N-type semiconductor
Encyclopedia
N-type semiconductors are a type of extrinsic semiconductor
where the dopant
atoms are capable of providing extra conduction electrons to the host material (e.g. phosphorus
in silicon
). This creates an excess of negative (n-type) electron charge carriers.
invented a type of diode
during the development of the radio. He used the rectifying properties of galena
crystal (lead sulfide
).
In 1909, Braun shared the Nobel Prize in physics with Guglielmo Marconi
for the development of wireless
telegraphy
.
Later, semiconductors found applications in transistor
s. A transistor is a solid-state electronic device that controls the flow of electric current. Until World War II, most systems used vacuum tubes for the amplification and control of electric current. Those vacuum tubes were bulky and fragile, consumed much power, and tended to overheat. The demands of radar, in particular during World War II, encouraged scientists to look for another method of amplifying and controlling electric current in communication devices.
because at their Fermi level
, there is a large density of energetically available states that each electron can occupy. Electrons can move quite freely between energy levels without a high energy cost. Metal conductivity decreases with temperature increase because thermal vibrations of crystal lattice disrupt the free motion of electrons. Insulators, by contrast, are very poor conductors of electricity because there is a large difference in energies (called a band gap
) between electron-occupied energy levels and empty energy levels that allow for electron motion.
Insulator conductivity increases with temperature because heat provides energy to promote electrons across the band gap to the higher electron conduction energy levels (called the conduction band
). Semiconductors, on the other hand, have an intermediate level of electric conductivity when compared to metals and insulators. Their band gap is small enough that small increase in temperature promotes sufficient number of electrons (to result in measurable currents) from the lowest energy levels (in the valence band
) to the conduction band. This creates electron holes, or unoccupied levels, in the valence band, and very loosely held electrons in the conduction band.
An intrinsic semiconductor
is made up ideally of one pure element, typically silicon. At room temperature, the conductivity of intrinsic semiconductors is relatively low. Conductivity is greatly enhanced by a process called doping
, in which other elements are added to the intrinsic crystal in very small amounts to create what is called an extrinsic semiconductor
. When the dopant provides extra electrons to the host, the product is called an n-type semiconductor. The process of doping is described as it introduces energy levels into band gap; those levels are filled with electrons and lie close to the conduction band so that even slight thermal agitation can release them into the conduction band.
It should be noted, that the negative charge of the electrons is balanced by an equivalent positive charge in the center of the impurity atoms. Therefore, the net electrical charge of the semiconductor material is not changed.
than one type of the host atoms. The most common example is atomic substitution in group IV solids (silicon
, germanium
, tin
which contain four valence electrons) by group V elements (phosphorus
, arsenic
, antimony
) which contain five loosely bound valence electrons. The situation is more uncertain when the host contains more than one type of atoms. For example, in III-V semiconductors like gallium arsenide, silicon can be a donor when it substitutes for gallium and acceptor when it replaces arsenic. Some donors have fewer valence electrons than the host, such as alkali metal
s, which are donors in most solids.
plays an important role in describing the behavior of doped semiconductors. A substance’s Fermi level is defined as the highest occupied energy level found in that substance at absolute zero
temperature (0 kelvins or -273 °C). At higher temperatures, energy from heat is available to promote electrons into slightly higher energy levels. However, picturing density of states to be filled to the Fermi level helps scientists understand different behaviors between insulators, metals, and intrinsic and extrinsic semiconductors. As seen in figure one, the Fermi level of n-type semiconductors is elevated from that of the corresponding un-doped intrinsic semiconductor. This makes the conduction band much more thermally accessible at temperatures above absolute zero.
is passed over the gallium arsenide, and sulfur is incorporated into the structure. This process is characterized by a constant concentration of sulfur on the surface. In the case of semiconductors in general, only a very thin layer of the wafer needs to be doped in order to obtain the desired electronic properties. The reaction conditions typically range from 600 to 800 °C for the n-doping with group VI elements, and the time is typically 6–12 hours depending on the temperature.
s. These are p-type and n-type semiconductors brought together in close contact, creating what is called the depletion region
. The importance of this contact or junction is the creating of a region between the p and n layers where p-type holes can recombine with n-type free electrons producing light, such as in light emitting diodes (LEDs). P-n junctions form the basis of how a lot of current technology works by creating diodes. In a diode
, current can flow easily in one direction but not the other, which is a basis for digital electronics.
is held at a positive potential, however, there is negligible current in the case where tungsten is held at a negative potential. The reverse is true in the case of 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. Schottky diode
s also allow for faster switching times compared to p-n junctions.
The n-type junction (Schottky diode) can also be used in producing a hydrogen fuel cell. Water and hydrogen would combine on a palladium
layer and generate a thermionic current sent into an n-type silicon carbide
semiconductor with a potential greater than the Schottky barrier
. These solid state
fuel cells can potentially allow for integration of fuel cells into portable electronics.
s have been of great research interest for use in low cost, ultra thin, and flexible products such as displays and solar energy conversion cells. While many p-type organic semiconductors have been thoroughly characterized, n-type organic semiconductors have proven hard to obtain. Both types are needed for the diodes and transistors that make desirable devices possible. N-type organic semiconductors were produced of the arylene diimide family that are resistant to thermal and environmental stresses, which is one of the largest challenges in the field. Several other compounds are being explored for n-type organic semiconductors for use in organic field-effect transistors (OFET), such as fullerene
(C60) and chemically modified oligothiophenes. Semiconductors are made from these compounds by reduction with electron withdrawing groups or, alternatively, by modifying the surface properties to control electron trapping. Organic thin film transistors (OTFTs) are being explored because their low synthesis temperatures allow them to be deposited on thin plastic substrates without damage, resulting in thin and flexible devices. Same compounds are often considered for use in OFETs and OTFTs.
Extrinsic semiconductor
An extrinsic semiconductor is a semiconductor that has been doped, that is, into which a doping agent has been introduced, giving it different electrical properties than the intrinsic semiconductor....
where the dopant
Doping (semiconductor)
In semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic...
atoms are capable of providing extra conduction electrons to the host material (e.g. phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
in silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
). This creates an excess of negative (n-type) electron charge carriers.
History of semiconducting systems
Around 1898, Karl Ferdinand BraunKarl Ferdinand Braun
Karl Ferdinand Braun was a German inventor, physicist and Nobel laureate in physics. Braun contributed significantly to the development of the radio and television technology: he shared with Guglielmo Marconi the 1909 Nobel Prize in Physics.-Biography:Braun was born in Fulda, Germany, and...
invented a type of 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...
during the development of the radio. He used the rectifying properties of galena
Galena
Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...
crystal (lead sulfide
Lead sulfide
Lead sulfide is an ionic compound of lead and sulfur, having two possible proportions:*Lead sulfide, the ionic compound containing lead in the +2 oxidation state*Lead sulfide, the ionic compound containing lead in the +4 oxidation state...
).
In 1909, Braun shared the Nobel Prize in physics with Guglielmo Marconi
Guglielmo Marconi
Guglielmo Marconi was an Italian inventor, known as the father of long distance radio transmission and for his development of Marconi's law and a radio telegraph system. Marconi is often credited as the inventor of radio, and indeed he shared the 1909 Nobel Prize in Physics with Karl Ferdinand...
for the development of wireless
Wireless
Wireless telecommunications is the transfer of information between two or more points that are not physically connected. Distances can be short, such as a few meters for television remote control, or as far as thousands or even millions of kilometers for deep-space radio communications...
telegraphy
Telegraphy
Telegraphy is the long-distance transmission of messages via some form of signalling technology. Telegraphy requires messages to be converted to a code which is known to both sender and receiver...
.
Later, semiconductors found applications in transistor
Transistor
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. A transistor is a solid-state electronic device that controls the flow of electric current. Until World War II, most systems used vacuum tubes for the amplification and control of electric current. Those vacuum tubes were bulky and fragile, consumed much power, and tended to overheat. The demands of radar, in particular during World War II, encouraged scientists to look for another method of amplifying and controlling electric current in communication devices.
Basic science of n-type semiconductors
Semiconductors are defined by their unique electric conductive behavior. Metals are good conductorsElectrical 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...
because at their Fermi level
Fermi level
The Fermi level is a hypothetical level of potential energy for an electron inside a crystalline solid. Occupying such a level would give an electron a potential energy \epsilon equal to its chemical potential \mu as they both appear in the Fermi-Dirac distribution function,which...
, there is a large density of energetically available states that each electron can occupy. Electrons can move quite freely between energy levels without a high energy cost. Metal conductivity decreases with temperature increase because thermal vibrations of crystal lattice disrupt the free motion of electrons. Insulators, by contrast, are very poor conductors of electricity because there is a large difference in energies (called a 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...
) between electron-occupied energy levels and empty energy levels that allow for electron motion.
Insulator conductivity increases with temperature because heat provides energy to promote electrons across the band gap to the higher electron conduction energy levels (called the 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...
). Semiconductors, on the other hand, have an intermediate level of electric conductivity when compared to metals and insulators. Their band gap is small enough that small increase in temperature promotes sufficient number of electrons (to result in measurable currents) from the lowest energy levels (in the valence band
Valence band
In solids, the valence band is the highest range of electron energies in which electrons are normally present at absolute zero temperature....
) to the conduction band. This creates electron holes, or unoccupied levels, in the valence band, and very loosely held electrons in the conduction band.
An intrinsic semiconductor
Intrinsic semiconductor
An intrinsic semiconductor, also called an undoped semiconductor or i-type semiconductor, is a pure semiconductor without any significant dopant species present. The number of charge carriers is therefore determined by the properties of the material itself instead of the amount of impurities...
is made up ideally of one pure element, typically silicon. At room temperature, the conductivity of intrinsic semiconductors is relatively low. Conductivity is greatly enhanced by a process called doping
Doping (semiconductor)
In semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic...
, in which other elements are added to the intrinsic crystal in very small amounts to create what is called an extrinsic semiconductor
Extrinsic semiconductor
An extrinsic semiconductor is a semiconductor that has been doped, that is, into which a doping agent has been introduced, giving it different electrical properties than the intrinsic semiconductor....
. When the dopant provides extra electrons to the host, the product is called an n-type semiconductor. The process of doping is described as it introduces energy levels into band gap; those levels are filled with electrons and lie close to the conduction band so that even slight thermal agitation can release them into the conduction band.
It should be noted, that the negative charge of the electrons is balanced by an equivalent positive charge in the center of the impurity atoms. Therefore, the net electrical charge of the semiconductor material is not changed.
Dopants
The doping atoms usually have one more valence electronValence electron
In chemistry, valence electrons are the electrons of an atom that can participate in the formation of chemical bonds with other atoms. Valence electrons are the "own" electrons, present in the free neutral atom, that combine with valence electrons of other atoms to form chemical bonds. In a single...
than one type of the host atoms. The most common example is atomic substitution in group IV solids (silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
, tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
which contain four valence electrons) by group V elements (phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
, arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
, antimony
Antimony
Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...
) which contain five loosely bound valence electrons. The situation is more uncertain when the host contains more than one type of atoms. For example, in III-V semiconductors like gallium arsenide, silicon can be a donor when it substitutes for gallium and acceptor when it replaces arsenic. Some donors have fewer valence electrons than the host, such as alkali metal
Alkali metal
The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
s, which are donors in most solids.
The Fermi level
The Fermi levelFermi level
The Fermi level is a hypothetical level of potential energy for an electron inside a crystalline solid. Occupying such a level would give an electron a potential energy \epsilon equal to its chemical potential \mu as they both appear in the Fermi-Dirac distribution function,which...
plays an important role in describing the behavior of doped semiconductors. A substance’s Fermi level is defined as the highest occupied energy level found in that substance at absolute zero
Absolute zero
Absolute zero is the theoretical temperature at which entropy reaches its minimum value. The laws of thermodynamics state that absolute zero cannot be reached using only thermodynamic means....
temperature (0 kelvins or -273 °C). At higher temperatures, energy from heat is available to promote electrons into slightly higher energy levels. However, picturing density of states to be filled to the Fermi level helps scientists understand different behaviors between insulators, metals, and intrinsic and extrinsic semiconductors. As seen in figure one, the Fermi level of n-type semiconductors is elevated from that of the corresponding un-doped intrinsic semiconductor. This makes the conduction band much more thermally accessible at temperatures above absolute zero.
Charge carriers
The concept of free electrons and corresponding holes in a solid semiconductor is a useful tool for understanding conduction through the solid. At temperatures above absolute zero, some electrons in an n-type semiconductor are expected to be excited from the energy level of the dopant into the conduction band. This creates a state with a few loosely held, very mobile electrons occupying some of the large amount of states available in the conduction band. The vacancies left in the dopant band group V ions contain a positively charged “hole” in their valence shell. While the amount of holes per filled levels in the dopant band is low, there is a large number of holes or levels to occupy per electron in the conduction band (see figure 2). The electrons can easily move between these available states and conduct a current. In the case of n-type semiconductors, the electrons are considered to be the majority charge carrier. The holes of the dopant band are considered minority carriers.Techniques of doping and synthesis
The synthesis of n-type semiconductors may involve the use of vapor-phase epitaxy. In vapor-phase epitaxy, a gas containing the negative dopant is passed over the substrate wafer. In the case of n-type GaAs doping, hydrogen sulfideHydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
is passed over the gallium arsenide, and sulfur is incorporated into the structure. This process is characterized by a constant concentration of sulfur on the surface. In the case of semiconductors in general, only a very thin layer of the wafer needs to be doped in order to obtain the desired electronic properties. The reaction conditions typically range from 600 to 800 °C for the n-doping with group VI elements, and the time is typically 6–12 hours depending on the temperature.
The p-n junction
Perhaps the most important current use of n-type semiconductors is in p-n junctionP-n junction
A p–n junction is formed at the boundary between a P-type and N-type semiconductor created in a single crystal of semiconductor by doping, for example by ion implantation, diffusion of dopants, or by epitaxy .If two separate pieces of material were used, this would...
s. These are p-type and n-type semiconductors brought together in close contact, creating what is called 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...
. The importance of this contact or junction is the creating of a region between the p and n layers where p-type holes can recombine with n-type free electrons producing light, such as in light emitting diodes (LEDs). P-n junctions form the basis of how a lot of current technology works by creating diodes. In 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...
, current can flow easily in one direction but not the other, which is a basis for digital electronics.
Rectifying junction
A rectifying junction can be made when certain metals are in contact with a semiconductor crystal. In the case of an n-type germanium crystal in contact with tungsten, current is allowed to flow when the tungstenTungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
is held at a positive potential, however, there is negligible current in the case where tungsten is held at a negative potential. The reverse is true in the case of 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. Schottky diode
Schottky diode
The Schottky diode is a semiconductor diode with a low forward voltage drop and a very fast switching action...
s also allow for faster switching times compared to p-n junctions.
The n-type junction (Schottky diode) can also be used in producing a hydrogen fuel cell. Water and hydrogen would combine on a palladium
Palladium
Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...
layer and generate a thermionic current sent into an n-type silicon carbide
Silicon carbide
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
semiconductor with a potential greater than the Schottky barrier
Schottky barrier
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...
. These solid state
Solid-state chemistry
Solid-state chemistry, also sometimes referred to as materials chemistry, is the study of the synthesis, structure, and properties of solid phase materials, particularly, but not necessarily exclusively of, non-molecular solids...
fuel cells can potentially allow for integration of fuel cells into portable electronics.
N-type semiconductors in organic devices
Organic semiconductorOrganic semiconductor
An organic semiconductor is an organic material with semiconductor properties. Single molecules, short chain and organic polymers can be semiconductive. Semiconducting small molecules include the polycyclic aromatic compounds pentacene, anthracene, and rubrene...
s have been of great research interest for use in low cost, ultra thin, and flexible products such as displays and solar energy conversion cells. While many p-type organic semiconductors have been thoroughly characterized, n-type organic semiconductors have proven hard to obtain. Both types are needed for the diodes and transistors that make desirable devices possible. N-type organic semiconductors were produced of the arylene diimide family that are resistant to thermal and environmental stresses, which is one of the largest challenges in the field. Several other compounds are being explored for n-type organic semiconductors for use in organic field-effect transistors (OFET), such as fullerene
Fullerene
A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and they resemble the balls used in association football. Cylindrical ones are called carbon nanotubes or buckytubes...
(C60) and chemically modified oligothiophenes. Semiconductors are made from these compounds by reduction with electron withdrawing groups or, alternatively, by modifying the surface properties to control electron trapping. Organic thin film transistors (OTFTs) are being explored because their low synthesis temperatures allow them to be deposited on thin plastic substrates without damage, resulting in thin and flexible devices. Same compounds are often considered for use in OFETs and OTFTs.
See also
- DopingDoping (semiconductor)In semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic...
- Electronic band structureElectronic band structureIn solid-state physics, the electronic band structure of a solid describes those ranges of energy an electron is "forbidden" or "allowed" to have. Band structure derives from the diffraction of the quantum mechanical electron waves in a periodic crystal lattice with a specific crystal system and...
- Extrinsic semiconductorExtrinsic semiconductorAn extrinsic semiconductor is a semiconductor that has been doped, that is, into which a doping agent has been introduced, giving it different electrical properties than the intrinsic semiconductor....
- P-I-N
- P/N
- P-type semiconductorP-type semiconductorA P-type semiconductor is obtained by carrying out a process of doping: that is, adding a certain type of atoms to the semiconductor in order to increase the number of free charge carriers ....
- SemiconductorSemiconductorA 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...