Saturation velocity
Encyclopedia
Saturation velocity is the maximum velocity a charge carrier in a semiconductor
, generally an electron
, attains in the presence of very high electric field
s. Charge carrier
s normally move at an average drift speed proportional to the electric field strength
they experience temporally. The proportionality constant is known as mobility
of the carrier, which is a material property. A good conductor
would have a high mobility value for its charge carrier, which means higher velocity, and consequently higher current values for a given electric field strength. There is a limit though to this process and at some high field value, a charge carrier can not move any faster, having reached its saturation velocity, due to mechanisms that eventually limit the movement of the carriers in the material.
Saturation velocity is a very important parameter in the design of semiconductor devices, especially field effect transistors, which are basic building blocks of almost all modern integrated circuits. Typical values of saturation velocity may vary greatly for different materials, for example for Si
it is in the order of 1×107 cm/s, for GaAs
1.2×107 cm/s, while for 6H-SiC
, it is near 2×107 cm/s. Typical electric field strengths at which carrier velocity saturates is usually on the order of 10-100 kV/cm. Both saturation field and the saturation velocity of a semiconductor material are typically strong function of impurities, crystal defects and temperature.
For extremely small scale devices, where the high-field regions may be comparable or smaller than the average mean free path
of the charge carrier, one can observe velocity overshoot
, or hot electron effects which has become more important as the transistor geometries continually decrease to enable design of faster, larger and more dense integrated circuits. The regime where the two terminals between which the electron moves is much smaller than the mean free path, is sometimes referred as ballistic transport
. There has been numerous attempts in the past to build transistors based on this principle without much success. Nevertheless, developing field of nanotechnology
, and new materials such as Carbon
nanotubes and graphene
, offers new hope.
Though in a semiconductor such as Si saturation velocity of a carrier is same as the peak velocity of the carrier, for some other materials with more complex energy band structures, this is not true. In GaAs
or InP
for example the carrier drift velocity reaches to a maximum as a function of field and then it begins to actually decrease as the electric field applied is increased further. Carriers which have gained enough energy are kicked up to a different conduction band
which presents a lower drift velocity and eventually a lower saturation velocity in these materials. This results in an overall decrease of current for higher voltage until all electrons are in the "slow" band and this is the principle behind operation of a Gunn diode
, which can display negative differential resistivity. Due to the transfer of electrons to a different conduction band involved, such devices, usually single terminal, are referred to as transferred electron devices, or TEDs.
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
, generally an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
, attains in the presence of very high electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
s. Charge carrier
Charge carrier
In physics, a charge carrier is a free particle carrying an electric charge, especially the particles that carry electric currents in electrical conductors. Examples are electrons and ions...
s normally move at an average drift speed proportional to the electric field strength
Field strength
In physics, the field strength of a field is the magnitude of its vector value.In theoretical physics, field strength is another name for the curvature form...
they experience temporally. The proportionality constant is known as mobility
Mobility
Mobility may refer to:* Mobility * "Mobiliy" , a song by Moby* Mobility * Mobility , the ability of military units or weapon systems to move to an objective-See also:* Academic mobility* Apprentices mobility...
of the carrier, which is a material property. A good conductor
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
would have a high mobility value for its charge carrier, which means higher velocity, and consequently higher current values for a given electric field strength. There is a limit though to this process and at some high field value, a charge carrier can not move any faster, having reached its saturation velocity, due to mechanisms that eventually limit the movement of the carriers in the material.
Saturation velocity is a very important parameter in the design of semiconductor devices, especially field effect transistors, which are basic building blocks of almost all modern integrated circuits. Typical values of saturation velocity may vary greatly for different materials, for example for Si
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...
it is in the order of 1×107 cm/s, for GaAs
Gaas
Gaas is a commune in the Landes department in Aquitaine in south-western France....
1.2×107 cm/s, while for 6H-SiC
SIC
Sic is a Latin word that means "thus" or, in writing, "it was thus in the source material".Sic may also refer to:* Sic, Cluj, a commune in Romania* SiC, Silicon carbide, a semiconducting material, also used to make metalworking tools from...
, it is near 2×107 cm/s. Typical electric field strengths at which carrier velocity saturates is usually on the order of 10-100 kV/cm. Both saturation field and the saturation velocity of a semiconductor material are typically strong function of impurities, crystal defects and temperature.
For extremely small scale devices, where the high-field regions may be comparable or smaller than the average mean free path
Mean free path
In physics, the mean free path is the average distance covered by a moving particle between successive impacts which modify its direction or energy or other particle properties.-Derivation:...
of the charge carrier, one can observe velocity overshoot
Velocity overshoot
Velocity overshoot is a result of transit times of a charge carrier going from source to drain being smaller than the time required to emit an optical phonon. The velocity therefore exceeds the saturation velocity, which leads to faster field-effect transistor switching. This notion is utilized...
, or hot electron effects which has become more important as the transistor geometries continually decrease to enable design of faster, larger and more dense integrated circuits. The regime where the two terminals between which the electron moves is much smaller than the mean free path, is sometimes referred as ballistic transport
Ballistic transport
Ballistic transport is the transport of electrons in a medium with negligible electrical resistivity due to scattering. Without scattering, electrons simply obey Newton's second law of motion at non-relativistic speeds....
. There has been numerous attempts in the past to build transistors based on this principle without much success. Nevertheless, developing field of nanotechnology
Nanotechnology
Nanotechnology is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with developing materials, devices, or other structures possessing at least one dimension sized from 1 to 100 nanometres...
, and new materials such as Carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
nanotubes and graphene
Graphene
Graphene is an allotrope of carbon, whose structure is one-atom-thick planar sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The term graphene was coined as a combination of graphite and the suffix -ene by Hanns-Peter Boehm, who described single-layer...
, offers new hope.
Though in a semiconductor such as Si saturation velocity of a carrier is same as the peak velocity of the carrier, for some other materials with more complex energy band structures, this is not true. In GaAs
Gaas
Gaas is a commune in the Landes department in Aquitaine in south-western France....
or InP
INP
INP may stand for:*INP , a Korean chemical company*INP , an early database system developed at the University of California, Berkeley*Integrated National Police, a forerunner of the Philippine National Police...
for example the carrier drift velocity reaches to a maximum as a function of field and then it begins to actually decrease as the electric field applied is increased further. Carriers which have gained enough energy are kicked up to a different conduction band
Conduction band
In the solid-state physics field of semiconductors and insulators, the conduction band is the range of electron energies, higher than that of the valence band, sufficient to free an electron from binding with its individual atom and allow it to move freely within the atomic lattice of the material...
which presents a lower drift velocity and eventually a lower saturation velocity in these materials. This results in an overall decrease of current for higher voltage until all electrons are in the "slow" band and this is the principle behind operation of a Gunn diode
Gunn diode
A Gunn diode, also known as a transferred electron device , is a form of diode used in high-frequency electronics. It is somewhat unusual in that it consists only of N-doped semiconductor material, whereas most diodes consist of both P and N-doped regions...
, which can display negative differential resistivity. Due to the transfer of electrons to a different conduction band involved, such devices, usually single terminal, are referred to as transferred electron devices, or TEDs.