Introduction Semi

8/3/2019 Introduction Semi

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IntroductionSemiconductors are amongst the most technologically important materials in existence today. With

the exception of extremely simple objects such as filament light bulbs, all of the electronic devices

that we use involve some semiconductor-based devices.

In order to appreciate how semiconductors can be used to create devices, it is important to have anunderstanding of the basic electronic properties of semiconductors. The first section of this TLP will

concentrate on describing what it is that makes a material a semiconductor, and how semiconductors

respond to an applied electric field. The second half of the TLP gives some specific examples of

semiconductor devices and where these devices are used.

Introduction to Energy BandsWhen two valence electron atomic orbitals in a simple molecule such as hydrogen combine to form a

chemical bond, two possible molecular orbitals result. One molecular orbital is lowered in energy

relative to the sum of the energies of the individual electron orbitals, and is referred to as the

'bonding' orbital. The other molecular orbital is raised in energy relative to the sum of the energies of

the individual electron orbitals and is termed the 'anti-bonding' orbital.

In a solid, the same principles apply. IfNvalence electron atomic orbitals, all of the same energy, are

taken and combined to form bonds, Npossible energy levels will result. Of these, N/2 will be lowered

in energy and N/2 will be raised in energy with respect to the sum of the energies of the Nvalence

electron atomic orbitals.

However, instead of forming N/2 bonding levels all of the exact same energy, the allowed energy

levels will be smeared out into energy bands. Within these energy bands local differences between

energy levels are extremely small. The energy differences between the levels within the bands are

much smaller than the difference between the energy of the highest bonding level and the energy of

the lowest anti-bonding level. Like molecular orbitals, each energy level can contain at most two

electrons of opposite spin.

The allowed energy levels are so close together that they are sometimes considered as being

continuous. It is very important to bear in mind that, while this is a useful and reasonable


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approximation in some calculations, the bands are actually composed of a finite number of very

closely spaced electron energy levels.

If there is one electron from each atom associated with each of the Norbitals that are combined to

form the bands, then because each resulting energy level can be doubly occupied, the 'bonding' band,

or valence band will be completely filled and the 'anti-bonding' band, or conduction band will be

empty. This is depicted schematically in the picture above by the grey shading of the valence band.

Electrons cannot have any values of energy that lie outside these bands. An electron can only move

('be promoted') from the valence band to the conduction band if it is given an energy at least as great

as the band gap energy. This can happen if, for example, the electron were to absorb a photon of

sufficiently high energy.

If, as in the above one-dimensional schematic, a band is completely filled with electrons, and the band

immediately above it is empty, the material has an energy band gap. This band gap is the energy

difference between the highest occupied state in the valence band and the lowest unoccupied state in

the conduction band. The material is either a semiconductor if the band gap is relatively small, or an

insulator if the band gap is relatively large.

Electrons in metals are also arranged in bands, but in a metal the electron distribution is different -

electrons are not localised on individual atoms or individual bonds. In a simple metal with one valence

electron per atom, such as sodium, the valence band is not full, and so the highest occupied electron

states lie some distance from the top of the valence band. Such materials are good electrical

conductors, because there are empty energy states available just above the highest occupied states,

so that electrons can easily gain energy from an applied electric field and jump into these empty

energy states.

The distinction between an insulator and a semiconductor is less precise. In general, a material with a

band gap of less than about 3 eV is regarded as a semiconductor. A material with a band gap of

greater than 3 eV will commonly be regarded as an insulator. A number of ceramics such as silicon

carbide (SiC), titanium dioxide (TiO2), barium titanate (BaTiO3) and zinc oxide (ZnO) have band gaps

around 3 eV and are regarded by ceramicists as semiconductors. Such ceramics are often referred to

as wide-band-gap semicondutors.

The distinction between semiconductors and insulators arises because in small band gap materials at

room temperature a small, but appreciable, number of electrons can be excited from the filled valence

bands into the unfilled conduction bands simply by thermal vibration. This leads to semiconducting

materials having electrical conductivities between those of metals and those of insulators.

The picture we have sketched here is only a very simple qualitative picture of the electronic structure

of a semiconductor designed to capture essential aspects of the band structure in semiconductorsrelevant to this TLP. More precise and quantitative approaches exist - see Going Further.Such

quantitative approaches are generally quite complex and require an understanding of quantum

mechanics. Fortunately, the very simple qualitative picture described above for semiconductors is all

that we need to be able to build upon and develop in this TLP.

An extension of the simple band energy diagram with only the vertical axis labelled as energy, with

the horizontal axis unlabelled, is to plot the energy vertically against wave vector, k. From de Broglie's
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relationshipp = kwherep is momentum and is Planck's constant, h, divided by 2. Such plots

therefore relate energy to momentum. The reason why such plots are useful lies in the more

quantitative methods referred to above, from which we shall simply quote useful results.

The energy of a classical, non-quantum, particle is proportional to the square of its momentum. This is

also true for a free electron, as in the most simple picture possible of valence electrons in metals

where the electrostatic potential from the nuclei is ignored. However, in a real crystalline solid the

periodicity of the lattice and the electrostatic potential from the nuclei together mean that in the

quantum world in a crystalline material the electron energy, E, is not simply proportional to the square

of the momentum, and so is not proprtional to the square of the wave vector, k.

In these E-kdiagrams, often called band diagrams, plotted in what is referred to as a reduced zone

scheme, the momentum that is plotted is actually a quantity called crystal momentum. The

distinction between momentum and crystal momentum arises from the periodicity of the solid.

Fortunately, this distinction is not important for understanding this TLP on semiconductors.

There are usually many different values of electron energy possible for any given value of the electronmomentum. Each possible energy value lies in one of the energy bands.

When plotted against the wave vector, k, the bands of allowed energy are not really flat. This means

that bands can overlap in energy, as the maximum value in one band may be higher then the

minimum value in another band. In this case the relevant maximum and minimum will occur for

different values ofkbecause energy bands never cross over each other. This is one way in which

metals can have partially filled energy bands. The available energy states are filled with electrons

starting with those lowest in energy. Such overlapping of bands as a function ofkdoes not occur in

semiconductors.

The FermiDirac DistributionElectrons are an example of a type of particle called a fermion. Other fermions include protons and

neutrons. In addition to their charge and mass, electrons have another fundamental property called

spin. A particle with spin behaves as though it has some intrinsic angular momentum. This causes


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each electron to have a small magnetic dipole. The spin quantum number is the projection along an

arbitrary axis (usually referred to in textbooks as thez-axis) of the spin of a particle expressed in

units of . Electrons have spin , which can be aligned in two possible ways, usually referred to as

'spin up' or 'spin down'.

All fermions have half-integer spin. A particle that has integer spin is called a boson. Photons, which

have spin 1, are examples of bosons. A consequence of the half-integer spin of fermions is that this

imposes a constraint on the behaviour of a system containing more then one fermion.

This constraint is the Pauli exclusion principle, which states that no two fermions can have the

exact same set of quantum numbers. It is for this reason that only two electrons can occupy each

electron energy level one electron can have spin up and the other can have spin down, so that they

have different spin quantum numbers, even though the electrons have the same energy.

These constraints on the behaviour of a system of many fermions can be treated statistically. The

result is that electrons will be distributed into the available energy levels according to the Fermi Dirac

Distribution:

where f() is the occupation probability of a state of energy , kB is Boltzmann's constant, (the Greek

letter mu) is the chemical potential, and Tis the temperature in Kelvin.

The distribution describes the occupation probability for a quantum state of energy Eat a temperature

T. If the energies of the available electron states and the degeneracy of the states (the number ofelectron energy states that have the same energy) are both known, this distribution can be used to

calculate thermodynamic properties of systems of electrons.

At absolute zero the value of the chemical potential, , is defined as the Fermi energy. At room

temperature the chemical potential for metals is virtually the same as the Fermi energy typically the

difference is only of the order of 0.01%. Not surprisingly, the chemical potential for metals at room

temperature is often taken to be the Fermi energy. For a pure undoped semiconductor at finite

temperature, the chemical potential always lies halfway between the valence band and the conduction

band. However, as we shall see in a subsequent section of this TLP, the chemical potential in extrinsic

(doped) semiconductors has a significant temperature dependence.

In order to understand the behaviour of electrons at finite temperature qualitatively in metals and

pure undoped semiconductors, it is clearly sufficient to treat as a constant to a first approximation.

With this approximation, the Fermi-Dirac distribution can be plotted at several different temperatures.

In the figure below, was set at 5 eV.


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From this figure it is clear that at absolute zero the distribution is a step function. It has the value of 1for energies below the Fermi energy, and a value of 0 for energies above. For finite temperatures the

distribution gets smeared out, as some electrons begin to be thermally excited to energy levels above

the chemical potential, . The figure shows that at room temperature the distribution function is still

not very far from being a step function.

Charge Carriers in SemiconductorsWhen an electric field is applied to a metal, negatively charged electrons are accelerated and carry the

resulting current. In a semiconductor the charge is not carried exclusively by electrons. Positively

charged holes also carry charge. These may be viewed either as vacancies in the otherwise filled

valence band, or equivalently as positively charged particles.

Since the Fermi-Dirac distribution is a step function at absolute zero, pure semiconductors will have all

the states in the valence bands filled with electrons and will be insulators at absolute zero. This is

depicted in the E-k diagram below; shaded circles represent filled momentum states and empty circles

unfilled momentum states. In this diagram k, rather than k, has been used to denote that the wave

vector is actually a vector, i.e., a tensor of the first rank, rather than a scalar.


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If the band gap is sufficiently small and the temperature is increased from absolute zero, some

electrons may be thermally excited into the conduction band, creating an electron-hole pair. This is as

a result of the smearing out of the Fermi-Dirac distribution at finite temperature. An electron may also

move into the conduction band from the valence band if it absorbs a photon that corresponds to the

energy difference between a filled state and an unfilled state. Any such photon must have an energy

that is greater than or equal to the band gap between the valence band and the conduction band, as

in the diagram below.


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Whether thermally or photonically induced, the result is an electron in the conduction band and a

vacant state in the valence band.

If an electric field is now applied to the material, all of the electrons in the solid will feel a force from

the electric field. However, because no two electrons can be in the exact same quantum state, an

electron cannot gain any momentum from the electric field unless there is a vacant momentum state


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adjacent to the state being occupied by the electron. In the above schematic, the electron in the

conduction band can gain momentum from the electric field, as can an electron adjacent to the vacant

state left behind in the valence band. In the diagram below, both of these electrons are shown moving

to the right.

The result of this is that the electrons have some net momentum, and so there is an overall

movement of charge. This slight imbalance of positive and negative momentum can be seen in the

diagram below, and it gives rise to an electric current.


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The vacant site in the valence band which has moved to the left can be viewed as being a particle

which carries positive electric charge of equal magnitude to the electron charge. This is therefore a

hole. It should be appreciated that these schematics do not represent electrons 'hopping' from site to

site in real space, because the electrons are not localised to specific sites in space. These schematics

are in momentum space. As such, holes should not be thought of as moving through the

semiconductor like dislocations when metals are plastically deformed it suffices to view them simply

as particles which carry positive charge.

The opposite process to the creation of an electron-hole pair is called recombination. This occurs

when an electron drops down in energy from the conduction band to the valence band. Just as the

creation of an electron-hole pair may be induced by a photon, recombination can produce a photon.

This is the principle behind semiconductor optical devices such as light-emitting diodes (LEDs), in

which the photons are light of visible wavelength.

Intrinsic and Extrinsic SemiconductorsIn most pure semiconductors at room temperature, the population of thermally excited charge carriers

is very small. Often the concentration of charge carriers may be orders of magnitude lower than for a

metallic conductor. For example, the number of thermally excited electrons cm3 in silicon (Si) at 298

K is 1.5 1010. In gallium arsenide (GaAs) the population is only 1.1 106 electrons cm3. This may be

compared with the number density of free electrons in a typical metal, which is of the order of 1028

electrons cm3.

Given these numbers of charge carriers, it is no surprise that, when they are extremely pure, silicon

and other semiconductors have high electrical resistivities, and therefore low electrical conductivities.

This problem can be overcome by doping a semiconducting material with impurity atoms. Even very

small controlled additions of impurity atoms at the 0.0001% level can make very large differences to

the conductivity of a semiconductor.


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It is easiest to begin with a specific example. Silicon is a group IV element, and has 4 valence

electrons per atom. In pure silicon the valence band is completely filled at absolute zero. At finite

temperatures the only charge carriers are the electrons in the conduction band and the holes in the

valence band that arise as a result of the thermal excitation of electrons to the conduction band.

These charge carriers are called intrinsiccharge carriers, and necessarily there are equal numbers of

electrons and holes. Pure silicon is therefore an example of an intrinsic semiconductor.

If a very small number of atoms of a group V element such as phosphorus (P) are added to the silicon

as substitutional atoms in the lattice, additional valence electrons are introduced into the material

because each phosphorus atom has 5 valence electrons. These additional electrons are bound only

weakly to their parent impurity atoms (the bonding energies are of the order of hundredths of an eV),

and even at very low temperatures these electrons can be promoted into the conduction band of the

semiconductor. This is often represented schematically in band diagrams by the addition of 'donor

levels' just below the bottom of the conduction band, as in the schematic below.

The presence of the dotted line in this schematic does not mean that there now exist allowed energy

states within the band gap. The dotted line represents the existence of additional electrons which may

be easily excited into the conduction band. Semiconductors that have been doped in this way will have

a surplus of electrons, and are called n-type semiconductors. In such semiconductors, electrons are

the majority carriers.

Conversely, if a group III element, such as aluminium (Al), is used to substitute for some of the atoms

in silicon, there will be a deficit in the number of valence electrons in the material. This introduces

electron-accepting levels just above the top of the valence band, and causes more holes to be

introduced into the valence band. Hence, the majority charge carriers are positive holes in this case.

Semiconductors doped in this way are termed p-type semiconductors.


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Doped semiconductors (either n-type orp-type) are known as extrinsic semiconductors. The

activation energy for electrons to be donated by or accepted to impurity states is usually so low that

at room temperature the concentration of majority charge carriers is similar to the concentration ofimpurities. It should be remembered that in an extrinsic semiconductor there is an contribution to the

total number of charge carriers from intrinsic electrons and holes, but at room temperature this

contribution is often very small in comparison with the number of charge carriers introduced by the

controlled impurity doping of the semiconductor.

Direct and Indirect Band Gap SemiconductorsThe band gap represents the minimum energy difference between the top of the valence band and the

bottom of the conduction band, However, the top of the valence band and the bottom of the

conduction band are not generally at the same value of the electron momentum. In a direct band

gap semiconductor, the top of the valence band and the bottom of the conduction band occur at the

same value of momentum, as in the schematic below.

In an indirect band gap semiconductor, the maximum energy of the valence band occurs at a

different value of momentum to the minimum in the conduction band energy:


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The difference between the two is most important in optical devices. As has been mentioned in the

section charge carriers in semiconductors, a photon can provide the energy to produce an electron-

hole pair.

Each photon of energy Ehas momentum , where cis the velocity of light. An optical photon

has an energy of the order of 1019 J, and, since c=3 108 ms1, a typical photon has a very small

amount of momentum.

A photon of energy Eg, where Egis the band gap energy, can produce an electron-hole pair in a direct

band gap semiconductor quite easily, because the electron does not need to be given very much

momentum. However, an electron must also undergo a significant change in its momentum for aphoton of energy Egto produce an electron-hole pair in an indirect band gap semiconductor. This is

possible, but it requires such an electron to interact not only with the photon to gain energy, but also

with a lattice vibration called a phonon in order to either gain or lose momentum.

The indirect process proceeds at a much slower rate, as it requires three entities to intersect in order

to proceed: an electron, a photon and a phonon. This is analogous to chemical reactions, where, in a

particular reaction step, a reaction between two molecules will proceed at a much greater rate than a

process which involves three molecules.

The same principle applies to recombination of electrons and holes to produce photons. The

recombination process is much more efficient for a direct band gap semiconductor than for an indirect

band gap semiconductor, where the process must be mediated by a phonon.

As a result of such considerations, gallium arsenide and other direct band gap semiconductors are

used to make optical devices such as LEDs and semiconductor lasers, whereas silicon, which is an

indirect band gap semiconductor, is not. The table in the next section lists a number of different

semiconducting compounds and their band gaps, and it also specifies whether their band gaps are

direct or indirect.
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Compound SemiconductorsIn addition to group IV elements, compounds of group III and group V elements, and also compounds

of group II and group VI elements are often semiconductors. The common feature to all of these is

that they have an average of 4 valence electrons per atom.

One example of a compound semiconductor is gallium arsenide, GaAs. In a compound semiconductorlike GaAs, doping can be accomplished by slightly varying the stoichiometry, i.e., the ratio of Ga

atoms to As atoms. A slight increase in the proportion of As produces n-type doping, and a slight

increase in the proportion of Ga producesp-type doping.

The table below list some semiconducting elements and compounds together with their bandgaps at

300 K.

MaterialDirect / Indirect

Bandgap

Band Gap Energy at 300 K

(eV)

Elements C (diamond)

Ge

Si

Sn (grey)

Indirect

Indirect

Indirect

Direct

5.47

0.66

1.12

0.08

Groups III-V compounds GaAs

InAs

InSb

GaP

GaN

InN

Direct

Direct

Direct

Indirect

Direct

Direct

1.42

0.36

0.17

2.26

3.36

0.70

Groups IV-IV compounds -SiC Indirect 2.99

Groups II-VI compounds ZnO

CdSe

ZnS

Direct

Direct

Direct

3.35

1.70

3.68

Behaviour of the Chemical PotentialThe Fermi-Dirac distribution was introduced in the section The Fermi-Dirac Distribution. The relevant

equation to describe the distribution is

so that for a chemical potential, , of 5 eV, the distribution takes the form
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as a function of temperature.

One feature that is very important about the Fermi-Dirac distribution is that it is symmetric about the

chemical potential. Hence for a simple intrinsic semiconductor, which has equal numbers of electrons

in the conduction band and holes in the valence band, and where the density of states is also

symmetric about the centre of the band gap, the chemical potential must lie halfway between the

valence band and the conductance band, regardless of the temperature, because each electron

promoted to the conduction band leaves a hole in the valence band. This is shown in the band diagram

below in which energy is plotted vertically against temperature horizontally.


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[Note that if the density of states is not exactly symmetric about the centre of the band gap, then the

chemical potential does not have to be exactly in the centre of the band gap. However, under such

circumstances, it will still be extremely close to the centre of the band gap whatever the temperature,

and for all practical purposes can be considered to be in the centre of the band gap.]

For an extrinsic semiconductor the situation is slightly more complicated. At absolute zero in an n-type

semiconductor, the chemical potential must lie in the centre of the gap between the donor level and

the bottom of the conduction band. At low temperatures in such a semiconductor there are more

conduction electrons than there are holes. If the donor level is more than half full, the chemical

potential must lie somewhere between the donor levels and the conduction band. At higher

temperatures, when the donor level is completely depleted of electrons, and the contribution from

intrinsic electrons to the overall electrical conductivity becomes more substantial, the chemical

potential tends towards that for an intrinsic semiconductor, i.e., halfway between the conduction and

valence bands, and therefore in the middle of the band gap.


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Forp-type semiconductors the behaviour is similar, but the other way around, i.e., the chemical

potential starts midway between the valence band and the acceptor levels at absolute zeo and

gradually increases in energy as the temperature increases, so that at high temperatures it too is in

the middle of the band gap.


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MetalSemiconductor Junction Ohmic ContactWhen a metal and an n-type semiconductor are joined and M < S, electrons will flow from the Fermi

energy level in the metal into the semiconductor conduction band to lower their energy. This willcause the chemical potential of the semiconductor to move up into equilibrium with that of the metal.

It will also deform the semiconductor bands, so that they curve upwards away from the metal. This

situation is depicted in the animation below. Use the tabs to navigate through the animation.


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Note: This animation requires Adobe Flash Player 8 and later, which can be downloaded here.

This type of contact yields a linear relationship between the voltage applied and the current that flows

across the junction. It is therefore called an Ohmic contact, because it obeys Ohm's law. This type of

contact is also described as metallization, and is used to supply electric current into semiconductor

devices.

The pn JunctionAs an alternative to the Schottky Barrier contact described in the sectionMetalSemiconductor

Junction - Rectifying Contact, a junction between an n-type semiconductor and ap-type

semiconductor can be used as a rectifying contact. To see why, browse through the animation below.

The various parts of the animation are discussed in detail later in this section, so do not be concerned

if you do not understand every stage. You can return to this animation as you read more about thep-

n junction.
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It should be noted in the above animation that the relative quantity of electrons in thep-type material

and the relative quantity of holes in the n-type semiconductor before they are joined together has

been greatly exaggerated for the purposes of illustration. Both of these are minority carriers in their

respective environments remember that electrons are the majority carriers in n-type semiconductors

and that holes are the majority carriers inp-type semiconductors.

When the two semiconductors are initially joined together, electrons will flow from the n-type

semiconductor into thep-type semiconductor, and holes will flow from thep-type semiconductor into

the n-type semiconductor. The chemical potentials of the two semiconductors will come to equilibrium,

and the band structures will be deformed accordingly. A depletion layer is formed at the interface

between the two types of doped semiconductor, in which numbers of electrons in the conduction band

and holes in the valence band are both significantly reduced.

In equilibrium, there is a potential barrier for electrons to diffuse from the n-type semiconductor into

thep-type semiconductor, and also for holes to move from thep-type semiconductor into the n-type

semiconductor. These are the majority carriers. In addition, there will be currents from minority

carriers, i.e., holes on the n-type side and electrons on thep-type side. For example, holes generated

as a result of thermal excitation of electrons in the n-type semiconductor finding themselves in the

depletion layer between the n- andp-type semiconductors will be swept over to thep-type side of the
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junction by the strong electric field within the depletion layer since the electric field deters electrons

from diffusing from the n-type side, it necessarily helps holes entering the depletion layer from the n-

type side.

At equilibrium, the total current across the junction has be the same in both directions, so that the

overall net current is zero. Any imbalance in current would mean that the system was not in

equilibrium, and the bands would have to deform until the system returned to equilibrium.

If the n-type region is now connected to the positive terminal of a d.c. source and thep-type to the

negative side, the bands will be further deformed at the interface, creating larger potential barriers for

both electrons and holes to move across the junction and a wider depletion layer (i.e., a wider space

charge region). In this situation of reverse bias, the only current is the very small contribution from

the drift current arising from the minority carriers on both sides of the junction.

When the n-type region is connected to the negative terminal of a d.c. source and thep-type to the

positive side, the depletion layer becomes narrower and the potential barriers are decreased in size.

For this forward biasing, there will be a large net flow of electrons from the n-type semiconductor into

thep-type semiconductor, and there will also be a net flow of holes moving into the n-typesemiconductor from thep-type semiconductor.

In ap-n junction rectifier, an increase in the strength of the reverse biasing will eventually lead to an

increase in the current that flows. This is because for a sufficiently high field electric field, dielectric

breakdown of the semiconductor occurs. The bias at which this occurs is called the breakdown voltage.

The overall current voltage characteristic of thep-n junction is shown in the diagram below.


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Bipolar TransistorImagine two p-n junctions being joined back-to-back. This is the basic structure of the bipolar

transistor. It is called bipolar because both electrons and holes carry current in the device. Bipolartransistors can occur in either npn orpnp configurations. A schematic of a device in the npn

configuration is displayed below. Use the buttons to navigate through the animation.



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The source and the drain are about 1 m apart. Metallized contacts are made to both source and

drain, generally using aluminium. The rest of the substrate surface is covered with a thin oxide film,

typically about 0.05 m thick. The gate electrode is laid on top of the insulating oxide layer, and the

body electrode in the above diagram provides a counter electrode to the Gate. The thin oxide film

contains silicon dioxide (SiO2), but it may well also contain silicon nitride (Si3N4) and silicon oxynitride

(Si2

N2

O).

Thep-type doped substrate is only very lightly doped, and so it has a very high electrical resistance,

and current cannot pass between the source and drain if there is zero voltage on the gate. Application

of a positive potential to the gate electrode creates a strong electric field across thep-type material

even for relatively small voltages, as the device thickness is very small and the field strength is given

by the potential difference divided by the separation of the gate and body electrodes.

Since the gate electrode is be positively charged, it will therefore repel the holes in thep-type region.

For high enough electrical fields, the resulting deformation of the energy bands will cause the bands of

thep-type region to curve up so much that electrons will begin to populate the conduction band. This

is depicted in the animation below which shows a cross section through the region of thep-type

material near the gate electrode. Click the button to increase the voltage applied to the gateelectrode.


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The population of thep-type substrate conduction bands in the region near to the oxide layer creates

a conducting channel between the source and drain electrodes, permitting a current to pass through

the device. The population of the conduction band begins above a critical voltage, VT, below which

there is no conducting channel and no current flows. In this way the MOSFET may be used as a

switch. Above the critical voltage, the gate voltage modulates the flow of current between source and

drain, and may be used for signal amplification.

This is just one type of MOSFET, called 'normally -off' because it is only the application of a positive

gate voltage above the critical voltage which allows it to pass current. Another type of MOSFET is the

'normally-on', which has a conductive channel of less heavily doped n-type material between the

source and drain electrodes. This channel can be depleted of electrons by applying a negative voltage

to the gate electrode. A large enough negative voltage will cause the channel to be closed off entirely.


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'Normally-off' MOSFETs are used in a wide variety of integrated circuit applications. AND gates, NOT

gates and NAND gates are all made from these type of MOSFETs and are essential components of

memory devices.

Summary

The purpose of this teaching and learning package has been to give a very basic introduction tosemiconductors.

By the end of this package you should be conversant with various aspects of terminology used to

describe semiconducting materials, such as n-type semiconductor,p-type semiconductor, electrons,

holes, band gap, majority carrier, minority carrier, work function, chemical potential, Fermi level,

electron affinity, forward bias, reverse bias, Schottky barriers and Ohmic contacts. You should have an

appreciation of what types of materials are semiconductors and what distinguishes wide-band-gap

semiconductors from insulators. You should also be able to appreciate how very simple devices made

from semiconducting materials such asp-n junctions and MOSFETs are able to respond to applied

voltages.

Finally, the textbooks listed in the Going further section should be consulted for greater detail about

the different topics covered in this TLP.

Please follow this link if you would like toprovide a short review for this TLP

http://www.doitpoms.ac.uk/tlplib/se

miconductors/questions.php
http://www.doitpoms.ac.uk/tlplib/semiconductors/links.phphttp://www.doitpoms.ac.uk/tlplib/review.php?tlp=Introduction+to+Semiconductorshttp://www.doitpoms.ac.uk/tlplib/review.php?tlp=Introduction+to+Semiconductorshttp://www.doitpoms.ac.uk/tlplib/semiconductors/links.phphttp://www.doitpoms.ac.uk/tlplib/review.php?tlp=Introduction+to+Semiconductors

Documents

Introduction Semi