Glossary for Semiconductors


Absolute Zero
The lowest possible temperature and energy of a thermodynamic system. It is equal to 0 Kelvin, -273.15° Celsius, or -459.67° F. Click here to see a primer on the three temperature scales.




Acceptor
An element that "accepts" an electron from a semiconductor atom.  Acceptors will have one fewer valence band than the semiconductor they accept from.  For example, Gallium (Z=31, 3 valence electrons) could be an acceptor for a semiconductor like silicon (Z=14) or germanium (Z=32), both of which have 4 valence electrons.  Acceptors facilitate conduction when used to dope semiconductors, since they provide an extra hole.  Semiconductors that are doped with acceptor atoms are called p-type Semiconductor.  Since semiconductors tend to fall into group IVA of the periodic table, acceptors will be found in group IIIA.




Angular Momentum
Generally denoted by L, the angular momentum about a given point equals the cross product of distance r from that point and linear momentum p:
L = r x p.

The figure on the left shows Rensselaer president Dr. Shirley Ann Jackson using a modified bicycle wheel to demonstrate how angular momentum is always conserved (never changes).

It is assumed that the user of these materials has some familiarity with angular momentum.  If not, see this link  (click "Visit Site")  for a ThinkQuest page explaining angular momentum.




Atomic Number (Z)
The number identifying an element in the periodic table, the atomic number of an element equals the  number of protons (which also equals the number of electrons) in the nucleus of a neutral atom.




Atomic Spectra
The collection of wavelengths (or their corresponding colors) of light that can be emitted or absorbed by an element.  Light is emitted (or absorbed) by an atom when electrons change from one energy state to another as the atom loses (or gains) energy.   (Disclaimer)  Since each element has its own characteristic set of allowed energy states, the set of allowed transitions between energy states is unique to a particular element.  Thus the atomic spectrum of an element can be used to identify it.  For an overview of atomic spectra, see this this page of the University of Colorado's Physics 2000 project.  Or, go to this page in the project to see an explanation of how spectra are formed.




(Energy) Band
The continuous range of allowed energies for electrons in a solid.  Individual atoms can have only certain quantized energies.  As atoms bond to form solids, each energy level "spreads" to accommodate the shifted levels of adjacent atoms.  Atoms in the solid can occupy any energy in one of the newly-formed "bands". This page of this module provides more detail and animations depicting the formation of energy bands.




Band Gap
The range of energies between two allowed energy bands in a solid.  Electrons in the solid can only take energies in the allowed bands, so electrons cannot have one of the energies in the gap.   (Disclaimer)  We are typically interested in the band gap falling between the conduction and valence bands in semiconductors, as depicted in the energy diagram to the right.




Bohr, Niels
The "father of the atom", Niels Bohr explained several experimental results with his solar-system-like atomic model.  He applied the idea of quantized energies to atoms, proposing that electrons in atoms can only take certain discrete, or "quantized", values of energy.   (Disclaimer)  They don't spiral into the nucleus because they can only emit certain values of energy (values equal to a difference between allowed energy levels) and are thus "stuck" in their orbits.  (See this word of caution about visualizing electron orbits.)  Bohr's quantization of energy in the atom also explained why elements only emit certain colors (and certain energies) of light.  These spectra are characteristic of an element and determined by the energy levels in atoms of that element.  Click here for more information on Niels Bohr.




Boltzmann Constant (kB)
A constant of nature, converting the temperature of a gas (in degrees Kelvin) to the kinetic energy (in Joules or electonVolts) associated with the thermal motion of the particles comprising the gas.  It has the numeric value of
kB = 1.38 x 10-23 J/K = 8.62 x 10-5 eV/K




Bound (States)
An electron is in a bound state of an atom (or of a solid) when it does not have enough energy to leave the vicinity of the atom (or solid).   (Disclaimer)  The lower the energy of the electron, the more "tightly" bound it is and the less likely it is to take part in interactions with electrons of other atoms or to stray from the atom to which it is bound.  In this author's convention, tightly-bound electrons in a solid are represented in an energy diagram (such as the one to the right) by blue circles.  The green circles represent more loosely-bound conduction electrons, which can move freely through a solid.




Charge Carrier
A particle having electric charge that can move freely through the material.  In conductors, electrons are free to move, so they are the charge carriers.  Holes are the charge carriers in p-type semiconductors.  Positive or negative ions can be charge carriers in some liquids.




Charge Carrier Number Density (n)
The number density (number per volume) of charge carriers in a material.  In conductors, electrons are free to move, so n for a conductor equals the number density of conduction electrons in the material.  Most conductors have one conduction electron per atom, but some have as many as four conduction electrons per atom.  One can use this property, along with the density of the solid and the atomic mass of the material, to calculate the number density of charge carriers per volume.  Charge is carried by both electrons and holes in intrinsic semiconductors, and some plasmas and solutions contain both positive and negative charge carriers as well.  One must add the density of holes (or other positive charge carriers) to the density of conduction electrons (or other negative charge carriers) to obtain the total charge carrier number density n.  Take note:  this usage of the symbol n should not be confused with the principle quantum number, which also uses the symbol n.




Conduction Band

The unfilled top Energy band in a solid. Since this band is not filled, electrons with energies in this band can move easily through the solid, creating an electric current.   (Disclaimer)  The image to the right follows this author's convention of representing electrons in the conduction band by green circles.  The blue circles represent electrons in filled (or mostly filled) bands, such as the valence band in the energy diagram to the right.




Conduction Electrons
Electrons that are free to move within a solid.  The motion of these electrons can give rise to the conduction of electricity by creating an electric current through the solid .  The energy diagram of an intrinsic semiconductor found to the right follows this author's convention of representing conduction electrons by green circles.  The blue circles represent tightly bound electrons that do not significantly contribute to conduction through a solid.




Conductor
A material with low resistivity used for contacts and interconnects in semiconductor processing.  Conductors have a partially filled valence band, through which electrons can move freely, as shown in the energy diagram to the left.   Thus, in a conductor, the conduction band is the same as the valence band, and the charge carriers are primarily electrons.  In keeping with this author's convention, the electrons in this partially-filled conduction band are represented by green circles.




Conductivity (s)
A measure of how freely current can flow through a material.  Copper, with its high conductivity of 5.95 x 107 W-1m-1, conducts electric current more freely than does aluminum, with its slightly lower conductivity of 3.77 x 107 W-1m-1.  Conductivity is the inverse of resistivityr:
s = 1/r.




Contact Potential
A potential difference that arises at the junction of two different conducting materials.  For example, aluminum has a higher Fermi energy than does copper, so it is easier to remove an electron from aluminum than from copper.  When copper and aluminum are placed in contact, the discontinuity in Fermi energy at the boundary is quickly smoothed out - some of the conduction electrons from the aluminum flow into the copper, until the Fermi energy is the same on either side of the boundary.  This produces a net negative charge on the copper and a net positive charge on the aluminum.  The separation of charge sets up a potential difference, or voltage, across the boundary.  The size of this "contact potential" difference is dependent upon the difference in Fermi energies between the materials.




Continuous
Smoothly varying; taking any value.  An electron moving through empty space can have any value of energy, so we say the allowed energies for this free electron are continuous.  An electron in an atom, however, can only have certain discrete, or quantized, energies, according to the quantum atomic theory first suggested by Niels Bohr. (Disclaimer)




(Atomic) Core
The nucleus along with the electrons in completely-filled energy shells.  These tightly-bound electrons stay near the nucleus and do not contribute to conduction or interactions with other atoms.  They are of less interest than the valence electrons of partially-filled shells.    In the cartoon to the right, the pink sphere surrounds the core of Silicon; only the four valence electrons are outside this core.   (Disclaimer)
Note:  Figure not to scale or, 
for that matter, an accurate rep-
resentation of electron behavior.  For 
details, see our word of caution.




Coulomb (C)
The SI unit of electric charge.  One coulomb is a fairly large amount of charge, equaling the charge of 6.25 x 1018 protons.




Covalent Bond
An interaction between atoms by which they "share" valence electrons in the outermost energy shell, thereby filling the outer shell of both atoms involved in the interaction.  The electrons orbit both nuclei, binding the atoms together.  For example, each hydrogen atom in a hydrogen gas molecule has a single electron.  The valence energy level of a hydrogen atom is filled by two electrons; sharing their electrons allows each hydrogen atom to fill its energy shell. A nice pictorial explanation of ionic and covalent bonding is found on this page by Access Excellence @ the national health museum.




Cross-Sectional Area (A)
The (2-dimensional) area formed when one "slices" through a (3-dimensional) solid.  When discussing electric current, we typically take a cross section that is perpendicular to the direction of current flow, like the two cross sections of the pipe shown to the left.  Link here for a more technical definition from Wolfram's MathWorld.




Crystalline Structure
Regularly repeating array of atoms forming a solid.  The image to the right shows how carbon atoms are arranged in 3-D to make diamond.  The red atoms are closest to you, and the violet atoms are the furthest away.  The pattern of carbon atoms is exactly duplicated many times.  Such a repeating arrangement of atoms is called a "crystal"; the crystalline structure is the pattern that is repeated.




(Electric) Current (I)
The rate at which electric charge flows past a given point.  The convention in physics is to discuss the flow of positive charges, even though we now know that the motion of (negatively-charged) electrons is what creates a current in conductors.  According to this convention, if a beam of electrons traveled from right to left across your screen, we would say the current flows from left to right.  The SI unit is the Ampere, or Amp; one Amp is equal to one Coulomb of charge per second of time.  For example, if particles carrying 5 Coulombs of net electric charge flowed into a light bulb each second, we would say that the current through the bulb was 5 Amps.  For a good introduction to current and other circuit properties, try this page of the All About Circuits site.



Degrees Kelvin (K)
One degree Kelvin represents the same change in temperature as one degree centigrade, or Celsius, but the Kelvin scale "starts" at a different temperature.  Zero degrees Kelvin is absolute zero, the lowest temperature anything could ever take.    In contrast, zero degrees Celsius is the temperature at which water freezes.  When measuring changes in temperature, the Kelvin and Celsius scales give the same result.  But when you need the absolute temperature, such as when calculating how much thermal energy an object has, Kelvin is the preferred unit.  One can convert temperature (T) between degrees Celsius (°C) or degrees Ferenheit (°F) and degrees Kelvin (K) using the following relationships:
T(K) = T(°C) + 273.15, and
T(K) = T(°F) * 5/9 +241.15
Click here to see a primer on the three temperature scales.




Diffuse
To spread out, particularly when substance was originally concentrated.  Consider a drop of red food coloring placed in a glass of water.  Initially, the dye is concentrated in the single drop.  As time elapses, however, the dye will diffuse, spreading out through the entire glass of water.




Diffusion Current
The current occurring at a p-n junction due to diffusion of charge carriers.  The n-side of the junction contains conduction electrons, while the p-side contains holes.  These electrons and holes will move across the junction as they diffuse, causing a net (positive) electric current from the p-side to the n-side.




Diode
A semiconductor device which allows current to flow in only one direction.  A  is composed of a single p-n junction, and the words can often be used interchangeably.  When a forward bias  is applied (positive terminal of a battery connected to the p-side of the junction and negative terminal to the n-side), current flows freely through the device.  But when the battery terminals are reversed and this reverse bias applied, no significant amount of current will flow. Operation of diodes is discussed on this page of this module.




Donor
An element that "donates" an electron to a semiconductor atom.  Donors will have one more valence electron than the semiconductor they accept from.  For example, Phosphorus (Z=15, 5 valence electrons) could be a donor for a semiconductor like silicon (Z=14) or germanium (Z=32), both of which have 4 valence electrons.  Donors facilitate conduction when used to dope semiconductors, since they provide an extra conduction electron.  Semiconductors that are doped with donor atoms are called n-type Semiconductors.  Since semiconductors tend to fall into group IVA of the periodic table, acceptors will be found in group VA.




Doping
The process of introducing impurities into a substance to enhance or control its properties.  For example, silicon is often doped with gallium or with phosphorus.  Both increase the conductivity of silicon; gallium increases the concentration of holes in the silicon, and phosphorus increases the concentration of conduction electrons.  Doping not only increases the conductivity but it can be used to produce diodes, transistors, and many other devices.  The discussion of doping starts on this page of this module.



Drift Velocity
The average velocity of charge carriers in a material.  This is not the same as the average speed.  In general, charge carriers move very quickly, but mostly randomly, colliding with each other and with the atomic cores around them.  When a voltage is applied, the random motion continues, but with an overall net "drift" in the direction of the force due to the applied voltage.  The average speed of a charge carrier may be on the order of 106 m/s, while the magnitude of the drift velocity is about 10-5 m/s.




Effective Mass
The value of mass one must use to model electronbehavior in solids as the motion of independent particles.  A conduction electron in a solid interacts with the atomic cores of nearby atoms, as well as with other conduction electrons in the solid.  The motion of all the electrons is very interconnected and complicated, yet we can successfully describe observed effects by treating each conduction electron as an independent particle with an "effective mass" determined by experiment and characteristic of a material.  Typically the effective mass is less than the mass of an electron in free space ( 9.11 x 10-31 kg).  See this page of the Wikpedia for more information on, and some representative values of, effective mass.


Electron
A fundamental particle with negative charge found in atoms.  It has a mass of 9.11 x 10-31 kilograms and a charge of - 1.6 x 10-19Coulombs.  Conduction occurs primarily from the movement of electrons through materials.  Click this link to go to the Particle Adventure webpage for more information on electrons.


ElectronVolt (eV)
A unit of energy, equal to the energy gained by an electron that passes through a potential difference of 1 Volt.  An electronVolt is related to the SI energy unit Joule by the charge of the electron e as follows:
1 eV = 1.6 x 10-19 J.




Energy
Energy comes in many different forms, such as "kinetic energy" (energy of motion), heat, and even mass.  Energy can never be created or destroyed; it can merely change from one form to another.  These materials assume the reader has some prior knowledge of energy as it is defined in science. If you are not confident in your prior knowledge, you can access Glenbrook High's Physics Classroom pages on energy, by clicking here.  If you only want an overview, try this nice site by the Danish Wind Industry Association.



Energy Diagram
A plot showing energies on the vertical axis, typically used to illustrate the specific quantized energies allowable for electrons (Disclaimer) in atoms and solids.  The energy diagram for a silicon atom shown on the left indicates the fourteen electrons in their lowest states.  (The Pauli exclusion principle determines how many electrons can "fit" into each value of energy.)  The band diagram on the right, shown for a pure semiconductor such as silicon, shows electrons populating the energy bands of a solid.  Energy is again the vertical axis; the horizontal axis represents location in one dimension.  Both diagrams follow this author's color convention:  blue electrons are tightly bound in filled states or bands and not likely to move or interact; green electrons are more loosely bound and participate in bonding (between atoms) or conduction (within a solid).



Energy Level
Essentially synonymous with energy shell, an energy level is a collection of energy states with the same energy (in the absence of electromagnetic fields) for electrons in atoms (Disclaimer).  According to the Pauli Exclusion Principle, electrons can never occupy identical states.  Electrons in different states within an energy level are distinguished by other characteristics, such as their angular momentum and spin.  In an atom, the lowest energy level (having principle quantum number n=1) has two distinct states, the second energy level (n=2) contains eight different states, and so on.  You may see a pattern emerging - the number of energy states within an energy level determines the shape of the periodic table.




(Energy) State
A complete set of variable values that a particle can exhibit.  For example, an electron in a hydrogen atom can be described by its location, its energy (Disclaimer), and its angular momentum.  The collection of these properties constitutes the "state" of the electron.  According to quantum theory, an electron in an atom can take only certain, quantized, values of energy and angular momentum, so the number of states in an atom is limited.




Excited State
Any energy state of an atom, or of a solid, above its ground state.  For our purposes, we are primarily interested in the energy levels of electrons in atoms and solids (Disclaimer), so when we refer to "excited state" we mean at least one of the electrons having an energy higher than its ground-state energy.  Excited states are not indefinitely stable, so the atom will eventually return to its ground state.



(Pauli) Exclusion Principle
First proposed by Wolfgang Pauli to explain the arrangement of electrons in atoms, the Exclusion Principle asserts that no two electrons can be in the same state.  In other words, two electrons in the same atom cannot have the same energy(Disclaimer), angular momentum and spin.  We now know that the Exclusion Principle applies not just to electrons, but to an entire class of particles called "fermions", which includes protons and neutrons. Click here to go to the Particle Adventure's website, or click here to go to the Physics 2000 page explaining the Exclusion Principle.




Faraday's Law
Michael Faraday (and Joseph Henry, working independently in America) performed a series of experiments proving that changing the magnetic field through a loop of wire will produce a current through the loop.  The direction of the current "induced" by the changing magnetic field depends upon the direction of change of the field.  Try this site for more information about Faraday's Law (Note:  this site employs calculus; non-calculus-savvy users may try this site or they might look up Faraday's Law a conceptual-based introductory physics text, such as the one by Hewitt).



Features
The individual pieces making up a circuit element.  For example, a transistor such as the one shown here has several features, including  the region(s) of n-type semiconductor (green), the region(s) of p-type semiconductor (blue), the gate(red), and the (yellow) region of insulating material that separates the gate from the semiconductor regions.  Of these four features, the insulation between the gate and the semiconductors is generally the narrowest, limited by the manufacturing process.  (Note:  the colors in this picture are fictitious, used only to aid the reader in identifying features.  True transistors are not multicolored but are pretty much gray all over.) To learn more about the manufacturing process, try this page of howstuffworks.




Fermi Energy
The energy of the highest occupied state in a solid in its ground state.  The Fermi Energy represents the kinetic energy of electrons responsible for conduction.  For more on the Fermi energy, try this Wikipedia page.




Forward Bias
A voltage applied to a diode (a p-n junction) in a direction that does produce electric current.  When a p-n junction is forward biased, the positive terminal of a battery is connected to the p-side of the diode, and the negative terminal to the n-side.  Biasing of a diode is discussed on this page of this module.




Gate (on Transistor)
The part of the transistor that controls the flow of electric current through the device. Applying a voltage to the (red) gate terminal in the transistor shown to the left allows conduction electrons to flow from one (green) n-type semiconductor region, through the (blue) p-type semiconductor region, to the other n-type region. For more on how transistors work, see this page (and the following) of this module.



Ground State
The energy state of an atom, or of a solid, when its total energy equals the minimum possible energy for that atom or solid.  For solids, this only occurs at a temperature of absolute zero.  Individual atoms may be in their ground state at non-zero temperatures, but they will not stay there indefinitely.  Instead, they can occasionally absorb thermal energy and move to an excited state for a while before they decay back to the ground state.  For our purposes, we are primarily interested in the energy levels of electrons in atoms and solids (Disclaimer), so when we refer to "ground state" we mean all the electrons having their lowest possible energy.




Group
A column in the periodic table.  Elements in the same group have the same number of valence electrons and so will tend to interact in similar manners.  For example, regular table salt consists of an atom of sodium (group IA) bound to an atom of chlorine (group VIIA).  Potassium is in the same group as sodium; it bonds with chlorine to make potassium chloride, which has many of the same properties as table salt.



Hole
Essentially the absence of an electron in an otherwise filled energy band, a hole can be treated as a positively-charged particle moving through the valence band of a solid.  Its effective mass can be experimentally determined. According to the Wikipedia entry for effective mass, a hole in solid silicon has an effective mass of about 81% of the rest mass of the electron, or 7.38 x 10-31 kg.  Holes are explained on this page of this module.




Impurity
Different type of atom scattered through an otherwise regular crystal.  The image to the right depicts silicon atoms (in red) in a regular array, with blue lines depicting shared electrons providing covalent bonds.  The lighter-colored atom could be gallium, an impurity that leads to p-type behavior in the solid.



Insulator
A material with very high resistivity (low conductivity) often used to prevent contact between conductors.  The poor conductivity is due to the completely full valance band, illustrated in the energy diagram to the right.  An insulator has no charge carriers at absolute zero and very few charge carriers at room temperature.  The energy diagram to the right follows this author's convention of representing conduction electrons by green circles.  The blue circles represent tightly bound electrons that do not significantly contribute to conduction through a solid.  The absence of a blue circle is a hole, which can contribute to conduction, as described on this page of this module.



Interconnect
A strip of conductor on a computer chip that connects circuit elements such as transistors and resistors to each other.




Intrinsic (Pure) Semiconductors
A semiconductor comprised of a single type of molecule, with no impurities.  Pure semiconductors are typically poor conductors of electricity.  In an intrinsic semiconductor, each conduction electron in the conduction band leaves behind a hole in the valence band that also contributes to conduction, as shown in the energy diagram to the right.  This diagram follows this author's convention of representing conduction electrons by green circles.  The blue circles represent tightly bound electrons that do not significantly contribute to conduction through a solid.  Each white circle represents a hole.




Ionic Bonding
An interaction between atoms in which one atom "donates" valence electron(s) to the other atom, resulting in filled energy shells for both atoms involved in the interaction.  The electrons originally from the donor atom move to the vicinity of the other "acceptor" atom, binding the atoms together.  For example, a chlorine atom has seven electrons, out of the eight needed to fill its valence band.  Sodium, on the other hand has a single electron in its valence band.  When sodium and chlorine bind, sodium donates its single valence electron to chlorine.  The remaining 10 electrons surrounding sodium comprise filled shells (2 in the lowest energy level, 8 in the next level), and the donated electron fills the valence shell for chlorine. A nice pictorial explanation of ionic and covalent bonding is found on this page by Access Excellence @ the national health museum.


Joule
The SI unit of energy, one Joule is a Newton-meter, or a kg m2/s2.




Kinetic Energy (KE)
The energy of motion, determined by the mass n of an object and its speed v, according to the following relationship:
KE = 1/2 mv2.




Light-Emitting Diode (LED)
A diodethat has been constructed a) to optimize the emission of light as conduction electrons and holes recombine at a p-n junction, and b) to maximize the fraction of the produced light that escapes the device and is seen.  LEDs are energy-efficient sources of light in widespread use as indicator lights and in traffic signals, among other applications.  The photo to the left shows three LEDs that produce different colors of light.  The color of the plastic surrounding the diode does not filter the light, since LED light is monochromatic when produced, but merely aids the user in identifying the LED.




Majority Carrier
The type of charged particle that is primarily responsible for current flow in a material.  In conductors, current is produced by the motion of electrons through the material, so electrons are the majority carrier in conductors.  But in some solutions and in p-type Semiconductors, the current is due to the motion of positive ions or holes.  In these cases, the positive ion or the hole would be the majority carrier.




Micron
One millionth of a meter, or one thousandth of a millimeter.  (In good SAT-question-style, a micron is to a millimeter what a millimeter is to a meter.)  At the time of this writing (2004), semiconductor devices can be manufactured to such precision that features on a chip can be smaller than a tenth of a micron!



Minority Carrier
The type of charged particle that does NOT contribute significantly to current flow in a material.  In n-type semiconductors, the number of conduction electrons is far larger than the number of holes, so holes are the minority carrier (and electrons are the majority carrier).  In p-type Semiconductors, the current is primarily due to the motion of hole, making conduction electrons the minority carrier.



Minority Current
Electric current due to the motion of minority carriers.



(Linear) Momentum
Generally denoted by p, linear momentum is the mass m of an object times its velocity v:
p = mv.
Since velocity has direction, so does momentum.  It is conserved (doesn't change) in the absence of forces.  It is assumed that the user of these materials has some familiarity with momentum.  If not, see this link to the Glenbrook High's Physics Classroom Webpage for a good introduction to the topic.




Neutron
A fundamental particle with no electric charge found in the nuclei of atoms.  It has a mass of 1.675 x 10-27 kilograms. Click this link to go to the Particle Adventure webpage for more information on neutrons.


Nucleus (Nuclei)
The positively-charged "center" of an atom, comprised of protons and neutrons.  In the cartoon of a silicon atom shown to the right, the protons are represented by red spheres, and the neutrons by turquoise spheres.  Electrons are blue (when in a completely filled energy level) or green (when in a partially full energy level).  These colors are purely a convention, unique to this author, and do not represent optical properties of the particles. 
Note:  Figure not to scale or, 
for that matter, an accurate rep-
resentation of electron behavior.  For 
details, see our word of caution.




Number Density (n)
The number of a particular type of object found in each unit volume.  For example, if 2500 cattle are fairly uniformly spread across 100 acres of grassland, the number density of cattle in the region is 25 cattle per acre.  In circuits, we often discuss the number density of charge carriers n, which denotes the average number of charge carriers per cubic meter of material.  Take note:  this usage of the symbol n should not be confused with the principle quantum number, which also uses the symbol n.



n-type Semiconductor
Consists of a semiconductor, such as silicon, which has been doped with atoms of an donor element, such as phosphorus.  The extra valence electrons of the donor atoms provide filled energy states in the band gap, just below the conduction bandElectrons from these "extra" states can easily move into the conduction band and become conduction electrons, as shown in the energy diagram to the right.  Replacing even one in every million silicon atoms with a phosphorus atom can increase the conductivity of the solid by a factor of five million.  Because the density of conduction electrons provided by the donor is much larger than the density of holes contributed by the semiconductor atoms, conduction in this material is primarily due to the motion of (negatively-charged) electrons.  Thus it is called an n-type semiconductor.



Ohmic (Materials)
Materials obeying Ohm's Law.  Ohmic materials have resistances that do not change over a wide range of voltages and currents.  For an ohmic device, a graph of voltage across the device vs the current through that device will yield a straight line, with constant slope equal to the resistance of the device.




Ohm's Law
An observation (first made by Georg Simon Ohm) that applies to certain materials or devices, called ohmic materials or devices.  States that many materials have resistances that are independent of voltage and current under regular operating conditions.  Ohm's Law is NOT the statement V = IR, as many believe.  That statement is instead the definition of resistance.  Ohm's Law instead says that, for many materials under a wide range of conditions, the voltage V and current I are linearly related, which implies a resistance R independent of V and I.  To read more about Ohm's Law, try this page, of the ohmslaw.com website.




(in) Parallel
Two segments of an electric circuit are "in parallel" if they offer distinct paths between the same two points. Because houses are wired in parallel, turning off one appliance does not keep current from flowing to other appliances - each is on its own parallel path.  The figure to the right depicts a circuit diagram of two resistors connected in parallel to a battery.  Notice how removing one resistor will not alter the current flow from the battery through the other resistor. To read more about circuits, try the links from this page by Electronics Lab.  Another good source of circuit information is AllAboutCircuits - try this link for their treatment of series and parallel circuits.



P-N Junction
An interface between a p-type semiconductor and an n-type semiconductor device which can be used for many different applications.  A single p-n junction acts as a diode, conducting current when a forward bias  is applied (positive terminal of a battery connected to the p-side of the junction and negative terminal to the n-side) but not when the terminals are switched to apply a reverse biasThis page of this module discusses the structure of p-n junctions.  p-n junctions also figure prominently in transistors, as discussed on this page of this module.  For an alternate look at p-n junctions, see this page of Georgia State's HyperPhysics project.




P-type Semiconductor
Consists of a semiconductor, such as silicon, which has been doped with atoms of an acceptor element, such as gallium. The unfilled valence shells of the acceptor atoms provide empty energy states in the band gap, just above the valence band.  Valence electrons can move into these "extra" states, leaving behind holes in the valence band, as shown in the energy diagram to the right.  Replacing even one in every million silicon atoms with a gallium atom can increase the conductivity of the solid by a factor of five million.  Because the density of holes provided by the acceptor is much larger than the density of conduction electrons contributed by the semiconductor atoms, conduction in this material is primarily due to the motion of (positively-charged) holes.  Thus it is called a p-type semiconductor.




Periodic Table
A table of all known elements, sorted into columns by their chemical properties and into rows by their relative masses within the columns.  Mendeleev was the first to organize elements in such a manner, and he successfully predicted the existence of elements that had not been previously detected or expected.  Click here for a nice on-line table coded by Michael Dayah.  Another nice on-line periodic table can be found at this site by webelements.com.  For a description of how Mendeleev organized the elements, try this Physics 2000 page.




Potential Energy
Energy that is stored in an item, potential energy can take many forms, including chemical, electrical, and gravitational.  This module assumes basic familiarity with the concept of potential energy.  If you need a review, try Glenbrook High's Physics Classroom treatment of energy.




(Principle) Quantum Number (n)
An integer that identifies the energy level of electrons in atoms  (Disclaimer). n=1 corresponds to the lowest energy possible for an electron in an atom.  Be careful not to confuse this use of the symbol n for other meanings, such as number density, of this common symbol. For more on quantum numbers and how they underlie the periodic table, go to this Physics 2000 site.


Proton
A fundamental particle with positive charge found in the nuclei of atoms.  It has a mass of 1.673 x 10-27 kilograms and a charge of 1.6 x 10-19 Coulombs.  Protons are fairly fixed in solids and so don't contribute to conduction. Click this link to go to the Particle Adventure webpage for more information on protons.




Quantized
Having only certain, discrete values.  For example, whole numbers are quantized.  They can only take the values 0, 1, 2, 3, etc. and will never fall between those discrete values.  Decimals, on the other hand, can take any value and thus are continuous rather than being quantized. (Disclaimer)




Resistance (R)
The ratio between the voltage applied to a device and the electric current I that flows through it:

R = V/I.

Certain materials (called ohmic materials) have resistance that is independent of voltage and current.  Typically, the word "resistor" refers to an ohmic device in a circuit. For a good introduction to resistance and other circuit properties, try this page of the All About Circuits site.




Resistivity (r)
The "part" of the resistance of an object independent of the geometry of the object.  Resistivity depends on the type of material used and on the temperature of the object.  For a homogeneous solid, resistivity r is related to resistance R in the following manner:

r = R A / L,

where A is the cross-sectional area of the solid, and L is its length, as shown to the right.  Resistivity is the inverse of conductivitys:

r = 1/s .
Copper, with its low resistivity of 1.68 x 10-8 Wm conducts electric current more freely than does aluminum, with its slightly higher resistivity of
2.65 x 10-8 Wm.




Reverse Bias

A voltage applied to a diode (a p-n junction) in a direction that does NOT produce significant electric current.  When a p-n junction is reverse biased, the positive terminal of a battery is connected to the n-side of the diode, and the negative terminal to the p-side.  Biasing of a diode is discussed on this page of this module.




Semiconductor
A subclass of insulators, semiconductors are materials with conductivity that can be controlled through methods such as doping or changing the temperature.  Like all insulators, they have a valence band that is completely full in the ground state.  A semiconductor has no charge carriers at absolute zero; pure semiconductors have very few charge carriers even at room temperature.  Conductivity can be increased through doping, creating either p-type semiconductors or n-type semiconductors.




Shell
Note:  Figure not to scale or,
for that matter, an accurate rep-
resentation of electron behavior.  For
details, see our word of caution.
Essentially synonymous with energy level, a shell is a collection of energy states with the same energy (in the absence of electromagnetic fields) for electrons in atoms (Disclaimer).  In an atom, the innermost energy shell (having principle quantum number n=1) can "hold" two electrons, as shown in the cartoon to the left.  The second shell (n=2) holds 8 electrons, and so on. 


Spin
A property of elementary particles having the units of angular momentum, spin was originally thought to be the angular momentum associated with an elementary particle's spinning on its axis.  More modern experiments have shown that this physical interpretation is in error, and spin is now considered an intrinsic property of a particle, like its mass and electric charge.  Click here to go to the Physics 2000 page that discusses spin.




Transistor
An electronic switch.  Transistors allow a (relatively) large amount of current to flow when a (relatively) small voltage is applied, just like a light switch can provide a large amount of electric energy to a lamp when a small amount of mechanical energy is expended to flip the switch.  Transistors in modern electronics are made from layered p- and n- type semiconductors.  This module's discussion of transistors starts on this page.




Valence Band
The outermost energy band that contains electrons when a solid is in the ground state.  An intrinsic semiconductor (or insulator) in its ground state will have a completely filled valence band, while the conduction band above the valence band is completely empty. The energy diagram to the right depicts a semiconductor in which some electrons have gained energy to jump to the conduction band, leaving behind holes in the valence band.   (Disclaimer)  These holes can move through the nearly-full valence band, just as the electrons can move through the nearly-empty conduction band.  The image follows this author's convention of representing electrons bound in the valence band by blue circles.  The green circles represent electrons that are free to move, such as the ones in the conduction band in the image to the right.




Valence Electron
An electron in the valence shell of an atom, or in the valence band of a solid.  Valence electrons are more active than non-valence electrons, so valence electrons are responsible for most of an atom's or solid's electrical and chemical properties.




Valence Shell
The outermost energy shell that contains electrons when an atom is in the ground state.  In the cartoon of a silicon atom to the right, the four "green" electrons occupy the valence shell.  The image follows this author's convention of representing tightly-bound electrons (those in inner shells) by blue circles; the green circles represent electrons that are free to interact.
Note:  Figure not to scale or,
for that matter, an accurate rep-
resentation of electron behavior.  For
details, see our word of caution.




Voltage (V), or Potential Difference
The change in energy per unit of electric charge, measured in the SI unit of Volt (V).  For example, a 1.0 Volt battery increases the energy of each Coulomb of charge by one Joule. For a good introduction to voltage and other circuit properties, try this page of the All About Circuits site.



Copyright © 2004 Doris Jeanne Wagner and Rensselaer Polytechnic Institute.  All Rights Reserved.