to properly switch it off. A clear advantage of such a device is that there is no need for channel
doping and expensive technological steps like
ion implantation
Ion implantation is a low-temperature process by which ions of one element are accelerated into a solid target, thereby changing the physical, chemical, or electrical properties of the target. Ion implantation is used in semiconductor device fab ...
and
high temperature annealings can be avoided, keeping the thermal budget low. However the band bending due to the voltage difference between drain and gate often injects enough carriers to make impossible a proper switch off of the device. Also, low on-currents due to the intrinsic resistance of the Schottky contacts are typical of this kind of device just like a very hard and unreliable scalability due to the difficult control of the junction area.
![G12922](https://upload.wikimedia.org/wikipedia/commons/b/be/G12922.png)
A
bipolar junction transistor
A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipola ...
with a Schottky barrier between the base and the collector is known as a
Schottky transistor
A Schottky transistor is a combination of a transistor and a Schottky diode that prevents the transistor from saturating by diverting the excessive input current. It is also called a Schottky-clamped transistor.
Mechanism
Standard transistor ...
. Because the junction voltage of the Schottky barrier is small, the transistor is prevented from saturating too deeply, which improves the speed when used as a switch. This is the basis for the Schottky and Advanced Schottky
TTL
TTL may refer to:
Photography
* Through-the-lens metering, a camera feature
* Zenit TTL, an SLR film camera named for its TTL metering capability
Technology
* Time to live, a computer data lifespan-limiting mechanism
* Transistor–transistor lo ...
families, as well as their low
power
Power most often refers to:
* Power (physics), meaning "rate of doing work"
** Engine power, the power put out by an engine
** Electric power
* Power (social and political), the ability to influence people or events
** Abusive power
Power may a ...
variants.
A
MESFET
A MESFET (metal–semiconductor field-effect transistor) is a field-effect transistor semiconductor device similar to a JFET with a Schottky (metal–semiconductor) junction instead of a p–n junction for a gate.
Construction
MESFETs are constr ...
or metal–semiconductor
FET
The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs (JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs contr ...
uses a reverse-biased Schottky barrier to provide a depletion region that pinches off a conducting channel buried inside the semiconductor (similar to the
JFET
The junction-gate field-effect transistor (JFET) is one of the simplest types of field-effect transistor. JFETs are three-terminal semiconductor devices that can be used as electronically controlled switches or resistors, or to build amplifier ...
where instead a
p–n junction provides the depletion region). A variant of this device is the
high-electron-mobility transistor
A high-electron-mobility transistor (HEMT), also known as heterostructure FET (HFET) or modulation-doped FET (MODFET), is a field-effect transistor incorporating a junction between two materials with different band gaps (i.e. a heterojunction) ...
(HEMT), which also utilizes a
heterojunction A heterojunction is an interface between two layers or regions of dissimilar semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction. It is often advantageous to engineer the electronic energy bands in ma ...
to provide a device with extremely high conductance.
A Schottky barrier
carbon nanotube FET uses the non-ideal contact between a metal and a carbon nanotube to form a Schottky barrier that can be used to make extremely small Schottky diodes, transistors, and similar electronic devices with unique mechanical and electronic properties.
Schottky barriers can also be used to characterize a semiconductor. In the
depletion region
In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobile ...
of the Schottky barrier, dopants remain ionized and give rise to a "space charge" which in turn give rise to a
capacitance
Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are ...
of the junction. The metal–semiconductor interface and the opposite boundary of the depleted area act like two capacitor plates, with the
depletion region
In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobile ...
acting as a
dielectric
In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...
. By applying a voltage to the junction it is possible to vary the
depletion width
In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobile ...
and vary the capacitance, used in
capacitance voltage profiling. By analyzing the ''speed'' at which the capacitance responds to changes in voltage, it is possible to obtain information about dopants and other defects, a technique known as
deep-level transient spectroscopy.
Electrowetting
In the subject of
microfluidics
Microfluidics refers to the behavior, precise control, and manipulation of fluids that are geometrically constrained to a small scale (typically sub-millimeter) at which surface forces dominate volumetric forces. It is a multidisciplinary field tha ...
,
electrowetting Electrowetting is the modification of the wetting properties of a surface (which is typically hydrophobic) with an applied electric field.
History
The electrowetting of mercury and other liquids on variably charged surfaces was probably first ex ...
can be observed at a
metal–semiconductor junction
In solid-state physics, a metal–semiconductor (M–S) junction is a type of electrical 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 ...
by using a
droplet
A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the lower end of a tube or other surface boundary, producing a hanging drop called a pendant ...
of
liquid metal
A liquid metal is a metal or a metal alloy which is liquid at or near room temperature.
The only stable liquid elemental metal at room temperature is Mercury (element), mercury (Hg), which is molten above −38.8 °C (234.3 K, −37.9&nbs ...
(
mercury) resting on crystalline
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
to form a Schottky barrier in a
Schottky diode
The Schottky diode (named after the German physicist Walter H. Schottky), also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltag ...
electrical setup. 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. This effect has been termed Schottky electrowetting, effectively linking electrowetting and semiconductor effects.
Modification of Schottky barrier height (SBH)
Incorporation of
nanoparticles
A nanoparticle or ultrafine particle is usually defined as a particle of matter that is between 1 and 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 1 ...
at the interface of a contact/
semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
interface can effectively modify the Schottky barrier height (SBH).
See also
*
Ohmic contact
An ohmic contact is a non-rectifying electrical junction: a junction between two conductors that has a linear current–voltage (I–V) curve as with Ohm's law. Low-resistance ohmic contacts are used to allow charge to flow easily in both direct ...
*
Schottky diode
The Schottky diode (named after the German physicist Walter H. Schottky), also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltag ...
*
Diode
A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other.
A diode ...
*
Metal-induced gap states In bulk semiconductor band structure calculations, it is assumed that the crystal lattice (which features a periodic potential due to the atomic structure) of the material is infinite. When the finite size of a crystal is taken into account, the wa ...
*
Memristor
A memristor (; a portmanteau of ''memory resistor'') is a non-linear two-terminal electrical component relating electric charge and magnetic flux linkage. It was described and named in 1971 by Leon Chua, completing a theoretical quartet of fu ...
*
Electrowetting Electrowetting is the modification of the wetting properties of a surface (which is typically hydrophobic) with an applied electric field.
History
The electrowetting of mercury and other liquids on variably charged surfaces was probably first ex ...
References
{{Reflist
Semiconductor structures
de:Schottky-Kontakt