Electronics the First and Second Lectures

Second weekهـ 1436/ 11/ 15 - 18

السلمي / سمر أ

Outline for today Grades

Office Hours

Curriculum

Syllabus

References

Chapter One

Electronics Concepts

Atomic Structure

Explain Energy Band Theory

Composition , Structure and Lattices , Chemical Bonds of Semiconductors

Charge Carriers and Conduction Mechanisms in Semiconductors

Attendance and participation

Lab

Homework ( from 3 to 5)

Periodic Exam ( 2 exams )

Final Exam:

Grades

5%

20%

15%

20%

40%--------------------

100%

Wednesday from 2 to 3, the other time is not specified yet

you can put any paper or homework in my mailbox in Faculty of Physics

Department

I will sent any announcement or apology by email, so please check your

university email ( go to Umm Al Qura University first, to students email. Second,

writ your university number as (s43106110@st.uqu.edu.sa example ) then your

Password

Time of Periodic Exams The first periodic exam in / 1 / 1437 - 1312هـ

The second periodic exam in 10-11 / 2 / 1437 هـ

Please everyone attend In her group

Office Hours

Conduction Mechanisms in Semiconductors

Junction Diode Physical Electronics

Bipolar Junction Transistors

Field Effect Transistors

Operational Amplifiers

Digital Electronics

Syllabus

Electronic Principles Physics , Models and Circuits by Geray and Searle

Textbook Of Electronic Devices By Floyd (4th Edition)

Electronic devices and circuits theory by Boylestad and Nashelstky.

Microelectronics by millman and Grabel,

Electronic devices, Discrete and integrated by Fleeman

/ شاووش عبدالله د ترجمة ودوائر ونماذج فيزياء االلكترونية المبادئ

جمال د و مصطفى يسرى د وترجمة فلويد توماس تأليف اإللكترونية األجهزة

الفردغ

– األول الجزء المعـــاصرة الشبــول : االلكترونيات أحمــد ياسين. قواسمة : مؤيــد الموصالت أشباه  االلكترونيات ابراهيم: مبادئ المجيد عبد معن

http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html

http://ar.wikipedia.org

References

Conduction Mechanisms in Semiconductors

Electronics Concepts

Electronics is the science of how to control electric energy, energy in which the

electrons have a fundamental role. Electronics deals with electrical circuits that

involve active electrical components such as vacuum tubes, transistors, diodes and

integrated circuits.

In the first Chapter, we will study conduction mechanisms in semiconductors,

which is one of the main material types as division in terms of different

conduction

Atomic Structure

An atom is the smallest particle of an element that retains the characteristics of

that element. Each of the known 109 elements has atoms that are different from

the atoms of all other elements. This gives each element a unique atomic

structure. According to classical Bohr model, atoms have a planetary type of

structure that consists of a central nucleus

surrounded by orbiting electrons, as in figure.

The electrons has negative charge. The nucleus

consists of positively charged called protons

and uncharged particles called neutrons.

Every atom has atomic number, which is the same

number of electrons and protons. The atomic

number for any element is different from other elements .

Atomic Structure

Each discrete distance (orbit) from the nucleus correspond to a certain energy

level. These orbits are grouped into energy bands known as shells. Each sell has

maximum number of electrons at permissible energy levels(orbits), as shown in

the left column of table. The differences in energy levels within a shell are much

smaller than the difference in energy between shells. The shells are designated 1,

2, 3 and so on, as its distance from the nucleus. In addition, some references

designate shells by the letters K, L , M, as shown in the table and figure.

Atomic Structure

Valence electrons are electrons that are in orbits farther from the nucleus have

higher energy and less tightly bound to the atom. Their shell is not complete their

electrons (expect 8th group in Periodic Table) as shown in figure for Si atom,

which consists 4 valence electrons

and its electron configuration

so electrons and shells divided in to

inter and outer (or valence).These valence

electrons contribute to chemical reactions

and bonding within the structure of

a material and determine its electrical

properties. As in the Periodic Table

Conduction Mechanisms in Semiconductors

Energy Band Theory

when we study material types, we know that the main material as division in

terms of different conduction are three types which are: Conductors, Insulators,

and Semiconductors.

how we compare between those materials? What are their properties ?

of course, you study some of those properties over the previous years in different

courses as: Electricity physics, Modern physics, and Solid State Physics 1

in Solid State Physics 1, you study Energy Band Theory. We will begin from

this theory as a review.

The electron configuration for Oxygen atom O8 then to two atoms

Source: Dr. Abdul Aziz  Kutub

What about the electron configuration for N atoms ( thus, according to the

number of atoms, the number of energy levels is consisted )

Source: Dr. Abdul Aziz  Kutub

The electron configuration for N atoms (those energy levels consists energy band)

Source: Dr. Abdul Aziz  Kutub

Explain Energy Band Theory

At crystal solid state consists energy levels (as of the case of isolated atoms ) .

But when there are N atoms consists energy bands( which are infinitesimal

numbers of energy levels that are near from each others , so it is difficult to

designated between them.

every level represents energy level occupied with electrons.

in the next example in next slide, we will focus at outer shell or valence levels.

Here the bonding orbital band is consisted in down part of valence levels and the

bonding antiorbital band is consisted in upper part of its. Between those two

bands is the energy gap, which consists Fermi level.

also we can notice that valence band is the top band from the bonding orbital

band ( which is filled with electrons) and conduction band is the bottom band

from the antibonding orbital band ( which is different according to material types

which either empty of or partially filled with electrons).

Another example Si, its electron configuration is

We will focus at valence shells only (from here materials classifies

conductors, insulators, and semiconductors according to Energy Band Theory)

Energy levels at outer orbitals :

The red box in the last slide

bonding orbital band

bonding antiorbital

band

Conduction and valence bands in solid:

Comparison of three materials

From last slide, we notes in terms of energy band theory and electrical

conductivity that :

Conductors Insulators Semiconductors

the energy gap is overlap that

means the part of conduction band

inside valence band . Because of

that, we find conduction electrons

in conduction band. Thus, it is a

good electronic conductor. There

are some conductors have very

narrow energy gap and we find

conduction electron in conduction

band.

There is a wide gap

between the conduction

band and valence band .

This gap is forbidden area

for the existence of

electrons. Thus, the

conduction band is empty

of electrons. This is what

makes it insulated on the

electrical conduction

the gap between the

conduction band and

valence band is narrower;

but at small degrees of

heat, electrons move across

the gap to the conduction

band and become

conductive material but at a

temperature of absolute

zero, its behave as insulator

Comparison of three materials Comparison Semiconductors Insulators Conductors

Example Germanium . Silicon. Lead sulfide PdS. Cadmium sulfide CdS

Glass. Quartz. Porcelain. Alaunit. Amber. Ceramic

Metals (silver. Copper. Iron. Lead ..

bonds Covalent or mixed bonds Ionic bond Metal bond

Resistivity Medium at normal temperature in the range of from 10-5 Ω.m to 106 Ω.m

very large at room temperature in the range of 106 Ω.m to 1016 Ω.m

From 10-5 Ω.m to 10-

8 Ω.m

Valence band Filled with electrons Filled with electrons Filled with electrons

Connection band

Completely empty of electrons in the degree of absolute zero, but it contains

a number of electrons in the normal temperature or greater than absolute

zero

Empty of free electrons at normal temperatures

Partially filled with electrons at normal

temperatures

Energy gap Medium from 0.7 eV to 2 eV very large 5 eV Very small 0.01 eV

The effect of raising the temperature on the resistivity and

conductivity

Resistance decreases dramatically. it has a moderate connectivity between metals and insulators. conductivity

increases with raising the temperature

Resistance decreases but remains so large that the material fused before it becomes conductive

Increasing resistance and conductivity decreases with

increasing temperature

elements: from the 4th group in

Periodic Table (IV) as Si and Ge

binary compounds : from the 3th and 5th

group (III- V) as GaAs

:also from the 2th and 6th group (II- VI) as

ZnS , 2th column here is Zn and Cd.

Composition of Semiconductors

From those three materials, we will focus at studying about semiconductor

because it is the main elementary in making vacuum tubes, transistors, ..etc.

Thus, semiconductor is the main forming in electronic science.

in this course, we will focus at most important structures

for semiconductors.

diamond structure : it has a cube shape. in which has eight

atoms at corners (eighth ), six atoms at center face of the

cube (half), and four atoms inside the cube (complete). It

almost as two face-centered lattices (fcc) direction (111).

The elements semiconductors from the 4th group take this

structure

Zincblende structure : it is similar to diamond lattice. The

different is arranged of atoms, in a why one element in one of

fcc lattice and the other element in the other fcc lattice) .

Compounds semiconductors from the III-V , IV-IV , and

some of II-VI take this structure.

diamond structure

zinc blende structure

Structure and Lattices of Semiconductors

As we mention at the comparison that semiconductors

consist covalent or mixed bonds

Covalent bond : its for elements the 4th group as Si and

Ge . it is exhibited by the diamond structure. In these

crystal, each atom shares its valence electrons with its four

neighbors.

Mixed bond : its for compounds as III-V and II-VI such

as GaAs and ZnS. it is exhibited by the Zincblende

structure. The mixed bond consist of covalent bond and

ionic bond). In which the ionic bond is between the two

ions in compounds , however covalent bond is between

one compound and the others.

Covalent bond in Si

mixed bond in GaAs

Chemical Bonds of Semiconductors

Positive Ion

negative Ion

In conductors : to understand the conduction mechanism in conductors, we can image that metal atoms immersed in sea of electrons. This electrons can move as group under the effect of electric energy.

For insulators: there is no conduction electrons in insulators because of the lack of electrons. So there are no swimming electrons in model of ionic compound ( as NaCl) ; also, because of strong of ionic bond in insulators.

Charge Carriers and Conduction Mechanisms in Semiconductors

Charge Carriers and Conduction Mechanisms in Semiconductors

In semiconductors :

there is no conduction electrons in semiconductor at normal condition at

absolute zero temperature similar to insulators because there are no swimming

electrons in model . As in covalent bond for Silicon, all atoms share with four

valence electrons to form the bond.

however, this situation changes ,and there

will be conduction electrons in semiconductor

according to two main factors:

rising temperature higher than absolute zero

adding impurity to intrinsic semiconductors

At rising temperature higher than absolute zero in semiconductors

We know that covalent bond is a weak bond. Thus, when the temperature raised,

the bond breaks. As a result, a electron generates. This electron is the charge

carrier in semiconductors similar to conductors . However, this electron leaves

behind a hole .This hole has a positive charge, and it moves opposite direction

than electron. Thus, by increasing rising temperatures, more electron – hole pair

generate. This process happens only in semiconductors, not in conductors.

Charge Carriers and Conduction Mechanisms in Semiconductors

At rising temperature higher than absolute zero in semiconductors:

The last explanation was about the models and chemical bonds . We can also

explain by energy bands. Thus, when the temperature raised, a electron receive

enough thermal, energy to excited from filled valence band to empty connection

band leaving behind a hole (another charge carrier). Thus, we will notice electron

– hole pair (EHP) appearance as we explain in last slide. Those charge carriers

are effect factors to conduction electron in semiconductors.

The different between electron and hole

We study about electrons but what about holes?

Electrons flow from minus to plus, however, holes flow

from plus to minus. Therefore, they are equal in

magnitude and opposite in the direction. Because of that, electron has negative

charge and hole has positive charge . Mobile hole has the same direction of

current and electric field

Note (hole movement is virtual but electron movement is real)

holes electrons

positive charge negative charge

Energy increases going down

Energy increases going up

The same direction of

electric field

The opposite direction of electric field

At adding impurity to intrinsic semiconductors

Another method to get charges carriers in semiconductors instead of giving

excitation energy (thermal energy) is by adding impurity or doping. (what is the

different between them?)

impurity is adding different atoms from the original atoms , and happened

automatically in natural. However, doping is similar but we add intentional

impurities in order to change its electronic properties as in increasing electron or

hole numbers.

in the case of elemental semiconductors as Si, impurities either be from 3ed

group (such as B) which called acceptor meaning it gives extra hole and accept

extra electron, or from 5th group (such as P) which called donor meaning it gives

extra electron

At adding impurity to intrinsic semiconductors

Also, acceptor is called extrinsic semiconductor p-type

which has higher holes concentration than electrons

concentration , and Fermi level is near valence band.

Donor is called extrinsic semiconductor n-type which

has higher electrons concentration than holes concentration

, and Fermi level is near conduction band.

The extra electron or hole

associates with positive or

negative ion by a weak bond.

Thus, it can swim freely in

crystal in case presence

thermal energy.

At adding impurity to intrinsic semiconductors

In case of no presence thermal energy in both n-

type and p-type, the presence of impurities contribute

in moving Fermi level near to two bands. As a result,

possibility of electron or hole existence in it.

In the presence of a small thermal energy in

extrinsic semiconductor p-type or n-type has effect in

transport electron from valence band to connection

band

Therefore, the presence of both impurities and

thermal energy together effect more conduction

mechanisms in semiconductors than the presence of

one of them.

At adding impurity to intrinsic semiconductors

the figure below illustrates in details the role of acceptor and donor levels in case

of no presence thermal energy both n-type (above) and p-type (below), and the

case of presence of small thermal energy. In room temperature , we obtains

complete ionization acceptor and donor atoms.

At adding impurity to intrinsic semiconductors

We studied in the case of elemental semiconductors such as Si and Ge

impurities either

be from 3ed group which called acceptor and extrinsic semiconductor p-type

or from 5th group which called donor and extrinsic semiconductor n-type

As in the case of binary compound semiconductors such as GaAs and ZnS

which consist mixed bond (ionic bond association between two elements of one

compound and the covalent bond between compounds)

in the case of (III- V) compound such as GaAs impurities either

be from 2nd group (such as Mg) which called acceptor and extrinsic

semiconductor p-type if it substitutes for column III (Ga) of GaAs.

or from 6th group (such as S) which called donor and extrinsic semiconductor

n-type if it substitutes for column V (As) of GaAs.

At adding impurity to intrinsic semiconductors

in the case of (III- V) compound such as

GaAs impurities either

be from 2nd group (such as Mg) which

called acceptor and extrinsic

semiconductor p-type if it substitutes for

column III (Ga) of GaAs.

or from 5th group (such as S) which

called donor and extrinsic semiconductor

n-type if it substitutes for column V (As)

of GaAs.

S

Mg

Charge conservation law in intrinsic and extrinsic semiconductors

Drift, diffusion, recombination and generation in semiconductors

Next class review

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University first, to students email. Second, writ your university

number as (s43106110@st.uqu.edu.sa example ) then your Password

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