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EE201 :

SEMICONDUCTOR DEVICES

Atomic structure. Material classification. Covalent bonds. Conduction in semiconductors. Extrinsic semiconductors. The p-n junction Biasing the p-n junction

Student Learning Outcomes

Upon completion of viewing this presentation, you should be able to: Understand the characteristics and electrical properties of semiconductors. Define a semiconductor and state that silicon and germanium are

semiconductor materials.

Explain the characteristics of N-type and P-type semiconductors. Understand the characteristics of P-N junction and its reaction towards

voltage biasing.

Illustrate the formation of a junction Illustrate the meaning of forward biased voltage and reverse biased voltage

supplied across a P-N junction.

Identify the effects when a P-N junction is supplied with forward biasedvoltage and reverse biased voltage on the following items:

Area of depletion region Junction resistance Current flow (including leakage current)

Explain why breakdown occurs when P-N junction is reversing biased.

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1.0 INTRODUCTION TO SEMICONDUCTOR1.1UNDERSTAND THE CHARACTERISTICS AND ELECTRICAL PROPERTIES OF

SEMICONDUCTOR

1.1.1 Define a semiconductor and state that silicon and germanium aresemiconductor materials.

Semiconductors are materials that essentially can be conditioned to act as good

conductors, or good insulators, or anything in between. Common elements such

as carbon, silicon, and germanium are semiconductors. Silicon is the best and

most widely used semiconductor.

A semiconductor is a substance, usually a solid chemical element or compound

that can conduct electricity under some conditions but not others, making it a

good medium for the control of electrical current. Its conductance varies

depending on the current or voltage applied to a control electrode, or on theintensity of irradiation by infrared (IR), visible light, ultraviolet (UV), or X rays.

The specific properties of a semiconductor depend on the impurities, or

dopants, added to it. An N-type semiconductor carries current mainly in the

form of negatively-charged electrons, in a manner similar to the conduction of

current in a wire. A P-type semiconductor carries current predominantly as

electron deficiencies called holes. A hole has a positive electric charge, equal and

opposite to the charge on an electron. In a semiconductor material, the flow of

holes occurs in a direction opposite to the flow of electrons.

Elemental semiconductors include antimony, arsenic, boron, carbon,

germanium, selenium, silicon, sulphur, and tellurium. Silicon is the best-known

of these, forming the basis of most integrated circuits (ICs). Common

semiconductor compounds include gallium arsenide, indium antimonide, and

the oxides of most metals. Of these, gallium arsenide (GaAs) is widely used in

low-noise, high-gain, and weak-signal amplifying devices.

A semiconductor device can perform the function of a vacuum tube having

hundreds of times its volume. A single integrated circuit (IC), such as a

microprocessor chip, can do the work of a set of vacuum tubes that would fill a

large building and require its own electric generating plant.

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1.1.2 Explain the characteristics of N-type and P-type semiconductors.The Structure of the Atom

Atomic Structure is based on the component parts of an Element. An Element is a

unique chemical substance found in nature that is made up of specific, identical

atoms. Examples of elements are iron, carbon, oxygen, sodium and chlorine.

An atom is the smallest particle that has the properties of the element. Each atom is

made up of three things:

Protons (P+) - the smallest positively charged unit of matter

Neutrons (N) - the smallest neutral unit of matter (no charge)

Electrons (e-) - the smallest negatively charged unit of matter

The structure of each atom, the Atomic Structure, has a ball shaped center called the

nucleus which contains the protons (P+) and the neutrons (N). Around this nucleus,

the electrons orbit like planets around the sun as shown in the picture above.

Each atom of the a specific element has a constant and unique number of protons.

Each element of an atom has an equal number of protons and electrons. Therefore,

the element and its physical properties are defined by the number of protons and

electrons specific to the element.

Atomic Number of an element is referred to as Z and is the total number of protons

(P+) that the element has. Since each element has the same number of electrons as

protons the Atomic Number is also equal to the number of electrons each element

has.

Atomic Mass of an element is the total mass (weight) of all the protons, neutrons

and electrons that make up the atom of an element. The protons and neutrons are

much bigger and heavier than the electrons. The protons and neutrons have an

individual Atomic Mass of about 1.

In the atom, there is a maximum of 7 layers of the orbit (or shell). Each layer is a

layer known as K, l, M, N, O, P and Q (Or 1-7). The maximum number of electrons inone shell is determined by the formula:

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2 x n2n is the number of shells position

so ... the maximum number of electrons in each shell are:

Shell of K (1): 2 x 12

= 2

Shell of L (2): 2 x 22

= 8

Shell of M (3): 2 x 32

= 18

Shell of N (4): 2 x 42

= 32

Shell of O (5): 2 x 52

= 50

Shell of P (6): 2x 62

= 72

Shell of Q (7): 2 x 72

= 98

The outermost shell for an atomic is called valence shell and the outmost electron

(at the valence shell) is called valence electrons. This layer will not accommodate

more than 8 valence electrons. Vales number of electrons is what will determine the

electrical properties of a material.

Semiconductors are defined by their unique electric conductive behavior. Metals are

good conductors 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 electronholes, 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

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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

called donor donates 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.

Doping atoms, donors, usually have one more valence electron 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 metals, which are

donors in most solids.

No of Electron Materials type Notes

1 - 3 valenceselectrons

Conductor The ability to conduct electricity. have low resistance to facilitate current flow Atomic tends to release valence electrons and become

free electrons move from one atom to another atom.

5 8 valences

electrons

Insulator Cannot conduct electricity. Have a high resistance. atomic tend to accept valence electrons from other

atoms to fill the valence shell of the atoms and make it

stable and able to stay away from any electrical or

chemical activity.

4 valences

electrons

Semiconductor The situation is intermediate between conductors andinsulators.

Not easy to remove / accept valence electrons fromother atoms.

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Intrinsic semiconductors

An intrinsic semiconductor is an undoped semiconductor. This means that holes in

the valence band are vacancies created by electrons that have been thermally

excited to the conduction band, as opposed to doped semiconductors where holes

or electrons are supplied by a foreign atom acting as an impurity.

Doped (extrinsic) semiconductors

An extrinsic semiconductor is a semiconductor doped by a specific impurity which is

able to deeply modify its electrical properties, making it suitable for electronic

applications (diodes, transistors, etc.) or optoelectronic applications (light emitters

and detectors).

N-type semiconductors

A N-type semiconductor is an intrinsic semiconductor (e.g. silicon Si) in which a

donor impurity (e.g. arsenic As in Si, or Si in GaAs) has been intentionally introduced.

The impurities are called donor impurities since they have to give an extra electron

to the conduction band in order to make all the bonds with neighboring atoms (As is

pentavalent while Si is tetravalent).

N-type semiconductors are a type of extrinsic semiconductor where the dopant

atoms (donors) 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.

The silicon doped with extra electrons is called an N type semiconductor. N is

for negative, which is the charge of an electron.

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P-type semiconductors

A P-type semiconductor is an intrinsic semiconductor (like Si) in which an impurity

acting as an acceptor (like e.g. boron B in Si) has been intentionally added. These

impurities are called acceptors since once they are inserted in the crystalline lattice;

they lack one or several electrons to realize a full bonding with the rest of the

crystal.

A p-type semiconductor (p for Positive) is obtained by carrying out a process of

doping by adding a certain type of atoms (acceptors) to the semiconductor in order

to increase the number of free charge carriers (in this case positive holes).

When the doping material is added, it takes away (accepts) weakly bound outer

electrons from the semiconductor atoms. This type of doping agent is also known as

an acceptor material and the vacancy left behind by the electron is known as a hole.

The purpose of p-type doping is to create an abundance of holes. In the case of

silicon, a trivalent atom (typically from Group 13 of the periodic table, such as boron

or aluminium) is substituted into the crystal lattice. The result is that one electron is

missing from one of the four covalent bonds normal for the silicon lattice. Thus the

dopant atom can accept an electron from a neighboring atom's covalent bond to

complete the fourth bond. This is why such dopants are called acceptors. The

dopant atom accepts an electron, causing the loss of half of one bond from the

neighboring atom and resulting in the formation of a "hole". Each hole is associated

with a nearby negatively charged dopant ion, and the semiconductor remains

electrically neutral as a whole. However, once each hole has wandered away into

the lattice, one proton in the atom at the hole's location will be "exposed" and no

longer cancelled by an electron. This atom will have 3 electrons and 1 hole

surrounding a particular nucleus with 4 protons. For this reason a hole behaves as a

positive charge. When a sufficiently large number of acceptor atoms are added, the

holes greatly outnumber thermal excited electrons. Thus, holes are the majority

carriers, while electrons become minority carriers in p-type materials.

Silicon doped with material missing electrons that produce locations called holes is

called P type semiconductor. P is for positive, which is the charge of a hole.

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1.2UNDERSTAND THE CHARACTERISTICS OF P-N JUNCTION AND ITS REACTIONTOWARDS VOLTAGE BIASING.

1.2.1 Illustrate the formation of a junction.a. Free electrons mobility.b. Formation of depletion region and its properties.c. Existence of threshold voltage and its values for silicon and

germanium.

P N Junction

Free electrons and hole will try to move to P and N region. Electrons in the N-type

material will be attracted to fill the hole in the P-type material. Therefore, the

crossing occurs of the electron N-type material to the P-type material

This combination causes nearby atoms combining into neutral (areas that have no

current carriers). The crossing of electron and hole will cease. The neutral area

(without current carriers) is known as the depletion region.

Positive ions and negative ions; surrounding combining causes a potential difference

there is between the two materials. Positive potential in the N-type material, and

negatively on the P-type material. This potential is the same as a small battery

which is known as voltage drop. The voltage drop is small. The voltage drop for the

semiconductor germanium about 0.3 V and silicon semiconductor value of

approximately 0.7V.

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1.2.2 Illustrate the meaning of forward biased voltage and reversebiased voltage supplied across a P-N junction.

FORWARD BIAS

In forward bias, the p-type is connected with the positive terminal and the n-type is

connected with the negative terminal.

REVERSE BIAS

Reverse biased usually refers to how a diode is used in a circuit. If a diode is reverse

biased, the voltage at the cathode is higher than that at the anode. Therefore, no

current will flow until the diode breaks down. Connecting the P-type region to the

negative terminal of the battery and the N-type region to the positive terminal,

corresponds to reverse bias.

1.2.3 Identify the effects when a P-N junction is supplied with forwardbiased voltage and reverse biased voltage on the following items:

a. Area of depletion regionb. Junction resistancec. Current flow (including leakage current)

A 'pn 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 (growing a layer of crystal

doped with one type of dopant on top of a layer of crystal doped with another type

of dopant). If two separate pieces of material were used, this would introduce a

grain boundary between the semiconductors that severely inhibits its utility by

scattering the electrons and holes.

PN junctions are elementary "building blocks" of most semiconductor electronicdevices such as diodes, transistors, solar cells, LEDs, and integrated circuits; they are

the active sites where the electronic action of the device takes place. For example, a

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common type of transistor, the bipolar junction transistor, consists of two pn

junctions in series, in the form npn or pnp.

The pn junction possesses some interesting properties that have useful applications

in modern electronics. A p-doped semiconductor is relatively conductive. The same

is true of an n-doped semiconductor, but the junction between them can become

depleted of charge carriers, and hence non-conductive, depending on the relative

voltages of the two semiconductor regions. By manipulating this non-conductive

layer, pn junctions are commonly used as diodes: circuit elements that allow a flow

of electricity in one direction but not in the other (opposite) direction. This property

is explained in terms of forward bias and reverse bias, where the term bias refers to

an application of electric voltage to the pn junction.

In forward bias, the p-type is connected with the positive terminal and the n-type is

connected with the negative terminal.

With a battery connected this way, the holes in the P-type region and the electrons

in the N-type region are pushed toward the junction. This reduces the width of the

depletion zone. The positive charge applied to the P-type material repels the holes,

while the negative charge applied to the N-type material repels the electrons. As

electrons and holes are pushed toward the junction, the distance between them

decreases. This lowers the barrier in potential. With increasing forward-bias voltage,

the depletion zone eventually becomes thin enough that the zone's electric field

cannot counteract charge carrier motion across the pn junction, as a consequence

reducing electrical resistance. The electrons that cross the pn junction into the P-

type material (or holes that cross into the N-type material) will diffuse in the near-

neutral region. Therefore, the amount of minority diffusion in the near-neutral zones

determines the amount of current that may flow through the diode.

Only majority carriers (electrons in N-type material or holes in P-type) can flow

through a semiconductor for a macroscopic length. With this in mind, consider the

flow of electrons across the junction. The forward bias causes a force on the

electrons pushing them from the N side toward the P side. With forward bias, the

depletion region is narrow enough that electrons can cross the junction and inject

into the P-type material. However, they do not continue to flow through the P-type

material indefinitely, because it is energetically favorable for them to recombine

with holes. The average length an electron travels through the P-type material

before recombining is called the diffusion length, and it is typically on the order of

microns.

Although the electrons penetrate only a short distance into the P-type material, the

electric current continues uninterrupted, because holes (the majority carriers) begin

to flow in the opposite direction. The total current (the sum of the electron and hole

currents) is constant in space, because any variation would cause charge buildup

over time (this is Kirchhoff's current law). The flow of holes from the P-type regioninto the N-type region is exactly analogous to the flow of electrons from N to P

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(electrons and holes swap roles and the signs of all currents and voltages are

reversed).

Therefore, the macroscopic picture of the current flow through the diode involves

electrons flowing through the N-type region toward the junction, holes flowing

through the P-type region in the opposite direction toward the junction, and the two

species of carriers constantly recombining in the vicinity of the junction. The

electrons and holes travel in opposite directions, but they also have opposite

charges, so the overall current is in the same direction on both sides of the diode, as

required.

The Shockley diode equation models the forward-bias operational characteristics of

a pn junction outside the avalanche (reverse-biased conducting) region.

Reverse-biased usually refers to how a diode is used in a circuit. If a diode is reverse-

biased, the voltage at the cathode is higher than that at the anode. Therefore, no

current will flow until the diode breaks down. Connecting the P-type region to the

negative terminal of the battery and the N-type region to the positive terminal

corresponds to reverse bias. The connections are illustrated in the following

diagram:

Because the p-type material is now connected to the negative terminal of the power

supply, the 'holes' in the P-type material are pulled away from the junction, causing

the width of the depletion zone to increase. Likewise, because the N-type region is

connected to the positive terminal, the electrons will also be pulled away from the

junction. Therefore, the depletion region widens, and does so increasingly with

increasing reverse-bias voltage. This increases the voltage barrier causing a high

resistance to the flow of charge carriers, thus allowing minimal electric current to

cross the pn junction. The increase in resistance of the pn junction results in the

junction behaving as an insulator.

The strength of the depletion zone electric field increases as the reverse-bias voltage

increases. Once the electric field intensity increases beyond a critical level, the pn

junction depletion zone breaks down and current begins to flow, usually by either

the Zener or the avalanche breakdown processes. Both of these breakdown

processes are non-destructive and are reversible, as long as the amount of currentflowing does not reach levels that cause the semiconductor material to overheat and

cause thermal damage.

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This effect is used to one's advantage in Zener diode regulator circuits. Zener diodes

have a certain low breakdown voltage. A standard value for breakdown voltage is

for instance 5.6 V. This means that the voltage at the cathode can never be more

than 5.6 V higher than the voltage at the anode, because the diode will break down

and therefore conduct if the voltage gets any higher. This in effect regulates the

voltage over the diode.

Another application where reverse biased diodes are used is in Varicap diodes. The

width of the depletion zone of any diode changes with voltage applied. This varies

the capacitance of the diode. For more information, refer to the Varicap article.

Forward Bias

With a battery connected this way, the holes in the P-type region and the electrons

in the N-type region are pushed towards the junction. This reduces the width of thedepletion zone. The positive charge applied to the P-type material repels the holes,

while the negative charge applied to the N-type material repels the electrons. As

electrons and holes are pushed towards the junction, the distance between them

decreases. This lowers the barrier in potential. With increasing forward-bias voltage,

the depletion zone eventually becomes thin enough that the zone's electric field

can't counteract charge carrier motion across the pn junction, consequently

reducing electrical resistance. The electrons which cross the pn junction into the P-

type material (or holes which cross into the N-type material) will diffuse in the near-

neutral region. Therefore, the amount of minority diffusion in the near-neutral zones

determines the amount ofcurrent that may flow through the diode.

Reverse Bias

Because the p-type material is now connected to the negative terminal of the power

supply, the 'holes' in the P-type material are pulled away from the junction, causing

the width of the depletion zone to increase. Similarly, because the N-type region is

connected to the positive terminal, the electrons will also be pulled away from the

junction. Therefore the depletion region widens, and does so increasingly with

increasing reverse-bias voltage. This increases the voltage barrier causing a high

resistance to the flow of charge carriers thus allowing minimal electric current to

cross the pn junction. The increase in resistance of the pn junction results in the

junction behaving as an insulator.

1.2.4 Explain why breakdown occurs when P-N junction is reversebiased.

Reverse Bias

Because the p-type material is now connected to the negative terminal of the power

supply, the 'holes' in the P-type material are pulled away from the junction, causingthe width of the depletion zone to increase. Similarly, because the N-type region is

connected to the positive terminal, the electrons will also be pulled away from the

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junction. Therefore the depletion region widens, and does so increasingly with

increasing reverse-bias voltage. This increases the voltage barrier causing a high

resistance to the flow of charge carriers thus allowing minimal electric current to

cross the pn junction. The increase in resistance of the pn junction results in the

junction behaving as an insulator.

Breakdown Voltage

The strength of the depletion zone electric field increases as the reverse-bias voltage

increases. Once the electric field intensity increases beyond a critical level, the pn

junction depletion zone breaks-down and current begins to flow cause the

semiconductor material to overheat and cause thermal damage. To overcome this,

make sure that reverse bias voltage that can be applied to the P-N junction must not

exceed the voltage breakdown point.

Leakage Current

It is a minority current within the material. It exists when the diode is given reverse

bias voltage. The electrons in the P-type material will be rejected by a bias voltage to

the negative terminal and continued to cross it. The current flow produced a very

small value. The current is referred to as Leakage current or reverse current. The

value depends on the temperature. The lower the temperature; the lower its value,

and vice versa.

Figure: Reversed biased P-N junction

If the voltage is reverse, a tiny leakage current of no more than a few microamps will

be get. At a certain voltage, anything from 10 V to 2000 V, depending on the doping,

the insulation of the barrier layer breaks down suddenly and a sudden increase in

current occurs. The breakdown voltage is the voltage at which this happens. In

many diodes, this would result in burn-out.