Superconductor and Semiconductors

7/24/2019 Superconductor and Semiconductors

1/45

Lecture notes on

ElectricalEngineering

Materials

Semiconductor Devices

Magnetic Properties of Materials

Optical Properties of Materials

Dielectric Properties

Introduction to IC Fabrication

2014

Keshav Raj Sigdel

2014-09-17Kathmandu University


2/45

Electrical Engineering Materials

EEEG 207

P a g e1

Semiconductor Devices

Bipolar Junction Transistor

A transistor is a semiconductor device used to amplify the signal. When a third doped element is

added to the crystal diode it becomes a transistor. Transistors are of two types (i) n-p-n transistor

and (ii) p-n-p transistor.

A transistor transfers a signal from low resistance to a high resistance. The prefix trans means

the signal transfer property of the device while istor classifies it as a solid state element (i.e.

resistor).

So transfer of resistor is a transistor.

Transfer + resistor =transistor

There are three regions and two junctions in a transistor.

(i) Emitter: The left side of transistor which supplies the charge carrier (i.e. electrons or

holes) is called emitter. It is highly doped and moderate in size. The emitter is always

forward bias with respect to base. In first p-n-p transistor hole to base and the second

n-p-n transistor it transfer electron to base.

(ii)Base: The middle section of the transistor which is lightly doped and very thin in size.

The base emitter junction is forward biased, allowing low resistance for emitter

circuit. The base collector junction is reversed biased and provides high resistance in

the collector circuit.

(iii)Collector: The section in the right side which collects the charge is called collector. It is

moderately doped and larger in size. The collector is always reverse biased. In p-n-p

it receives hole that flow in the output circuit. In n-p-n transistor it receives electron.

P Pn

ForwardReverse

Fig (i) PNP

n np

ForwardReverse

Fig (ii) NPN

E C

B

E C

B


3/45


EEEG 207

P a g e2

The Bipolar Junction Transistor (BJT) operates by the injection and collection of minority

carriers. Since the action of both electrons and holes is important in this device, it is called a

bipolar transistor.

The two operations of a transistor are amplification and switching.

Current amplification factor

The ratio of change in output current to the change in input current of a transistor is called

current amplification factor.

Common Base Common Emitter Common Collector

C

E

I

I

C

B

I

I

E

B

I

I

Also, E C BI I I and C B CEOI I I

Where 1CEO CBOI I is the leakage current (Collector emitter current with open

base).

Note: p-n-p (pointed in) & n-p-n (not pointed in)

Load line

Consider a common emitter n-p-n transistor circuit where no signal is applied only dc source are

present. The output characteristics are shown by figure.

From thisCC CE C C

CE CC C C

V V I R

V V I R

As VCCandRCare fixed values therefore it is first degree equation and can be represented by a

straight line on output characteristics. This is known as dc load line.

No signal

IB

IC

IE

+ -

RCVCE

+-

VBB VCC

IB

IC

IE

+ -

RCVCE

+-

VBB VCC

Fig (i)

Cot off region

Saturation region

VCE

B

A

VCC0

Load line

Q

Active regionCC

C

V

R

IC

IB=5A

IB=10A

IB=15A

IB=20A

IB=0A

Fig (ii)


4/45


EEEG 207

P a g e3

When IC= 0 Maximum VCE= VCC

When VCE= 0 MaximumIC= VCC/RC. By joining these two points load line is obtained.

Cutoff region: The region below IB=0 is called cutoff region. In this region both junctions are

reverse bias.

Saturation region: The region left to the curve is saturation region. In this region both junctions

are forward bias.

Active region: The region where the parallel curves are drawn is called active region. In this

region emitter base junction is forward bias and emitter collector junction is reverse bias. The

transistor will function normally in this region.

The zero signal value of ICand VCEare known as the operating point. The point where the load

line and characteristics curve intersect and satisfy the zero signal value of IC and VCE .(i.e. Q-

point.) It is also called operating point because the variation ofICand VCEtake place about thispoint when signal is applied. It is also called quiescent (silent) point or Q-point.

Minority carrier distribution and terminal current

Assumptions

(i) Holes diffuse from emitter to collector; drift is negligible in the base region.

(ii)Emitter injection efficiency is 1.

(iii)Collector saturation is negligible.

(iv) Active part of base and two junctions have same cross section.

(v) All currents and voltages are steady state.

p n pIE IC

IB

+VEB- -VCB+

Ep

Cp Wb

xnFig: Simplified p-n-p transistor geometry


5/45


EEEG 207

P a g e4

Since injected holes are assumed to flow emitter to collector by diffusion. Neglecting

recombination in the two depletion regions, the hole current entering the base at emitter junction

isIEand hole current leaving the base at collector is IC. The base width between two depletion

regions is wband uniform cross section area isA.

The excess hole concentration at the edge of the emitter depletion region is

1EBqV

kTE np p e

(1)

And the excess hole concentration at the collector side of the base is

1CBqV

kTC np p e

(2)

Since the emitter base junction is forward bias EBkT

V q and collector junction is strongly

reversed bias 0CBV . Then

EBqVkT

E np p e (3)

C np p (4)

Also from continuity equation

2

2 2

n n

n p

d p x p xdx L

(5)

The solution of this equation is

1 2n n

p p

x xL L

np x C e C e

(6)

Where Lp is the diffusion length of holes in the base region. To solve this equation we use

boundary condition to found constants C1and C2.

1 20n Ep x p C C (7)

1 2b b

p p

W WL L

n b CP x W p C e C e

(8)

Solving equations (7) and (8) we get


6/45


EEEG 207

P a g e5

1

b

p

b b

p p

WL

C E

W WL L

p p eC

e e

(9)

& 2

b

p

b b

p p

WL

E C

W WL L

p e pC

e e

(10)

By putting these values of C1 and C2 in equation (6) and 0Cp for strongly reversed bias

junction

b n b n

p p p p

b b

p p

W x W xL L L L

n E W WL L

e e e ep x p

e e

i.e. excess hole distribution

1 2n n

p p

x xL L

n E Ep x M p e M p e

Where 1 1&

b b

p p

b b b b

p p p p

W WL L

W W W W L L L L

e eM M

e e e e

Here, the excess electron concentration in the p+emitter is shown to decay exponentially to zero

corresponding to a long diode. This is due to the minority carrier electron diffusion length is

often shorter than the thin emitter region at high emitter doping level. Othrewise, the narrow

diode expressions must be used in the emitter region.

Fig: Distribution of injected carrier in active region

np x

nx

1 EM p

2 EM p

1

n

px

L

EM p e

2

n

p

xL

EM p e

p Straight line approximation

bW


7/45


EEEG 207

P a g e6

Evaluation of terminal current

We have solution of excess hole distribution in the base region

1 2n n

p p

x xL L

np x C e C e

(1)

The emitter and collector current are obtained from the gradient of hole concentration at each

depletion region edge.

We have n

p n p

n

d p xI x qAD

dx

(2)

At 0nx hole component of emitter current is

2 10p

EP p np

D

I I x qA C CL (3)

Similarly collector reverse saturation current IC is made up of entirely of holes entering the

collector depletion region from the base is

2 1b b

p p

W WL Lp

C p n b

p

DI I x W qA C e C e

L

(4)

Substituting the value of C1and C2we get

2b b

p p

b b

p p

W WL L

E C

p

EP W WL Lp

p e e pD

I qAL

e e

ctnh csch & csch ctnhp pb b b b

EP E C C E C

p p p p p p

D DW W W WI qA p p I qA p p

L L L L L L

If E EPI I for amplification factor 1 , then the base current is

ctn h csch

tanh2

p b bB E C E C

p p p

p bE C

p p

D W WI I I qA p p

L L L

D WqA p p

L L


8/45


EEEG 207

P a g e7

Field Effect Transistor

A field effect transistor is a three terminal semiconductor device in which the output current is

controlled by the applied electric field. The current in a FET is carried only by one type of

majority charge carriers electrons or holes. It is also called unipolar transistor. The current in the

device from source to drain can be controlled by the application of an electric potential (fortransverse electric field) introduced by gate, the device is known as field effect transistor (FET).

The field effect transistors are of two types:

(i) Junction field effect transistor (JFET)

(ii) Metal oxide semiconductor field effect transistor (MOSFET)

Construction

D

S

G

Fig: Symbol of n-channelD

S

G

Fig: Symbol of p-channel

p p

n

c

Chanel

Depletion

Drain (D)

Gate (G)

Source (S)

Fig: n-channel

p p

n

n

Drain (D)

Gate G

Source (S)

Fig: n-channel JFET

n

n

p

p

Drain (D)

Gate G

Source (S)

Fig: p-channel JFET


9/45


EEEG 207

P a g e8

A JFET consists of a p-type or n-type silicon bar containing two p-n junctions as shown in

figure. The bar forms the conducting channel for the charge carriers. If the bar is of n-type, it is

called n-channel JFET and if the bar is of p-type it is called p-channel JFET. The two p-n

junctions forming diodes are connected internally and a common terminal called gate. Other

terminals are source and drain taken out from the bar. Thus the FET consists of essentially three

terminals.

(i) Source (S): It is the terminal through which majority carriers inter the bar. Since carriers

come from it is called source.

(ii)Drain (D): It is the terminal through which majority carriers (electrons) leave the bar. The

drain to source voltage VDSdrives the drain current ID.

(iii)Gate (G): There are two internally connected heavily doped regions which form two p-n

junctions. The gate source voltage VGSreverse biases the gate.

(iv)

Channel (C): It is the space between two gates through which majority carriers pass from

source to drain when VDSis applied.

Working principle of JFET

It consists of n-type silicon bar the upper contact is known as drain (D) and lower contact as the

source (S). A current is established in the bar by an applied voltage 0-30 volts with negative

terminal to the source electrode. The electrons which are majority carriers in n-type bar leave the

bar through the drain electrode. The conventional current IDenters the bar at D.

p p

n

n

D

S

VGG

ID

VDD

VDS

VGS


10/45


EEEG 207

P a g e9

Two small regions of polarity opposite to that of crystal are created near the center at opposite

side of a bar. These two leads are called gate. The region between the gates is called channel.

If p-n junction is reversed biased, width of depletion zone (region) is increased and greater the

reverse bias voltage. Thus by applying the reverse bias the depletion layer becomes wider and

cross sectional area of channel is decreased. This will increase the resistivity and decrease theflow of current. If the varying reverse biased voltage is applied to FET. The current flowing in

source drain circuit is varied in inverse proportion. The biasing voltage is considered as electric

field. Hence strength of resistivity can be determined by electric field. From this concept we use

the name junction field effect transistor.

Characteristics of JFET

The external batteries VDD and VGG are connected in drain and gate respectively such that the

gate source junction is reversed biased. The characteristics may be obtained by investigating how

the current is flowing between the drain and source. The drain current varies with the drain tosource voltage for fix value of VGS.

Let VGS be initially fixed to zero at the initial position 0, VDS is zero. The thickness of the

depletion region around p-n junction is uniform. If VDSis positive the depletion region are thin at

low VDSvalues, the drain current increases with the voltage as shown by the curve OA. In this

case the depletion layer is thicker towards the drain side as the voltage V DSis further increased,

the p-n junction at the gate becomes more reversed biased. The current is now force to flow in a

channel which increases lightly.

At this stage, the increasing current due to channel is narrowing and the curve practically

horizontal AB. The drain voltage at which the cross sectional area of the channel becomes

minimum is called the pinch-off voltage. The FET is said to be in pinch-off region AB. When

VDDis further increased current IDincreases rapidly due to avalanche multiplication of electron

caused by breaking of covalent bond of Si atom in the depletion region between gate and drain.

The voltage at which break down occurs is denoted by VDGO.

VGG

VDD

RVDS

VGS

ID

0

A

B

C

VP VDS=VDGO VDS

Pinch off

(Saturation region Break down regionOhmic region


11/45


EEEG 207

P a g e10

Metal oxide semiconductor field effect transistor (MOSFET)

Metal oxide semiconductor field effect transistor is an important semiconductor device and is

widely used in many circuit applications. The input impedance of a MOSFET is much more than

that of JFET because of very small gate leakage current. The MOSFET can be used in any of the

circuits covered for the JFET.

Construction

It is similar to JFET except with the following modifications

(i) There is only a single p-region. This region is called substrate.

(ii)A thin layer of silicon oxide is deposited over the left side of the channel. A metallic gate

is deposited over oxide layer. As silicon dioxide is an insulator, therefore gate is

insulated from the channel. From this reason MOSFET is sometimes called insulated

gate FET.

(iii)Like JFET, a MOSFET has three terminals viz. source, gate and drain.

D

S

G

Fig: Symbol

Substratep

n

n

Drain (D)

Gate (G)

Source (S)

Fig: n-channel MOSFET

Oxide layer

Substrate


12/45


EEEG 207

P a g e11

Working principle of MOSFET

Instead of gate diode as in JFET, here gate is formed as a small capacitor. One plate of this

capacitor is the gate and the other plate is the channel with metal oxide as the dielectric. When

negative voltage is applied to the gate, electrons accumulate on it. Those electrons repeal the

conduction band electrons in the n-channel. Therefore, lesser number of conduction electrons is

made available for current conduction through the channel. The greater is the negative voltage on

the gate, the lesser the current conduction from source to drain. If the gate is given positive

voltage, more electrons are made available in the n-channel. Consequently, the current from

source to drain is increases.

Phototransistor

The phototransistor is a more sensitive semiconductor device than the p-n photo diode. The

phototransistor is usually connected a common emitter configuration with open base and

radiation is concentrated on the region near the collector junction JC as shown in figure. The

operation of this device can be understood if we recognize that the junction J Eis slightly forward

biased and the junction JC is reversed biased (i.e. the transistor is biased in the active region).

Assume first, that there is excitation of radiation. Under these circumstances minority carriers are

generated thermally and the electrons crossing from the base to the collector as well as the holes

crossing from the collector to the base, constitute the reverse saturation current ICO.

The collector current is given by

1 0C CO BI I I

p

n

n

D

S

ID

VDS

G

VGG

+

-

+

-


13/45


EEEG 207

P a g e12

If the light is now turned on, additional minority carriers are photo generated, and these

contribute to the reverse saturation current in exactly the same manner as do the thermally

generated minority charges. If the component of the reverse saturation current due to the light is

designated IL, the total collector current is

1C CO LI I I

We note that, due to transistor action the current caused by the radiation is multiplied by the

large factor 1 .

Typical volt-ampere characteristics are shown in graph for an n-p-n planar phototransistor for

different values of illumination intensities. Note the similarity between this family of curves and

these for the CE transistor output characteristics with base current as a parameter. The current I C

is then increased by the term BI .

n

n

p

JC

JE

+

-VCE

C

E

IC

Radiation

Fig: Phototransistor

VCE0

IC (mA)

Fig: Output characteristics of the

MRD 450 n-p-n transistor


14/45


EEEG 207

P a g e13

Magnetic Properties of Materials

Introduction

Magnetism was observed as early at 800B.C. in a naturally occurring material called load stone.

In the modern concept, all materials, viz., metals, semiconductors and insulators are said toexhibit magnetism though of different nature. Materials in which a state of magnetism can be

induced are called magnetic materials. The magnetic properties of solids originate in the motion

of electrons and in the permanent magnetic moments of the atoms and electrons. The ability of

certain metals like iron, cobalt and nickel and some of their alloys and compounds to acquire

large permanent magnetic moments is of prime importance.

Intensity of Magnetization (I)

( )

( )

2

2

Magnetic moment MI

Volume V

m l m

a l a

Where m pole strength

2l effective length of magnet

a cross section area

Hence the intensity of magnetization is also defined as the magnetic pole strength per unit area of

cross section.

Magnetic Susceptibility m

m

Intensity of Magnetisation I

Maetic field H

It is pure number, sinceIandHhave the same units. Its value for vacuum is zero, because there

is no magnetization in vacuum.

We can classify the materials in terms of m .

If m is negative, the material is diamagnetic and the magnetic induction is weakened by the

presence of material.

If m is small positive, the material is paramagnetic and the magnetic induction is strengthened

by the presence of material.


15/45


EEEG 207

P a g e14

Ifm

is large positive, the material is ferromagnetic. However in ferromagnetic materials I is not

accurately proportional to H and so m is not constant.

Magnetic Permeability

Let us consider a relation 0B H I

From the definition of susceptibility

mI H

0

0 1

m

m

B H H

H

If we write 0 1 m then we have B H

The constant is called the magnetic permeability of the material. It may be defined as the ratio

of magnetic inductionBto the magnetizing fieldH.

For vacuum 0m and 0 . Hence magnetic induction in vacuum is 0 0B H

The ratio00

B

B

is called relative permeability ( r ). Obviously; 1r m .

We may also clarify magnetic materials in terms of relative permeability r .

1

1

1

r

r

r

Diamagnetic

Paramagnetic

Ferromagnetic

Diamagnetic Materials

Those substances which when placed in magnetizing field are magnetized feebly in the opposite

direction of applied field. This property is found in the substance whose outermost orbit has aneven number of electrons. Since the electrons have spins opposite to each other, the net magnetic

moment of each atom is zero. If these materials are brought close to the pole of a powerful

electromagnet they are repelled away from a magnet.

Diamagnetic materials have small negative susceptibility; relative permeability is less than unity.

For example: Bismuth, Antimony, gold, water, alcohol, quartz, hydrogen etc.


16/45


EEEG 207

P a g e15

Paramagnetic Materials

Those substances which when placed in magnetizing field are magnetized feebly in the direction

of magnetizing field are called paramagnetic substances. This property is found in the substance

whose outermost orbit has an odd number of electrons. If these substances are brought close to

the pole of a powerful electromagnet they get attracted towards the magnet.

Paramagnetic materials have small positive susceptibility; relative permeability is little greater

than unity.

For example: Platinum, aluminum, chromium, manganese, copper sulphate, liquid oxygen,

solutions of salt of irons and nickel.

Ferromagnetic Materials

Those substances which when placed in magnetizing field are strongly magnetized in the

directions of magnetizing field are called ferromagnetic substances. This property is found in thesubstances which are generally like paramagnetic materials. If these substances are brought close

to the pole of a powerful electromagnet they are strongly attracted towards the magnet.

Ferromagnetic materials have large positive susceptibility; relative permeability is much greater

than unity (few thousands).

For example: Iron, Nickel, Cobalt, gadolinium, and their alloys.

Anti-ferromagnetic Materials

Anti-ferromagnetic materials are crystalline materials. In these materials, the dipole moments ofneighboring dipoles are equal and opposite in orientation so that the net magnetization vanishes.

If they are placed in the magnetic field, they are feebly magnetized in the direction of field. Such

materials are called anti-ferromagnetic materials.

Susceptibility of those materials varies with temperature. It increases in temperature and reaches

a maximum at a particular temperature called Neel temperature (TN). Above this temperature

these materials behave like paramagnetic materials.

For example: MnO, FeO, CaO, NiO, MnO4, MnS etc.

Ferri Magnetic Materials

If the spin of the atoms are such that there is a net magnetic moment in one direction, the

materials are called ferri magnetic materials.

For example: ferrites i.e. Fe2O3


17/45


EEEG 207

P a g e16

Classification of magnetic materials based on atomic dipoles

Those atoms which have permanent magnetic dipole moment is absent is called diamagnetic.

These atoms which have permanent magnetic dipole exists even in the absence of any external

field may be paramagnetic, ferromagnetic, anti-ferromagnetic and ferrimagnetic depending on

the interaction between the individual dipole.

Thus if the interaction between the atomic permanent dipole moments is zero or negligible then

the material will be paramagnetic.

Fig: Schematic illustration of paramagnetic

arrangement of spin.

If a dipole interacts in such a manner that they tend to line up in parallel, the material will be

ferromagnetic.

Fig: Schematic illustration of ferromagneticarrangement of spin.

If the neighboring dipoles tends to line up so that they are anti-parallel, the material is anti-

ferromagnetic or ferrimagnetic depending on magnitudes of dipoles on the two sub-lattices as

indicated schematically for a one dimensional mode.

Fig: Schematic illustration of anti-ferromagnetic


Fig: Schematic illustration of ferrimagnetic


Classification Permanent dipoles Interactions between neighboring dipoles

Diamagnetic No -----

Paramagnetic Yes negligible

Ferromagnetic Yes Parallel orientation

Anti-ferromagnetic Yes Anti-parallel orientation of equal moment

Ferrimagnetic Yes Anti-parallel orientation of unequal moment


18/45


EEEG 207

P a g e17

Atomic Magnetic Moment

An electron revolving in an orbit about the nucleus of an atom is a minute current loop and

produces a magnetic field. Thus it behaves like a magnetic dipole.

Let us consider an electron of mass m and charge e moving with speed v in a circular Bohr orbitof radius r as shown in figure.

It constitute a current of magnitude

eI

T

where Tis the orbital period of electron.

Now2 2 r

T

v

And so2

evI

r

From electromagnetic theory, the magnitude of orbital magnetic

dipole moment l

for a currentI in a loop of areaAis

lIA

and its direction is perpendicular to the plane of the orbit as shown. Substituting the value of Ifrom above and taking 2A r . We get

2

2 2l

ev evr r

r

(1)

Because the electron has negative charge, its magnetic dipole moment l

is opposite in direction

to its orbital angular momentum L

whose magnitude is given by

L mvr (2)

Dividing equation (1) by (2) we get

2

l e

L m

(3)

L

v

e

r

e


19/45


EEEG 207

P a g e18

Thus the ratio of the magnitudel

of the orbital magnetic dipole moment to the magnitude Lof

the orbital angular momentum for the electron is constant, independent of the details of the orbit.

This constant is called the gyromagnetic ratio for the electron.

We can write equation (3) in vector form

2l

eL

m

The minus sign means that l

is in opposite direction ofL

. The unit of electromagnetic moment

is ampere-m2or joule/Tesla.

Bohr Magneton

From wave mechanics, the permitted scalar values of L

are given by

12

hL l l

where lis the orbital quantum number. Therefore the magnitude of the orbital magnetic moment

of the electron is

14

l

ehl l

m

The quantity4

eh

mforms a natural unit for the measurement of atomic magnetic dipole moments,

and is called the Bohr magneton, denoted byB

.

Thus, 1l Bl l

where

19 34

31

24 2

1.6 10 6.6 10

4 4 3.14 9.1 10

9.28 10

B

C Jseh

m Kg

Amper m


20/45


EEEG 207

P a g e19

Electron spin and magnetic moment

An electron not only revolves on a circular orbit around the positive nucleus but also rotates

around an axis of its own. The magnetic moment associated with spinning of electron is called

spin magnetic moment s .

If we consider the simple case of an electronic charge being spread over a spherical volume, then

the electron spin would cause different charge elements of this sphere to form closed currents.

This will result a net spin magnetic moment. This net magnetic moment, obviously, would

depend upon the detailed structure of the electron and its charge distribution. It can be shown

that s is connected with spin angular momentum Sas

2s e Sm

Where the coefficient is known as the spin gyromagnetic ratio and depends on the structure of

the spinning particles and its charge distribution. The experimental value of for an electron is

-2.0024. Here the negative sign indicates that s is in a direction opposite to that of S.

Since2

hS

for an electron,

19 34

31

24 2

2 4

1.6 10 6.6 10

2.0024 8 3.14 9.1 10

9.4 10

s

hem

C Js

Kg

Amper m

Thus the magnetic moments due to the spin and the orbital motions of an electron are of the same

order of magnitude. It should be noted that spin and s are intrinsic properties of an electron and

exist even for a stationary electron (L=0).

Since the magnitude of the spin magnetic moment is always same, the external field can only

change its direction. If the electron spin moments are free to orient themselves, they will orient

themselves in the direction of the applied field B. Thus, paramagnetic is the result of spinmagnetic moments.


21/45


EEEG 207

P a g e20

Magnetic moment due to nuclear spin

In addition to electronic contribution, nuclear spin also contributes to magnetic moment of

atoms. By analogy with Bohr magneton, the nuclear magneton arises due to spin of the nucleus.

It is given by

4n

p

eh

m

where pm represents the mass of the proton. By putting the known values, we get

27 25.05 10n Amper m

Obviously, the nuclear magnetic moments are smaller than those associated with electron.

Thus the permanent dipoles originate: (i) the orbital motion of the electron, (ii) electron spin, and

(iii) the nuclear spin.

Curie-Weiss law

The temperature dependence of many paramagnetic materials is governed by the experimentally

found Curie law, which states that the susceptibility m is inversely proportional to the absolute

temperature T.

1m m CT T

Where2

3

B

B

nC

k

is called Curie constant.

For many other substance, a more general relationship is observed, which is known as Curie-

Weiss law.

m

C

T

where is another constant that has the same unit as the temperature and may have positive as

well as negative values.


22/45


EEEG 207

P a g e21

Fig: Schematic representation of (a) the Curie law, (b) and (c) the Curie-Weiss law and (d) the

diamagnetic behavior is also shown for comparison.

Anti-ferromagnetic materials are paramagnet above Neel temperature TN. i.e. they obey there a

linear1

m

T f

law. Below TNhowever the inverse susceptibility may rise with decreasing

temperature. The extrapolation of paramagnetic line to1

0

yields a negative . Thus the

Curie-Weiss law needs to be modified for anti-ferromagnetic as

m

C C

T T

The Neel temperature is often below room temperature. Most anti-ferromagnetic are found

among ionic compounds. They are insulators or semiconductors. No practical application for

anti-ferromagnetic is known at this time.

Fig: Schematic representation of the temperature dependence of a poly crystalline anti-

ferromagnetic material law.

1

m

T

(d)Dia

(b)Ferro

(a)Para

(C)Anti-ferro

1

m

TTN

Paramagnetic

Anti-ferro

0

Curie-Weiss


23/45


EEEG 207

P a g e22

Hysteresis loss in magnetic materials

When a magnetic material is subjected to a gradually increasing magnetizing field, the intensity

of magnetization Iincrease with the increase in strength of magnetizing field Halong the pathOA. This curve is known as virgin or initial magnetization curve.

At H=H0 the intensity of magnetization assumes a steady value Imax. The magnetic material

cannot be magnetized more strongly than this and at this stage the material is said to have

reached the magnetic saturation limit.

Now if the magnetizing fieldHis gradually decreased the intensity of magnetization Iwill not

decrease the same path OA, but will decrease along the path AB such that whenHbecomes zero

Iwill not become zero but has a definite valueI=OB.

The value of intensity of magnetization of the magnetic material even when the magnetizingfield is reduced to zero is called its retentivity or remanence or residual magnetism.

Now if the direction of magnetizing field is reversed the intensity of magnetization takes along

the path BC till it become zero at C. Thus to reduce the residual magnetism to zero, a

magnetizing field is equal to the value OC has to be applied in reverse direction. The value of

reverse magnetizing field require to reduce the residual magnetism to zero is called the coercive

force or coercivity.

When the magnetizing field is further increased in reverse direction, the intensity of

magnetization increases along the path CD and acquires the magnetic saturation limit at point D.If the magnetizing fieldHis now reduced to zero, the intensity of magnetizationIfollow the path

DE. Finally if His increased in the original direction I follow the path EFA and a closed curve

ABCDEFA is obtained. This closed curve is known as hysteresis loop. On repeating the process

the same closed curve is obtained again and again but never the portion OA. It is seen that I

always lags behindH. This lagging ofIbehindHis called hysteresis.


24/45


EEEG 207

P a g e23

The shape of this loop varies from one material to another. Some ferrites have an almost

rectangular hysteresis loop. These ferrites are used in digital computers as magnetic information

storage device. The area of the loop represents energy loss (hysteresis loss) per unit volume

during one cycle of the periodic magnetization of the ferromagnetic materials. This energy loss is

in the form of heat. Therefore it is desirable that materials used in electronic generators, motors

and transformers should have a tall but narrow hysteresis loop for maximum losses.

Permanent magnets (hard magnetic materials) are device which retain their magnetic field

indefinitely i.e. coercivity and area of hysteresis loop are large.

Eddy current loss in magnetic materials

Fig: Solid transformer core with eddy current Iein a cross sectional area A.

The core loss is the energy which is dissipated in the form of heat with in the core of

electromagnetic devices when the core is subjected to an alternating magnetic field. Several

types of losses such as hysteresis and eddy current loss also happen.

Consider a transformer whose primary and secondary coils are wounded around the lags of a

rectangular iron yoke as shown in figure. An alternating electric current in the primary coil

causes an alternating magnetic flux in the core. This in turn induces the secondary coil an

alternating emf e proportional tod

dt

i.e.d dB

e e Adt dt

Concurrently, an alternating emf is induced within the core itself as shown in figure. This emf

gives rise to the eddy current Ie. The eddy current is larger, then larger the permeability , the

larger the conductivity of the iron core material, the higher the applied frequency and the

larger the cross-sectional areaAof the core.

eI

A

N


25/45


EEEG 207

P a g e24

In order to decrease the eddy current, first core can be made of an insulator in order to

decrease . Ferrites are thus effective but also expensive materials to build magnetic cores.

There are indeed used for high frequency applications. Secondly, the core can be manufactured

from iron powder where by each particle is covered by an insulating coating. However the

decrease in , in this case is at the expense in large decrease in . Thirdly, the most widely

applied method to reduce eddy currents is the utilization of cores made out of thin sheets which

are electrically insulated from each other. This way the cross-sectional area Ais reduced which

in turn reduce emf. These losses are however less than 1% of the total energy transferred.

Application Of magnetic materials

Electrical devices like power transformers, motors, generators, electromagnets etc use soft

magnetic materials. Electrical steels are use as core materials in them. For retaining magnetic

fields of permanent magnets, hard magnetic materials are used in fabrications.

Magnetic materials find significant use in the storage information. Credit cards are properly usedwhich also have magnetic strips. To store large quantities of information of low cost, computers

are usually backed up with magnetic disks. The recording head consisting of laminated

electromagnet is made soft ferric having 0.3m wide air gap. Here the data written by the

electrical signal generates a magnetic field across the gap with in the coil. Finally the stored

information is read using the same head, and an alternating emf is induced in the coil of the head

by moving tap or disk in the read or play back mode. This emf is amplified, filtered and fed to a

suitable output device (loudspeaker).

Fig: Schematic arrangement of recording (playback) head and magnetic tape. The gap width isexaggerated (Recording mode). The plastic substrate is about 25m thick.

Magnetic material is also used for making medical devices such as thin motors in implantablepumps and valves.

N

Magneticlayer

Substrate

Tapemotion


26/45


EEEG 207

P a g e25

****Notes****

Eddy Current- An induced electric current formed with in the body of a conductor in a varying

magnetic field. An eddy current is a current that is induced in the iron core (iron being a

conductor as well as having a high permeability). The current flows back and forth in the iron

core as the alternating current in the windings changes directions. Eddy current does not usefulwork. They cause the core to heat up. So the energy in these induced eddy currents is lost as heat.

But induced current is useful energy. It can be used to run a motor, computer etc. The eddy

current causes random motion of atoms in the iron core so we cant get at that energy as easily in

order to do useful work.

Optical Properties of Materials

Introduction

The interaction of light with the valance electrons of a material is responsible for optical

properties. The optical measurements that give the fullest information on the electronic systemare measurements of the reflectivity of light at normal incidence on single crystal.

Most recently, a number of optical devices such as lasers, photo detectors, waveguides, etc. have

gained considerable technological importance. They are used in communication, fiber optics,medical diagnostics, night viewing, solar applications, optical computing or for other

optoelectronic purposes.

Refractive index or n

In optics the refractive index or index of refraction n of a substance (optical medium) is a

dimensionless number that describes how light, or any other radiation, propagates through thatmedium. It is defined as

cn

v

Where cis the speed of light in vacuum and vis the speed of light in the substance. For example,the refractive index of water is 1.33, means that light travels 1.33 times as fast in vacuum as itdoes in water.

The historically first occurrence of the refractive index was in Snells law of refraction, n1sin1=n2sin2, where 1and 2are the angles of incidence of a ray crossing the interface between twomedia with refractive indices n1and n2.

Refractive index of materials varies with the wavelength. This is called dispersion; it causes thesplitting of white light in prisms and rainbows, and chromatic aberration in lenses. In opaque


27/45


EEEG 207

P a g e26

media, the refractive index is a complex number: while the real part describes refraction, theimaginary part accounts for absorption.

The concept of refractive index is widely used within the full electromagnetic spectrum, from x-

rays to radio waves. It can also be used with wave phenomena other than light (e.g., sound). In

this case the speed of sound is used instead of that of light and a reference medium other thanvacuum must be chosen.

Penetration depth (W)

Penetration depth is a measure of how deep light or any electromagnetic radiation can penetrate

into a material. It is defined as the depth at which the intensity of the radiation inside the materialfalls to 1/e (about 37%) of its original value at (or more properly, just beneath) the surface.

When electromagnetic radiation is incident on the surface of a material, it may be (partly)

reflected from that surface and there will be a field containing energy transmitted into the

material. This electromagnetic field interacts with the atoms and electrons inside the material.Depending on the nature of the material, the electromagnetic field might travel very far into thematerial, or may die out very quickly. For a given material, penetration depth will generally be a

function of wavelength.

The intensity of light sensitive device (such as photo detector)Iequals to the square of the field

strength .

2

0

2exp

wkI I z

c

where kis damping constant.

We define characteristics penetration depth (W) is that distance at which the intensity of lightwave travels through a material.

When 1

0

1Ie

I e

This definition yields2 4 4

c cW

wk fk k

The inverse of Wis called the exponential or attenuation or absorbance.

4 2k wk

c

It is measured in cm-1.

Reflection, Transmission, and Absorption

Reflection is the process by which electromagnetic radiation is returned either at the boundary

between two media (surface reflection) or at the interior of a medium (volume reflection),

whereas transmission is the passage of electromagnetic radiation through a medium. Both

processes can be accompanied by diffusion (also called scattering), which is the process of

deflecting a unidirectional beam into many directions. In this case, we speak about diffuse


28/45


EEEG 207

P a g e27

reflection and diffuse transmission. When no diffusion occurs, reflection or transmission of a

unidirectional beam results in a unidirectional beam according to the laws of geometrical optics.

In this case, we speak about regular reflection and regular transmission (or direct transmission).

Reflection, transmission and scattering leave the frequency of the radiation unchanged.

Absorption is the transformation of radiant power to another type of energy, usually heat, by

interaction with matter.

ReflectivityR

When light radiation passes from one medium into another having a different index of refraction,

some of the light is scattered at the interface between the two media even if both are transparent.

The reflectivityRrepresents the fraction of the incident light that is reflected at the interface, or

0

RI

RI

whereI0andIRare the intensities of the incident and reflected beams respectively. If the light is

normal (or perpendicular) to the interface, then

2

2 1

2 1

n nR

n n

where n1and n2are the indices of refraction of the two media. If the incident light is normal if

the incident light is not normal to the interface, Rwill depend on the angle of incidence. When

light is transmitted from a vacuum or air into a solid, then

2

2

2

1

1

nR

n

Since the index of refraction of air is very nearly unity. Thus higher the index of refraction of the

solid greater is the reflectivity. For typical silicate glasses the reflectivity is approximately 0.05.

Just as the index of refraction of a solid depends on the wavelength of incident light and hence

reflectivity vary with wavelength.

Transmissivity T

Transmissivity is the ratio between the transmitted intensity ITand impinging light intensity I0.

Then 1 2

2

0 1 2

4TI n nT I n n

Notice that 1R T as conservation of energy.

For instance, when light passes from air 1 1n into glass 2 1.5n then 0.04 and 0.96R T .

i.e. most of the light is transmitted.


29/45


EEEG 207

P a g e28

Dielectric Properties

Introduction

A dielectric is an insulating material in which all the electrons are tightly bound to the nuclei of

the atoms and there are no free electrons available for the conduction of current. Therefore theelectrical conductivity of a dielectric is very low. The conductivity of an ideal dielectric is zero.

On the basis of band theory, the forbidden gap Egis very large in dielectrics. Materials such as

glass, polymers, mica, oil and paper are examples of dielectrics. They prevent flow of current

through them. Therefore they can be used for insulating purpose.

Dielectric Constant

It is found experimentally that the capacitance of a capacitor is increased if the space between its

plates is filled with a dielectric material. To understand this fact, Faraday took two identical

capacitors, one who was evacuated and the other was filled with dielectric material as shown in

the figure.

Then these two capacitors were charged with a battery of some potential difference. He found

that the charge on capacitor filled with a dielectric is larger than that of filled with air. If C0be

the capacitance in vacuum and C the capacitance when the space is filled with a dielectric

material then dielectric constant of the material

0

CK

C

Thus the dielectric constant of a material is the ratio of the capacitance of a given capacitor

completely filled with that material to the capacitance of the same capacitor in vacuum. In other

words, the ratio of permittivity of medium to the vacuum is also known as dielectric constant.

0

rK

This is also known as relative permittivity. It is found to be independent of the shape and size

and dimension of the capacitor.

Vacuum Dielectric Material


30/45


EEEG 207

P a g e29

Non Polar Dielectric

A non polar molecule is the one in which the center of gravity of the positive charge (protons)

and negative charge (electrons) coincide. So such molecules do not have any permanent dipole

moment. For example O2, N2andH2

Polar Dielectric

A polar molecule is the one in which the centre of gravity of positive charges is separated by

finite distance from that of negative charges. Unbalanced electric charges, using valance

electrons of such molecules result in dipole moment and orientation. Therefore those molecules

possess permanent electric dipole. For exampleN2O, H2OandHCL

Polarization of Dielectrics

When an electric field is applied to a dielectric material, it exerts a force on each charged particle

and charges is displaced in its orientation while the negative charges is displaced in opposite

direction as shown in figure (a). Consequently the center of positive and negative charges of each

atom displaced from their equilibrium positions. Such a molecule (or atoms) is then called as

induced electric dipole and this process is known as dielectric polarization.

0E

Fig (a)

Fig (c)

0E

pE

iq

iq

Dielectric slab

0E

Vacuum

Fig (b)


31/45


EEEG 207

P a g e30

We consider a parallel plate capacitor which has vacuum initially between its plates. When it is

charged with a battery, the electric field of strengthE0is set up between the plates of capacitor as

shown in figure (b). If and are the surface charge densities of the two plates of thecapacitor, then the electric field developed between the plate is given by

0

0

E

If now a slab of dielectric material is placed between the two plates of capacitor as shown in

figure (c), then it becomes electrical polarized. Hence its molecule becomes electric dipole

orientation in the direction of field. Because of this the center of positive and negative charges

gets displaced from each other. Therefore in the interior of dielectric as marked by dotted lines

these charges cancel. However the polarization charges on the opposite faces of dielectric slab

are not cancelled. These charges produce their own electric field Ep, which opposes the external

applied field E0. Under this situation, the net electric field in the dielectric is given by

0 pE E E

Polarization Density

The induced dipole moment developed per unit volume in a dielectric slab on placing it inside an

electric field is known as polarization density. It is denoted by the symbol P

. If p

is induced

dipole moment of individual atom and N is the number of atoms in a unit volume, the

polarization density as

P Np

The induced dipole moment of an individual atom is found to be proportional to the applied

electric field E

and is given by

0p E

q

q

iq

iq

Dielectric slab

AreaA AreaA

d


32/45


EEEG 207

P a g e31

Where is the proportionality constant also known as atomic polarizability.

Then 0P N E

Suppose A is the area of each plate of the capacitor, d is the separation between them. Then

volume of dielectric slab isAd. Sinceqiand +qiare the induced charges developed on the twofaces of the dielectric slab, the total dipole moment of the slab will be equal to qid. From the

definition of polarization density

i ip

q d qtotaldipolemomentP

volumeofslab Ad A

On placing the dielectric material between the plates of the capacitor, the reduced value of

electric field may be evaluated as

0

0 0 0 0

p p PE E

Or0

0

pE P

E P

The quantity 0E P

is of special significance and is known as electric displacement vector D

given by 0D E P

Relation between Dielectric constant and electric susceptibility

The polarization density of a dielectric is proportional to the effective value of electric field E

and is given by

0P E

Where is constant of proportionality and is known as susceptibility of dielectric material.

We have

00 0 0

0 0

0

0

1

1

1

EPE E E E E

E E

E

E

K


33/45


EEEG 207

P a g e32

Types of Polarization

Electronic Polarization

Under the action of an external field, the electron clouds of atoms are displaced with respect to

heavy fixed nuclei to a distance less than the dimension of atom. This is called electronic

polarization, which does not depend on temperature. The electronic polarization, is represented

as below

e eP N E

Ionic Polarization

This type of polarization occurs in ionic crystals for example in sodium chloride crystal. In the

presence of an external electric field, the positive and negative ions are displaced in opposite

directions until ionic bonding forces stop the process. This way the dipoles get induced. The

ionic polarization does not depend upon temperature.

Orientation Polarization

No field applied

Applied electric field E

E

Absence of field Presence of field

E




34/45


EEEG 207

P a g e33

This type of polarization is applicable in polar dielectrics. In the absence of an external electric

field, the permanent dipoles are oriented randomly such that they cancel the effects of each other.

When an electric field is applied, these dipoles tend to rotate and align in the direction of applied

field. This is known as orientation polarization which depends upon temperature.

In view all the polarizations; the total polarization is sum of electronic, ionic, and orientationpolarizations. This is given by

0e iP P P P

Dielectric Losses

When a dielectric material is placed in an alternating electric field, a part of energy is wasted,

which is known as dielectric loss i.e. the absorption of electrical energy by a dielectric material is

called dielectric losses.

Let us consider an alternating electric field is applied to a dielectric material given by

0 cosE E t

where is the applied frequency.

Then there will be two cases

(i) For the electric displacement vector is in the phase with applied field

0 cosD D t

Then the electric displacement current is

0 sinD

J D tt

(1)

The energy loss in time period T. i.e. average energy loss is

2

0

0

0

2

T

T

J Edt

W J Edt

dt

2

0 0

0

sin cos2

0

D t E tdt

shows that the energy loss is zero when displacement vector is in phase with the applied field.


35/45


EEEG 207

P a g e34

(ii)When electric displacement vector is out of phase with applied field.

If 0 cosE E t

then 0 cosD D t

Where is the possible phase difference between E

andD

. So average energy

loss

0

0

T

T

J Edt

W

dt

But 0 sinD

J D tt

2

0 0

0

2

0 0

0

22

2

0 0

0

2

0 0

0 0

sin cos2

sin cos cos sin cos2

sin cos2

1 2sin

2 2

1sin

2

D t E tdt

D E t t tdt

D E tdt

D E

D E

(2)

If we consider electric field and electric displacement vector is complex the dielectric constant is

also complex.

0 0

0 0

0

0

cos sin

i t i

i t

D e D eD

E E e E

Di

E

So 00

cos sinD

i iE

(3)

Where and are the real and imaginary part of dielectric constant.

So from equation (3) equating the real and imaginary part we get


36/45


EEEG 207

P a g e35

0

0

cosD

E

and 0

0

sinD

E

So tan

(angle form)

This tan is known as the loss tangent. Since and are frequency dependent so is alsofrequency dependent.

0 0sinD E

So equation (2) becomes

0 0

20

2

0 0

1

2

12

2r

W E E

W E

W E

Thus the absorption of energy is proportional to the imaginary part of complex dielectric

constant. Whenever there is energy dissipated in the medium called dielectric losses.

Ferroelectric material

For the dielectric materials, the polarization is a linear function of applied field. The polarization

in these materials is not a unique function of the field strength. In particular these materialsexhibits hysteresis effects similar to those obtained in ferromagnetic material, they are therefore

ferroelectric material.

Piezoelectricity

Piezoelectricity is the charge which accumulates in certain solid materials in response of

mechanical strain. The word piezoelectricity means electricity resulting from pressure.

The piezoelectric effect is the linear electromagnetically interaction between the mechanical and

electrical state in crystalline materials. It is found in useful applications such as the production

and detection of sound, generation of high voltages, electronic frequency generator etc.


37/45


EEEG 207

P a g e36

Introduction to Integrated Circuit Fabrication

Introduction

An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or

a microchip) is a set of electronic circuits on one small plate ("chip") of semiconductor material,normally silicon. This can be made much smaller than a discrete circuit made from independent

components. An integrated circuit is one in which circuit components such as transistors, diodes,

resistors, capacitors etc are automatically part of semiconductor chip.

In the early 1960s, a new field of microelectronics was born primarily to meet the requirements

of the Military which wanted to reduce the size of electronic equipment to approximately one-

tenth of its then existing volume. This device for extreme reduction in the size of electronic

circuits has led to the development of microelectronic circuits called integrated circuits (ICs)

which are small that their actual construction is done by technicians using microscopes.

Integrated circuits are used in virtually all electronic equipment today and have revolutionized

the world of electrons. Computers, mobile phones, and other digital home appliances are now

inextricable parts of the structure of modern societies, made possible by the low cost of

producing integrated circuits.

ICs can be made very compact, having up to several billion transistors and other electronic

components in an area the size of a fingernail. The width of each conducting line in a circuit can

be made smaller and smaller as the technology advances; in 2008 it dropped below 100

nanometers and in future it is expected to be in the tens of nanometers and even more.

Advantages

1.

Increased reliability due to lesser number of connections.

2. Extremely small size due to the fabrication of various circuit elements in a single ship of

semiconductor materials.

3. Lesser weight

4.

Low power required.

5. Reduced cost

6. Suitability for small-signal operation

7. Easy replacement

Disadvantages

1. If any component of ICs goes out of order, the whole IC has to be replaced by a new one.

2. In an IC fabrication, it is neither convenient nor economical to fabricate capacitance

exceeding 30pF.

3. It is not possible to fabricate inductors and transformers.

4. It is not possible to produce high power ICs (greater than 10W).


38/45


EEEG 207

P a g e37

Types

Four basic types of constructions are employed in the manufacture of integrated circuits namely,

1. Monolithic 2. Thin-film 3. Thick-film 4. Hybrid

Since it combines both active (eg- diodes, transistors) and passive elements (eg- resistors,

capacitors) in a monolithic structure, the complete unit is called integrated circuits.

IC Terminology

1. Bonding- attachment of wires to an IC.

2. Chip- an extremely small part of silicon wafer on which IC is fabricated. One silicon

wafer of 2cm diameter may contain up to 1000IC chips.

3. Circuit probing- to check the proper electrical performance of each IC with the help of

probes.

4. Die- same as chip.

5. Diffusion- introduction of controlled small quantities of a material into the crystal

structure for modifying its electrical characteristics.

6. Diffusion mask- it is a glass plate with the circuit pattern drawn on it. Impurities can

diffuse through its light areas but not through its dark ones.

7. Encapsulation- putting a cap over the IC and sealing it in an inert atmosphere.

8. Epitaxy- physical placement of materials on a given surface.

9. Etching- removal of surface material from a chip by chemical means.

10.Metallization- providing ohmic contacts and inter connections by evaporating aluminum

over the chip.

11.Photoresist- a photo-sensitive emulsion which hardens when exposed to ultraviolet light.

12.Scribing- incising or cutting with a sharp point.

13.

Wafer- a thin slice of a semiconductor material either circular or rectangular in shape in

which a number of ICs are fabricated simultaneously.


39/45


EEEG 207

P a g e38

Making monolithic IC

A monolithic IC is one in which all circuit components and their interconnections are found on a

single thin wafer called the substrate.

The basic production processes for monolithic ICs are as follows

Step-1: P-substrate (Crystal growth)

This is the first step in making of an IC. A cylindrical p-type silicon crystal is grown having

typical dimensions 25cm long and 2.5cm diameter shown in figure (i).

The crystal is then cut by a diamond saw into many thin wafers like figure (ii), the typical

thickness of the wafer being 200m. One side of wafer is polished to get rid of surface

imperfections. This wafer is called the substrate. The ICs are produced in this wafer.

Step-2: Epitaxial growth of n-layer

The next step is to put the wafer in a diffusion furnace. A gas mixture of silicon atoms and

pentavalent atoms is passed over the wafers. This forms a thin layer of n-type semiconductor on

the heated surface of the substrate as shown figure. This layer is called the epitaxial layer and is

about 10m thick. It is in thin layer that the whole integrated circuit is formed.

Step-3: Oxidation

P-Substrate

n

200m

10m

Epitaxial layer

1m

P-Substrate

n

SiO2layer

200m

2.5cm

P-Substrate

(ii) P-type Silicon crystal

(i) P-type Silicon crystal

25cm

2.5cm


40/45


EEEG 207

P a g e39

In order to prevent the contamination of epitaxial layer, a thin SiO 2 layer about 1m thick is

deposited over the entire surface as shown in figure. This is achieved by passing pure oxygen

over the epitaxial layer. The oxygen atom combines with silicon atoms to form a layer of SiO2.

Step-4: Photolithography

Selective removal of oxide is done by the process of photolithography. We cover the entire

surface of oxidized silicon with a photosensitive material called photoresist. Now portion ofphotoresist is remove away by subjecting ultraviolet radiation.

After removal of photoresist the oxide is etched. HCl solution etches the SiO2layer.

After this we remove the remaining photoresist from the rest of the portion.

Step-5: Diffusion

To dope through the window we usually do diffusion.

After diffusion we remove rest of oxide.

Step-5: Contact Metallization

Metallization needs selective deposition of metals over the p and n type i.e. contact outside.

P-Substrate

n

Photoresist

P-Substrate

n

UV radiations

P-Substrate

n

SiO2

P-Substrate

n

P-Substrate

n+

P-Substrate

n+


41/45


EEEG 207

P a g e40

Fabrication of components on monolithic IC

The notable feature of an IC is that it comprises a number of circuits elements inseparably

associated in a single small package to perform a complete electronic function. This differs from

discreet assembly where separately manufactured components are joined by wires. Some of the

circuit elements (e.g. diode, transistor, resistor, capacitor etc.) can be constructed in the IC form.

Diodes

P-Substrate

n

Exposed

(i)

P-Substrate

n

(ii)

P-Substrate

n

(iii)

P-Substrate

n

(iv)

Window

P-Substrate

n

(v)

p

P-Substrate

n

(vi)

p

P-Substrate

n

(vii)

p

1 2

1 2

(viii)


42/45


EEEG 207

P a g e41

Figure above shows how a diode is formed on a portion of substrate of monolithic IC. Part of

SiO2layer is etched off, exposing the epitaxial layer as shown in figure (i). The wafer is then put

into a furnace and trivalent atoms are diffused into the epitaxial layer. The trivalent atoms change

the exposed epitaxial layer from n-type material under SiO2layer as shown in figure (ii).

Next pure oxygen is passed over the wafer to from a complete SiO2layer as shown in figure (iii).A hole is then etched at the center of this layer; thus exposing the n-epitaxial layer as shown in

figure (iv). This hole in SiO2layer is called window. Now we pass trivalent atoms through the

window. The trivalent atoms diffuse into the epitaxial layer to form an island of p-type material

as shown in figure (v). The SiO2layer is again formed on a wafer by blowing pure oxygen over

the wafer as shown in figure (vi). Thus a p-n junction diode is formed on a substrate.

The last step is to attach the terminals. For this purpose, we etch the SiO2 layer at the desired

locations as shown in figure (vii). By depositing metal at these locations, we make electrical

contact with the anode and cathode of the integrated diode. Figure (viii) shows the electrical

circuit of the diode.

Transistors

Figure below shows how a transistor is formed on a portion of substrate of monolithic IC. Part of

SiO2layer is etched off, exposing the epitaxial layer as shown in figure (i). The wafer is then put

into a furnace and trivalent atoms are diffused into the epitaxial layer. The trivalent atoms change

the exposed epitaxial layer from n-type material under SiO2layer as shown in figure (ii).

Next pure oxygen is passed over the wafer to from a complete SiO2layer as shown in figure (iii).

A hole is then etched at the center of this layer; thus exposing the n-epitaxial layer as shown in

figure (iv). This hole in SiO2layer is called window. Now we pass trivalent atoms through the

window. The trivalent atoms diffuse into the epitaxial layer to form an island of p-type material

as shown in figure (v). The SiO2layer is again formed on a wafer by blowing pure oxygen over

the wafer as shown in figure (vi).

A window is now formed at the center of SiO2 layer, thus exposing the p-epitaxial layer as

shown in figure (vii). Then we pass pentavalent atoms through the window. The pentavalent

atoms diffuse into the epitaxial layer to form an island of n-type material as shown in figure

(viii). The SiO2layer is reformed over the wafer by passing pure oxygen as shown in figure (ix).

The terminals are processed by etching the SiO2layer at appropriate locations and depositing the

metal as these locations as shown in figure (x). In this way, we get the integrated transistor.

Figure (xi) shows the electrical circuit of a transistor.


43/45


EEEG 207

P a g e42

P-Substrate

n

Exposed

(i)

P-Substrate

n

(ii)

P-Substrate

n

(iii)

P-Substrate

n

(iv)

Window

P-Substrate

n

(v)

p

P-Substrate

n

(vi)

p

P-Substrate

n

(vii)

p

Window

P-Substrate

n

(viii)

p

n

P-Substrate

n

(ix)

p

n

P-Substrate

n

(x)

p

n

E BC

C

E

B

(xi)


44/45


EEEG 207

P a g e43

Resistors

Figure above shows how a resistor is formed on a portion of substrate of monolithic IC. Part of

SiO2layer is etched off, exposing the epitaxial layer as shown in figure (i). The wafer is then putinto a furnace and trivalent atoms are diffused into the epitaxial layer. The trivalent atoms change

the exposed epitaxial layer from n-type material under SiO2 layer as shown in figure (ii).Next

pure oxygen is passed over the wafer to from a complete SiO2layer as shown in figure (iii).

A window is now formed at the center of SiO2 layer, thus exposing the n-epitaxial layer as

shown in figure (iv). Then we diffuse a p-type material into the n-type area as shown in figure

P-Substrate

n

Exposed

(i)

P-Substrate

n

(ii)

P-Substrate

n

(iii)

P-Substrate

n

(iv)

Window

P-Substrate

n

(v)

p

P-Substrate

n

(vi)

p

P-Substrate

n

(vii)

p

1 2

21

(viii)


45/45


44

(v). The SiO2layer is re-formed over the wafer by passing pure oxygen as shown in figure (vi).

The terminals are processed by etching SiO2 layer at the two points above the p island and

depositing the metal at these locations as shown in figure (vii). In this way, we get an integrated

resistor. Figure (vii) shows the electrical circuit of a resistor.

The value of resistor is determined by the material, its length and area of cross section. The highresistance resistors are long and narrow while low-resistance resistors are short and greater the

cross section.

Capacitors

Figure above shows the process of fabricating a capacitor in the monolithic IC. The first step is

to diffuse an n-type material into the substrate which forms one plate of the capacitor as shown

in figure (i). Then SiO2 layer is reformed over the wafer by passing pure oxygen as shown in

figure (ii).The SiO2layer formed acts as the dielectric of the capacitor. The oxide layer is etched

and terminal 1 is added as shown in figure (iii). Next a large (compared to the electrode atterminal 1) metallic electrode is deposited on the SiO2 layer and forms the second plate of the

capacitor. The oxide layer is etched and terminal 2 is added. This gives an integrated capacitor.

The value of capacitor formed depends upon the dielectric constant of SiO2 layer, thickness of

SiO2layer and the area of cross section of the smaller of the two electrodes.

P-Substrate

n

Exposed

(i)

P-Substrate

n

(ii)

P-Substrate

n

(iii)

P-Substrate

n

(iv)

1 2

21

(v)

Superconductor and Semiconductors

Documents

Transcript of Superconductor and Semiconductors