lecture 1 sharil

8/3/2019 Lecture 1 Sharil.

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Chapter 1INTRODUCTION


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Learning Outcomes

explained the basic structure of atoms

Differentiated the properties of insulators,conductors, and semiconductors

Understand covalent bonding

Identify the properties of both p and ntype materials

Understand both forward and reverse

biasing of a p-n junction

Explained the basic operation of a diode

At the end of this topic, students should be able to:


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Bohr model of an atom

As seen in thismodel, electronscircle the nucleus.Atomic structureof a materialdetermines itsability to conduct

or insulate.


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The ability of a material to conduct current isbased on its atomic structure.

The orbit paths of the electrons surrounding

the nucleus are called shells.

The less complete a shell is filled to capacity themore conductive the material is.

Each shell has a defined number of electrons itwill hold. This is a fact of nature and can bedetermined by the formula, 2n2.

The outer shell is called the valence shell.

Consist of 3 partical: electron (-ve charged),proton (+ve charged), neutron (uncharged)


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Ionization

The process of losing a valenceelectron

Atom loses its valence electronand becomes a positive ion

Escaped valence electron , the

atom becomes a negative ion


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Conductor easily conducts electrical current . Singleelement material, such as copper, silver gold andaluminum

Insulator - does not conduct electrical current

Semiconductor pure (intrinsic) state is neither a goodconductor nor a good insulator, such as silicon,germanium and carbon. Characterized by atoms withfour valence electrons

Conductor, Insulator, and Semiconductor


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The valence shell determines the ability of material toconduct current.

A Copper atom has only 1 electron inits valence ring. This makes it agood conductor. It takes 2n2

electrons or in this case 32 electronsto fill the valence shell.

A Silicon atom has 4 electrons in itsvalence ring. This makes it asemiconductor. It takes 2n2 electrons

or in this case or 18 electrons to fillthe valence shell.

Conductors, Insulators, and Semiconductors


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Covalent Bonding

Covalent bonding is a bonding of two or more atoms by the

interaction of their valence electrons to strengthen and stablingthe atom bonding structure.


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Certain atoms will combine in this way to form a crystal

structure. Silicon and Germanium atoms combine in thisway in their intrinsic or pure state.

Covalent Bonding


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N-type and P-type Semiconductors

Other atoms with 5 electrons suchas Antimony are added to Silicon toincrease the free electrons.

Other atoms with 3 electrons such asBoron are added to Silicon to create adeficiency of electrons or hole charges.

The process of creating N and P type materials is called

doping. Will improve the electrical characteristics of Si & Ge

N-type P-type


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Introduction

Forward bias

Current flows

Reverse Bias

No current flow

The basic function of a diode is to restrict current flow to one

direction. Constructed by joining an n and p type material together.


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The Depletion Region

With the formation of the p

and n materials combinationof electrons and holes at thejunction takes place.

This creates the depletion

region and has a barrierpotential. This potentialcannot be measured with avoltmeter but it will cause asmall voltage drop.

p region n region p region n region


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Forward and Reverse Bias

Voltage source or bias connections are+ to the p material and to the nmaterial

Bias must be greater than .3 V forGermanium or .7 V for Silicon diodes.

The depletion region narrows.

Voltage source or bias connections areto the p material and + to the n material

Bias must be less than the break downvoltage.

Current flow is negligible in most cases.

The depletion region widens.

Forward Bias Reverse Bias


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Forward Bias Measurements With SmallVoltage Applied

In this case withthe voltage

applied is lessthan the barrierpotential so thediode for allpractical purposes

is still in a non-conducting state.Current is verysmall.


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Forward Bias Measurements With AppliedVoltage Greater Than the Barrier Voltage.

With the appliedvoltageexceeding thebarrier potentialthe now fullyforward biaseddiode conducts.

Note that theonly practical lossis the .7 Voltsdropped acrossthe diode.


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Ideal Diode Characteristic Curve

In thischaracteristiccurve we do notconsider thevoltage drop orthe resistiveproperties. Currentflow proportionally

increases withvoltage.


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Practical Diode Characteristic Curve

In most cases weconsider only theforward bias voltagedrop of a diode. Once

this voltage isovercome the currentincreasesproportionally withvoltage.This drop isparticularly importantto consider in lowvoltage applications.


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Complex Characteristic Curve of a Diode

The voltage dropis not the onlyloss of a diode.In some caseswe must takeinto accountother factorssuch as the

resistive effectsas well asreversebreakdown.


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Troubleshooting DiodesTesting a diode is quite simple, particularly if the multimeter

used has a diode check function. With the diode check functiona specific known voltage is applied from the meter across thediode.

K A A K

With the diode check

function a good diode willshow approximately .7 V or.3 V when forward biased.

When checking in reversebias the full applied testingvoltage will be seen on thedisplay.


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Troubleshooting Diodes

An ohmmeter can be used to check theforward and reverse resistance of a diode if

the ohmmeter has enough voltage to force thediode into conduction. Of course, in forwardbiased connection low resistance will be seenand in reverse biased connection highresistance will be seen.


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Troubleshooting Diodes

Open Diode

In the case of an open diodeno current flows in eitherdirection which is indicated by the full checking voltagewith the diode check function or high resistance using an

ohmmeter in both forward and reverse connections.

Shorted Diode

In the case of a shorted diodemaximum current flowsindicated by a 0 V with the diode check function or lowresistance with an ohmmeter in both forward and reverseconnections.


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Diode Packages

Diodes come in a variety of sizes and shapes. Thedesign and structure is determined by what type ofcircuit they will be used in.


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Summary

P-materials are doped with trivalent impurities

N-materials are doped with pentavalent impurities

P and N type materials are joined together to form aPN junction.

A diode is nothing more than a PN junction.

At the junction a depletion region is formed. Thiscreates barrier which requires approximately .3 V for a

Germanium and .7 V for Silicon for conduction to takeplace.

Diodes, transistors, and integrated circuits areall made of semiconductor material.


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Summary

When reversed biased a diode can only withstandso much applied voltage. The voltage at which

avalanche current occurs is called reverse breakdownvoltage.

There are three ways of analyzing a diode. Theseare ideal, practical, and complex. Typically we use a

practical diode model.

A diode conducts when forward biased and does notconduct when reverse biased

lecture 1 sharil

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