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ELECTRONIC PRINCIPLES ELECTRONIC PRINCIPLES (BEE 2113) (BEE 2113)

By:By:NABIAH BT ZINALNABIAH BT ZINAL

Department of Electronic Engineering Department of Electronic Engineering Faculty of Electrical and Electronic EngineeringFaculty of Electrical and Electronic Engineering

Kolej Universiti Teknologi Tun Hussein OnnKolej Universiti Teknologi Tun Hussein Onn

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CHAPTER 1CHAPTER 1: : SEMICONDUCTOR SEMICONDUCTOR

PN JUNCTION PN JUNCTION THEORYTHEORY

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Lecture ContentsLecture ContentsAtomic theory and energy bandAtomic theory and energy band

Insulator and semiconductor materialsInsulator and semiconductor materials

Formation of PN junctionFormation of PN junction

Distribution of holes and electronsDistribution of holes and electrons

Forward-biased and reversed-biased PNForward-biased and reversed-biased PN

junctionjunctionIV characteristics of forward-biased IV characteristics of forward-biased and reversed-biased PN junctionand reversed-biased PN junction

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1.1 Atomic Theory The atom has 3 basic particles:i. Proton

• positive charge• Same magnitude but different pole with

electronii. Electron

• negative charge• Same magnitude but different pole with hole

iii. Neutron• neutral

Protons and neutrons form the nucleus Electrons appear in fixed orbits around the nucleus.

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Cont…• For each atom;

No. of proton in nucleus = no. of electron

ATOM IS NEUTRAL• If an atom losses 1 valence electron - +ve• If an atom gains 1 valence electron - -ve

1.1.1 Bohr Model

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Cont…• The orbital paths or shells are identified using K through M.• The innermost shell- K shell.• The outermost atom- valence shell.• Valence shell – determines the conductivity of atom.• The conductivity of atom depends on the number of electron in valence shell (valence electrons).

Orbital shells

K L M

The orbital shells for an atom

7 Atomic StructureAtomic Structure

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1.1.2 Atomic structures

The Periodic Table

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The Atomic Structure

Cont…

Element in periodic table are arranged according to atomic number.

The atomic number of an element = the number of protons (which also equals the number of electrons) in the nucleus of a neutral atom.

Atomic number, often represented by the symbol Z.

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Cont.. Shells are divided into sub shells :

i. s – max 2 electrons

ii. p – max 6 electrons

iii. d – max 10 electrons

iv. f – max 14 electrons

Example:

The structure for nickel atom

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1.2 Energy Band• Electron energy level in valence shell is

changing depend on the atomic force.• Electron energy level always stated as energy

band.• In any material, there are 2 energy band;

i. Valence band – the outermost shell that determines the conductivity

ii. Energy band – the band outside the valence shell.

• The 2 bands are separated by one energy gap called – forbidden gap.

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Example:

Energy band in Silicon Atom

• The valence band contains with electrons.• The electrons can move to the conduction band if it have enough energy ( eg: light or heat)• When the electron absorbs enough energy to jump from valence band to the conduction band, the electron is said to be in excited state.

Cont…

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INSULATORCONDUCTOR

SEMICONDUCTOR

The energy band gap for conductor, insulator and semiconductor

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1.3 Insulator, Semiconductor and

Conductor

Energy Diagram for Three Types of Material

The concept of energy bands is particularly important in classifying materials as conductors, semiconductors, and insulators

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Cont…• Insulator - very wide energy gap. The wider

this gap, the greater the amount of energy required to move the electron from the valence band to the conduction band.

• Therefore, an insulator requires a large amount of energy to obtain a small amount of current.

• The insulator "insulates" because of the wide forbidden band or energy gap.

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Cont…• Semiconductor - has a smaller forbidden band

and requires less energy to move an electron from the valence band to the conduction band.

• Therefore, for a certain amount of applied voltage, more current will flow in the semiconductor than in the insulator.

• Conductor - no forbidden band or energy gap and the valence and conduction bands overlap.

• With no energy gap, it takes a small amount of energy to move electrons into the conduction band; consequently, conductors pass electrons very easily.

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Cont..• The valence shell determines the ability of material to

conduct current.• The number of valence electron in valence shell:

1 e – perfect conductor ( < 4e)

(Easy to drift or move to other atom)

8 e – insulator

4 e – semiconductor

Note: conductivity decreases with an increase in the number of valence electrons

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1.3.1 Conductor• Most of the conductors used in electronics

are metals like copper, aluminum and steel. • Conductors are materials that obey Ohm's

law and have very low resistance. • They can also carry electric currents from

place to place without dissipating a lot of power.

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1.3.2 Insulator• i.e: glass, most polymers (plastics), rubber

and wood.• Materials which will refuse to carry an electric

current.• Useful for jobs like coating electric wires to

prevent them from 'shorting together' or giving a shock.

• Silk and cotton are also good insulators (when they're dry!!)

• Modern insulators like PVC (Polyvinylchloride) are much better and safer.

• Insulators are also very useful to fill the 'gap' in between the metal plates of a capacitor.

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1.3.3 Semiconductor• Special class of elements having a conductivity

between that of a good conductor (like cooper) and that of an insulator (like plastic).

• Most of the transistors, diodes, integrated circuits, etc. used in modern electronics are built using a range of semiconductors.

• The basic property of a semiconductor is given away by its name - it 'conducts a little bit'.

• A semiconductor will carry electric current, but not as easily as a normal conductor.

• The semiconductor atoms complete their valence shells by sharing valence electrons with other atoms – covalent bonding.

• For low temperature, semiconductor material will act as an insulator.

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Cont…• In room temperature, the stability of atom is

threatened. Some of the electrons free from its bonding and jump to forbidden gap.

• When the temperature increases, more valence electrons (free electron) jump to conduction band and increase the conductivity.

• When the covalent bonding break, the hole is created by free electrons in valence bands.

• The thermal energy (heat) causes the constant creation of electron – hole pairs.

• Recombination occurs when the free electrons loss their energy and fall down to valence band (fill the hole).

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1.4 Types of Semiconductor• Semiconductors are mainly classified into two

categories:

i. Intrinsici. Intrinsic

ii. Extrinsicii. Extrinsic

IntrinsicIntrinsic - - chemically very pure and possesses chemically very pure and possesses poor conductivity.poor conductivity.- It has equal numbers of negative It has equal numbers of negative carriers (electrons) and positive carriers (electrons) and positive carriers (holes).carriers (holes). - Impurities do not affect its electrical Impurities do not affect its electrical behavior. behavior.

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Cont…Extrinsic - improved intrinsic semiconductor with

a small amount of impurities added by a process, known as doping process, which alters the electrical properties of the semiconductor and improves its conductivity.

- Introducing impurities into the semiconductor materials (doping process) can control their conductivity.

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1.4.1 Intrinsic Semiconductoro The pure semiconductor material without

impurities atoms.o Example: Silicon and Germanium.

The Silicon bonding

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1.4.2 Extrinsic Semiconductor

• Adding impurities atom into intrinsic semiconductor = extrinsic semiconductor.

• The process of adding specific types of atoms to a semiconductor to favorably alter electric characteristics - Doping

• 2 types of extrinsic (impure) semiconductor;– N-type– P-type

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Cont…

• When an impurity increases the number of free electrons, the doped semiconductor is NEGATIVE or N-TYPE.

• An impurity that reduces the number of free electrons, causing more holes, creates a POSITIVE or P-TYPE semiconductor.

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N– type materialN– type material

Antimony (Sb) impurity in n-type material

- Diffused impurities with 5 valence electrons are called donor atoms.

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P-type materialP-type material

Boron (B) impurity in p-type material

-The diffused impurities with 3 valence electrons are called acceptor atoms.

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1.5 PN Junction Formation• A PN junction is fabricated from a single slice of

semiconductor.• One side doped with acceptor impurity atoms – p region • One side doped with donor impurity atoms –

n region• The interface separating the n and p regions is referred

as the metallurgical junction.

The PN junction

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Majority and minority carriersMajority and minority carriers

a) n-type material b) p-type material

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Diffusion Process

The movement of holes and electrons in diffusion process.

• In trying to neutralize charges; - free electrons in n-type diffuse

across junction to p-type - free holes in p-type diffuse to n-type - electrons & holes close to junction recombine.

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Depletion regionE-field force on holes

E-field force on electrons

E-field

P N

A depletion region formation due to electrons and holes movement in diffusion process and electric field.

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Forward biased narrows the depletion region and produces a voltage drop across the PN junction equal to the barrier potential.

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Reverse biased condition in PN junction.

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The IV characteristics in forward biased and reverse biased.