final touhid

8/9/2019 Final Touhid

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Under the supervisor

of

Md. Mizanur RahmanAssistant Professor

ECE Discipline

Khulna University

Khulna


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Submitted By

A.K.M. Touhidur Rahman

Examination Roll no. 090918


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Discussion on

Introduction to metal, semiconductor & insulator,

types of semiconductor: p-type and n-type; p-njunction diode

& its characteristics, p-n junction diode as

rectifiers: half wave and full wave;


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INSULATOR:

An insulator is a material that offers a very low of conductivityunder pres-sure from an applied voltage source. Some examples ofinsulators include plastic, rubber, glass, porcelain, air, paper, cork, mica,ceramics and certain oils.

CONDUCTOR:

The term conductor is applied to any material that willsupport generous flow of charge when a voltage source of limited magnitudeis applied across its terminal. All metals are conductors and someexamples include copper, aluminium, brass, platinum, silver, gold andcarbon.

SEMICONDUCTOR :

A semiconductor is a material that has a conductivity levelsomewhere between the extremes of an insulator and a conductor. Someexamples of semiconductors include Ge, Si, GaAs, CdS, GaN.

Introduction of metal


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There are two classes of semiconductor materials.

1. Single crystal:----------- Ge, Si.

2. Compound crystal:----- GaAs, CdS.

The three semiconductors used most frequently are Ge, Si, GaAs.

SEMICONDUCTORS

Intrinsic semiconductor Extrinsic semiconductor

N-type semiconductor P-type semiconductor

Type of semiconductor:


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Intrinsic semiconductorA semiconductor in an extremely pure form is know as an

intrinsic semiconductor.In an intrinsic semiconductor, even at room temperature

,hole-electron pair are created. When electric field is applied across

an intrinsic semiconductor , the current conduction takes place by

two process, namely; by free electron and hole as show in fig:01

Free electron

Hole

A B

FIG:01


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The free electrons are produced due to the breaking up of some covalent

bonds by thermal energy . At the same time, holes are created in thecovalent bonds.

Extrinsic semiconductorWhen a small amount of suitable impurity(trivalent or pentavalent)

is added to a pure semiconductor is called extrinsic semiconductor.

The process of adding impurities to a semiconductor is knows as doping.

Depending upon the type of impurity added, extrinsic semiconductors are

classified into:(i)n-type semiconductor (ii)p-type semiconductor


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N-type semiconductor When a small amount of pentavalent impurity is added to a puresemiconductor , it is know as n-type semiconductor.

To produce a n-type semiconductor a small amount of pentavalent impurity i

added to a pure semiconductor . The addition of pentavalentimpurity provides a large number of free electrons in the semiconductor.

Examples: A small amount of pentavalent impurities like arsenic is added to pure

Germanium crystal . There exists a large number of electrons in the crystal

as show in fig:02


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FREE ELECTROPN

PENTAVALENTIMPURITY ATOM

FIG:02:Arsenic impurity in n-type semiconductor

N-type conductivity:When potential difference is applied to the n-type semiconductor, the free

electrons in the crystal will be directed towards the positive terminal,constituting electric current as the current flow through the crystal is by free

electron which are carriers of negative charge . This type of conductivity iscalled n-type conductivity.


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FIG:03:Conduction in an N-type semiconductor


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P-type semiconductor When a small amount of trivalent impurity is added to a pure semiconductor ,

it is know as p-type semiconductor.

To produce a p-type semiconductor a small amount of trivalent impurity is

added to a pure semiconductor . The addition of trivalent impurity provides a

large number of free electrons in the semiconductor.

Examples: A small amount of trivalent impurities like boron is added to pure silicon

crystal . There exists a large number of electrons in the crystal as show in fig:04


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FIG:04:Boron impurity in p-type material

P-type conductivity:When potential difference is applied to the p-type semiconductor, the conductorthe hole are shifted from one covalent bond to other. As the holes are positively

charged , they are directed towards the negative terminal, constituting what is

know as hole current . It may be noted that valence electrons move from one co-

valent bond to other.


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FIG:05:Conduction in an N-type semiconductor


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Majority and Minority CarriersA p-type semiconductor by the addition of an acceptor impurity which adds a large

number of holes. Here a p-type material contains a largenumber of holes and very smallnumber of thermally generated electron. In p-type material the number of holes is much

more that of electrons.In such a material holes constitute majority carriers and electron

are minority carriers.

Similarly in a n-type material,the number of electrons are much more than that the

thermally generated holes. In such a material holes are minority carriers and electron

are majority carriers.The majority and minority carriers as shown in fig:06

FIG:06

majority carriers

minority carriersminority carriers

miajority carriers


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PN junctionWhen a p-type semiconductor is suitably joined to n-type semiconductor ,the

contact surface is called PN junction.

Most semiconductor devices contain one or more PN junction .The PN

junction is of great importance because it is in effect, The control element forsemiconductor devices.Properties of PN junctionTo explain the properties of a PN junction, consider two types of materials;one P-type and other N-type as shown in fig:06. In this figure, lefT side ma-

rial is a N-type semiconductor having negative acceptor ions and positive

charged hols.The right side material is P-type semiconductor having positive

donor ions and free electrons.

FIG:06


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If we joined the n and p materials together by one of the processes mentioned earlier,

all the holes and electrons would pair up. On the contrary, this does not happen. Instead

the electrons in the n material diffuse(move or spread out) across the junction into the p

material and fill some of the holes. At the same time, the holes in the p material diffuse

across the junction into the n material and are filled by n material electrons. Thisprocess, called junction recombination reduces the number of free electrons and holes

in the vicinity of the junction. Because there is a depletion, or lack of free electrons and

holes in this area, it is known as the depletion region.

The loss of an electron from the n-type material created a positive ion in the n

material, while the loss of a hole from the p material created a negative ion in that

material. These ions are fixed in place in the crystal lattice structure and cannot move.

Thus, they make up a layer of fixed charges on the two sides of the junction as shown infigure 07 . On the n side of the junction, there is a layer of positively charged ions; on

the p side of the junction, there is a layer of negatively charged ions. An electrostatic

field is established across the junction between the oppositely charged ions. The

diffusion of electrons and holes across the junction will continue until the magnitude of

the electrostatic field is increased to the point where the electrons and holes no longer

have enough energy to overcome it, and are repelled by the negative and positive ions

respectively. At this point equilibrium is established and, for all practical purposes, the

movement of carriers across the junction ceases. For this reason, the electrostatic fieldcreated by the positive and negative ions in the depletion region is called a barrier.


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FIG:07:The PN junction

a) distribution of the charge carriersbefore the diffusion

b) distribution of the charge carriers

after the diffusion of the charge

carriers

c) junction barrier

d) charge distribution in thejunction barrier


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Applying Voltage Across PN JunctionThe potential difference across a PN junction can be applied in two ways namely:Forward bias and Reverse bias.

Forward bias:An external voltage applied to a PN junction is called bias. If, for example,

a battery is used to supply bias to a PN junction and is connected so that

its voltage opposes the junction field , it will reduce the junction barrier and ,

therefore, aid current flow through the junction. This type of bias is knownas forward bias , and it causes the junction to offer only minimum resistanceto the flow of current.

FIG:08:Forward bias


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In this arrangement, electrons in the ptype material near the positive terminal of the

battery break their electronpair bonds and enter the battery, creating new holes. At

the same time, electrons from the negative terminal of the battery enter the ntype

material and diffuse toward the junction. As a result, the space charge region becomes

effectively narrower, and the energy barrier decreases to an insignificant value. Excesselectrons from the ntype material can then penetrate the space charge region, flow

across the junction, and move by way of the holes in the ptype material toward the

positive terminal of the battery. This electron flow continues as long as the external

voltage is applied. Under these conditions, the junction is said to be forwardbiased.

Reverse bias:If the battery mentioned earlier is connected across the junction so that its

voltage aids the junction, it will increase the junction barrier and thereby offer

a high resistance to the current flow through the junction. This type of bias isknown as reverse bias.


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FIG:09:Reverse-biased PN junction.In this arrangement, the free electrons in the ntype material are attracted toward the

positive terminal of the battery and away from the junction. At the same time, holes

from the ptype material are attracted toward, the negative terminal of the battery

and away from the junction. As a result, the spacecharge region at the junction

becomes effectively wider, and the potential gradient increases until it approaches

the potential of the external battery. Current flow is then extremely small because

no voltage difference ( electric field ) exists across either the ptype or the ntype

region. Under these conditions, the pn junction is said to be reversebiased.


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VoltageCurrent Characteristic Of PN Junction

FIG:10:Typical Germanium diode V-I characteristics

Knee voltage(GERMANIUM)


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For the Forward-and reverse-bias region:

ID=IS(eKVD/TK-1)

Where , IS=reveres saturation current.

k=11,600/ with =1 for Ge and =2 for Si for relatively low levels

of diode current and =1 for Ge and Si for higher levels of diodecurrent (in the rapidly increasing section of the curve)

TK=TC+273

FIG:07

Forward-bias region

VD >0V, ID>0mA

Forward-bias region

VD


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Rectifier

Definition : The circuits which convert an ac voltage into dc voltage

is called rectifier.The classification of rectifier is given bellow :

Rectifier

Halfwave rectifier Full-Wave Rectifier

Bridge rectifier Centre- Tap Full-Wave Rectifier


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Half wave rectifierAccording to the Fig: the a.c voltage across the secondary winding AB

changes polarities after every half-cycle. During the positive half-cycle ofinput a.c. voltage end A becomes positive w.r.t. end B . This makes the

diode forward biased and hence it conducts current. During the negative

half-cycle, end A is negative w.r.t. end B . Under this condition, the diode

is reverse biased and it conducts no current. Therefore, current flows

through the diode during positive half-cycle of input a.c. voltage only ; it is

blocked during the negative half-cycle. In this way , current flows throughload R always in the same direction. Hence d.c. output is obtained acrossR.


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Centre- Tap Full-Wave Rectifier

According to the Fig: . during the positive half-cycle of secondary voltage,

the end A of secondary winding becomes positive and end B negative. This makesthe diode D1 forward biased and diode D2 reversed biased. therefore, diode D1 isactive while D2 is not. The conventional current flow is through diode D1, loadresister R and the upper half of secondary winding . During the negative half-cycle ,end A of the secondary winding becomes negative and end B positive. Therefore,diode D2 is active while D1 is not. The conventional current flow through diode D2 ,load R and the lower winding . It may be seen that current in the load R is in thesame direction both half-cycles of input a.c. voltage. Therefore , d.c. is obtained

across the load R.


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Bridge rectifier

According to the Fig: A bridge rectifier requires four diode instead of two , but avoids

the need for s centre-tapped transformer . During the positive half-cycle of thesecondary voltage , diodes D2 and D4 are conducting and diodes D1and D3 are nonconducting . Therefore , current flows through the secondary winding , diode D2 ,load resister R and diode D4. during negative half-cycles of the secondary voltage,diodes D1 and D3 conduct , and the diode D2 and D4 do not conduct . The currenttherefore flows through the secondary winding , diode D1 , load resister R, and diodeD3 . In both cases , the current passes through the load resister in the same direction. Therefore , a fluctuating , unidirectional voltage is developed across the load.


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CLIPERSClippers are network that employ diodes to clip away a portion of an

input signal without distorting remaining part of the applied waveform.Depending on the orientation of the diode , the positive or negative region

the input signal is clipped off.

There are two categories of clippers: series and parallel

The series configuration is defined as one where the diode is in series with

the load, while the parallel variety has the diode in a parallel to the load.


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Clamping networks have a capacitor connected directlyfrom input to output with a resistive element in parallel

with the output signal. The diode is also parallel with the

output signal but may or may not have a series dc supply

as an added element.

If the DC value of a signal needs to be changed, a

capacitor can be charged with the appropriate value.

When connected in series with the signal source, it willthen provide the desired DC level.

CLAMPERS


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For positive values of the input signal, the diode

immediately conducts, allowing the capacitor to be

charged. The RC time constant is small because the

only resistor present is the small internal resistor of the

diode (less than 1 ). For negative values of the input

signal, the diode is reverse-biased, so the capacitor

cannot be discharged, maintaining the potential. Ingeneral, if we use a battery of voltage V , the output

signal will be: Vout = Vp + V + Vp sinwt = Vp(sinwt 1)+ V


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