Schottky barrier
Encyclopedia
A Schottky barrier, named after Walter H. Schottky
, is a potential barrier formed at a metal–semiconductor junction which has rectifying characteristics, suitable for use as a diode
. The largest differences between a Schottky barrier and a p–n junction are its typically lower junction voltage, and decreased (almost nonexistent) depletion width in the metal.
Not all metal–semiconductor junctions form Schottky barriers. A metal–semiconductor junction that does not rectify current is called an ohmic contact
. Rectifying properties depend on the metal's work function
, the band gap
of the intrinsic semiconductor, the type and concentration
of dopant
s in the semiconductor, and other factors. Design of semiconductor devices requires familiarity with the Schottky effect to ensure Schottky barriers are not created accidentally where an ohmic connection is desired.
s, where their lower junction voltage is used for circuit protection (among other things).
Because one of the materials in a Schottky diode is a metal, lower resistance devices are often possible. In addition, the fact that only one type of dopant is needed may greatly simplify fabrication. And 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.
.
A bipolar junction transistor
with a Schottky barrier between the base and the collector is known as a Schottky 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
families, as well as their low power
variants.
A MESFET
, or Metal–Semiconductor FET, is a device similar in operation to the JFET
, which utilizes a reverse biased Schottky barrier to provide the depletion region. A particularly interesting variant of this device is the HEMT
, or High Electron Mobility Transistor, which also utilizes a heterojunction
to provide a device with extremely high conductance.
Schottky barriers are commonly used also in semiconductor electrical characterization techniques. In fact, in the semiconductor, a depletion region
is created by the metal electrons, which "push" away semiconductor electrons (simplification, see depletion region
article). In the depletion region, dopants remain ionized and give rise to a "space charge" which, in turn, give rise to a capacitance
of the junction. The metal-semiconductor interface and the opposite boundary of the depleted area act like two capacitor plates, with the depletion region
acting as a dielectric
.
By applying a voltage to the junction it is possible to vary the depletion width: if we reverse bias the junction, the dopants electrons will be emitted and pushed away; if we forward bias the junction, the electrons will be captured.
By analyzing the emission and capture of electrons by dopants (or, more frequently, by crystallographic defects or dislocations, or other electron traps) is possible to characterize the semiconductor material.
The most popular electrical characterization techniques that use this type of junction are DLTS and CV profiling
.
A Schottky barrier carbon nanotube FET uses the nonideal contact between a metal and a carbon nanotube (CNT) to form a Schottky barrier that can be used to make Schottky diodes or transistors, or so on. The scaling of semiconductor devices to ever-smaller sizes is rapidly approaching fundamental limits. Carbon nanotubes may become a practical alternative to customary devices due to their small size and unique mechanical and electronic properties.
Walter H. Schottky
Walter Hermann Schottky 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 and the pentode in 1919 while working at Siemens, and later made many significant contributions in the areas of...
, is a potential barrier formed at a metal–semiconductor junction which has rectifying characteristics, suitable for use as a diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
. The largest differences between a Schottky barrier and a p–n junction are its typically lower junction voltage, and decreased (almost nonexistent) depletion width in the metal.
Not all metal–semiconductor junctions form Schottky barriers. A metal–semiconductor junction that does not rectify current is called an ohmic contact
Ohmic contact
An ohmic contact is a region on a semiconductor device that has been prepared so that the current-voltage 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 Schottky contact...
. Rectifying properties depend on the metal's work function
Work function
In solid-state physics, the work function is the minimum energy needed to remove an electron from a solid to a point immediately outside the solid surface...
, the band gap
Band gap
In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...
of the intrinsic semiconductor, the type and concentration
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
of dopant
Dopant
A dopant, also called a doping agent, is a trace impurity element that is inserted into a substance in order to alter the electrical properties or the optical properties of the substance. In the case of crystalline substances, the atoms of the dopant very commonly take the place of elements that...
s in the semiconductor, and other factors. Design of semiconductor devices requires familiarity with the Schottky effect to ensure Schottky barriers are not created accidentally where an ohmic connection is desired.
Advantages
Schottky barriers, with their lower junction voltage, find application where a device better approximating an ideal diode is desired. They are also used in conjunction with normal diodes and transistorTransistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
s, where their lower junction voltage is used for circuit protection (among other things).
Because one of the materials in a Schottky diode is a metal, lower resistance devices are often possible. In addition, the fact that only one type of dopant is needed may greatly simplify fabrication. And 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.
Devices
A metal–semiconductor junction that forms a Schottky barrier as a device by itself is known as a Schottky diodeSchottky diode
The Schottky diode is a semiconductor diode with a low forward voltage drop and a very fast switching action...
.
A bipolar junction transistor
Bipolar junction transistor
|- align = "center"| || PNP|- align = "center"| || NPNA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons...
with a Schottky barrier between the base and the collector is known as a Schottky transistor
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:...
. 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
Transistor-transistor logic
Transistor–transistor logic is a class of digital circuits built from bipolar junction transistors and resistors. It is called transistor–transistor logic because both the logic gating function and the amplifying function are performed by transistors .TTL is notable for being a widespread...
families, as well as their low power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
variants.
A MESFET
MESFET
MESFET stands for metal semiconductor field effect transistor. It is quite similar to a JFET in construction and terminology. The difference is that instead of using a p-n junction for a gate, a Schottky junction is used...
, or Metal–Semiconductor FET, is a device similar in operation to the JFET
JFET
The junction gate field-effect transistor is the simplest type of field-effect transistor. It can be used as an electronically-controlled switch or as a voltage-controlled resistance. Electric charge flows through a semiconducting channel between "source" and "drain" terminals...
, which utilizes a reverse biased Schottky barrier to provide the depletion region. A particularly interesting variant of this device is the HEMT
HEMT
High electron mobility transistor , also known as heterostructure FET or modulation-doped FET , is a field effect transistor incorporating a junction between two materials with different band gaps as the channel instead of a doped region, as is generally the case for MOSFET...
, or High Electron Mobility Transistor, which also utilizes a heterojunction
Heterojunction
A heterojunction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction...
to provide a device with extremely high conductance.
Schottky barriers are commonly used also in semiconductor electrical characterization techniques. In fact, in the semiconductor, a depletion region
Depletion region
In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region, is an insulating region within a conductive, doped semiconductor material where the mobile charge carriers have diffused away, or have been forced away by an...
is created by the metal electrons, which "push" away semiconductor electrons (simplification, see depletion region
Depletion region
In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region, is an insulating region within a conductive, doped semiconductor material where the mobile charge carriers have diffused away, or have been forced away by an...
article). In the depletion region, dopants remain ionized and give rise to a "space charge" which, in turn, give rise to a capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
of the junction. The metal-semiconductor interface and the opposite boundary of the depleted area act like two capacitor plates, with the depletion region
Depletion region
In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region, is an insulating region within a conductive, doped semiconductor material where the mobile charge carriers have diffused away, or have been forced away by an...
acting as a dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
.
By applying a voltage to the junction it is possible to vary the depletion width: if we reverse bias the junction, the dopants electrons will be emitted and pushed away; if we forward bias the junction, the electrons will be captured.
By analyzing the emission and capture of electrons by dopants (or, more frequently, by crystallographic defects or dislocations, or other electron traps) is possible to characterize the semiconductor material.
The most popular electrical characterization techniques that use this type of junction are DLTS and CV profiling
CV profiling
Capacitance-voltage profiling is a technique for characterizing semiconductor materials and devices. The applied voltage is varied, and the capacitance is measured and plotted as a function of voltage...
.
A Schottky barrier carbon nanotube FET uses the nonideal contact between a metal and a carbon nanotube (CNT) to form a Schottky barrier that can be used to make Schottky diodes or transistors, or so on. The scaling of semiconductor devices to ever-smaller sizes is rapidly approaching fundamental limits. Carbon nanotubes may become a practical alternative to customary devices due to their small size and unique mechanical and electronic properties.
See also
- Ohmic contactOhmic contactAn ohmic contact is a region on a semiconductor device that has been prepared so that the current-voltage 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 Schottky contact...
- Schottky diodeSchottky diodeThe Schottky diode is a semiconductor diode with a low forward voltage drop and a very fast switching action...
- DiodeDiodeIn electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
- Metal-induced gap statesMetal-induced gap statesIn bulk semiconductor band structure calculations, it is assumed that the crystal lattice of the material is infinite. When the finite size of a crystal is taken into account, the wavefunctions of electrons are altered and states that are forbidden within the bulk semiconductor gap are allowed at...
- MemristorMemristorMemristor is a passive two-terminal electrical component envisioned by Leon Chua as a fundamental non-linear circuit element relating charge and magnetic flux linkage...