Lecture 2

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Types of MaterialTypes of Material

CHAPTER 2CHAPTER 2::

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2.1 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 - -ve1.1.1 Bohr Model

Cont…

Electrons orbit the nucleus of an atom at certain distance from the nucleus

Each orbit correspond to a certain energy level Electrons near the nucleus has less energy than

those in more distant orbits The orbits are grouped into energy bands known

as shells An atom has a fixed number of shells Each shell has a fixed maximum number of

electrons Energy differences within shells are much

smaller than the energy differences between shells

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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

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Atomic StructureAtomic Structure

Cont…

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2.1.2 Atomic structures

The Periodic Table

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The Atomic Structure

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

Cont…

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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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2.2 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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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…

Energy Levels

Valence Band

2nd Band1st BandNucleus

Energy Gaps (No electrons)

Energy Level Outermost shell

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INSULATORCONDUCTOR

SEMICONDUCTOR

The energy band gap for conductor, insulator and semiconductor

Cont…

Energy Gap

Many materials have a significant energy separation between the valence electron energy levels and the conduction electron energy levels

Unless a valence electron can get significantly more energy in some way, it stays in the lower valence energy band

A material with all its electrons in the valence band is not a good electric conductor (no moveable conduction electrons)

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2.3 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.

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.

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

Cont…

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

Two Important material Categories

Solid materials fall into two important categories:

1. Those with an electron energy gap Insulators (both electrical and thermal, in general) Semiconductors are a sub-class of Insulators, as we

will see2. Those with no energy gap

Conductors (both electrical and thermal conductors) Note: there are a few peculiar non-metal

materials (for example, Beryllium Oxide) which are moderately good thermal conductors and yet are electrical insulators.

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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.

Conductors

Best Metal Conductors (in order)

Silver: resistivity 16 n•m (nano-Ohm-meters) too costly for most applications. Sometimes used as a surface

plating over copper or brass for certain purposes (electrical or decorative)

Copper: resistivity 17 n•m widely used in pure or alloy form (Brass, etc.); forms a surface oxide

which is a relatively low resistance semiconductor Gold: resistivity 24 n•m

not the best conductor, but it does not form surface oxides or otherwise corrode, so it is often used as a protective metal surface plating on copper or brass for connectors, etc.

Aluminum: resistivity 28 n•m inexpensive and lighter than copper, but forms a surface oxide

which is a high resistance (insulator). Bad mechanical joints in aluminum wire (from loose holding screws, etc.) permit oxidation, local heating, and in some cases this heat initiates fires in nearby combustible materials.

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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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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.. Semiconductor 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).

Why Distinguish Insulators from Semiconductors?

When we examine the room temperature specific resistivity* of many materials, we find:all metals have relatively low resistivity, and many insulators (glass, sulphur, most plastics, etc.) have very high resistivity (many millions of times bigger than the resistivity of metals)Some materials appear to have resistivity somewhat larger than the metals, but much lower than the standard insulators at room temperature, we call these materials (silicon, germanium, etc.) semi-conductors* Resistivity is measured in ohm•meters, and is the resistance measured between two opposite faces of a 1 meter cube sample. For practical purposes, the ohm•centimeter unit is often used also.

Ionization

Atom can absorb energy from heat or light sources, hence, when an electron gains energy it moves to higher orbit

Electrons in the valence band can easily jump to higher orbits because they have more energy and are loosely bound to the nucleus

A valence electron can gain sufficient energy and be removed completely from the influence of the atom

The process of losing an electron is known as Ionization

The atom becomes positively charged and it becomes a positive ion designated H+

The escaped valence electron is called Free Electron

Covalent Bonds

When atoms combine to form a solid material they arrange themselves in a fixed pattern called a Crystal

Atoms within crystal structure are held together by Covalent bonds

The bonds are created from interaction of the valence electrons of the adjacent atoms

Silicon (Si) and Germanium (Ge) are semiconductor materials used to manufacture electronic devices

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SemiconductorsSemiconductors

CHAPTER 3CHAPTER 3::

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3.1 Types of Semiconductor Semiconductors are mainly classified into two categories: i. i. Intrinsic ii. ii. Extrinsic Intrinsic - - chemically very pure and possesses poor

conductivity.- It has equal numbers of negative carriers (electrons) and

positive carriers (holes). - Impurities do not affect its electrical behavior. . 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

Pure (Intrinsic) Si Crystal

Si has 4 valence electrons

SiSi

SiSi

Si

Each Si atom shares one valence electron with each of its four closest neighbors so that its valence band will have a full 8 electrons.

Covalent bonds hold the atoms together

Each shared electron is attracted equally by two adjacent atoms

SiSi+4+4

SiSi+4+4

SiSi+4+4

Si Si +4+4

SiSi+4+4

SiSi+4+4

SiSi+4+4

SiSi+4+4

SiSi+4+4

Conduction in Si Crystal

Pure Silicon crystal at room temperature absorbs heat energy from surrounding air

Some valence electrons will gain enough energy and become free electrons

The valence electrons will jump into conduction band and become conduction electrons

For each electron raised to conduction band a Hole is left in the valence band

Conduction in Si Crystal

Si

Si Free Electron

Hole

Heat energy

Valence Band

2nd Band1st BandNucleus

Conduction Band

Energy

N-type and P-type Semiconductors

Semiconductor materials do not conduct well and they are of little value in their intrinsic (pure) state

Pure silicon (or germanium) must be modified to increase the free electrons and holes to increase its conductivity and make it useful in electronic devices

This is done by adding impurities to the semiconductor material, a process known as DOPING

Doping produces n-type semiconductor or p-type semiconductor depending on the type of impurity used

N-type semiconductor has increased number of conduction electrons while P-type semiconductor has increased number of holes

N-Type semiconductor material

N-Type Material: To increase the number of

electrons pentavalent (Group V) atoms are added

Pentavalent atom has five valence electrons, the most commonly used dopants are arsenic, antimony and phosphorus.

Four of the pentavalent atom valence electrons are used to form the covalent bond with four adjacent silicon atoms, the extra electron becomes conduction electron

+4+4+4+4

+5+5

+4+4

+4+4+4+4+4+4

+4+4+4+4

N-Type semiconductor material

Antimony (Sb) Valence Electrons

SiSi

SiSi

Sb

Free Electron (Conduction)

Conduction in N-type Semiconductor

The number of conduction electrons due to doping can be controlled by the number of impurity atoms added to silicon

Doping process does not leave a hole in the valence band

If the N-type semiconductor is heated, electrons in the valence band will gain energy and move to conduction band and leave holes in the valence band

Conduction in N-type Semiconductor

Current is defined as flow electrons

Conduction Band

Valence Band

Electrons from Doping

Thermally generated Electrons

Thermally generated Holes

Conduction in N-type Semiconductor

There is conduction due the free electrons in the conduction band known as Majority Carriers

Electrons in the valance band can move and occupy the available few holes. Therefore holes in the valance band contribute in the conduction hence known as Minority Carries

Hence, for N-type semiconductor Electrons are the Majority carriers and Holes are the Minority carries

P-Type semiconductor material

P -Type Material: To increase the number of electrons trivalent (Group III) atoms are added

Trivalent atom has three valence electrons, the most commonly used dopants are dopants are aluminum, boron, and gallium.

All three valence electrons are used to form the covalent bond and since four electrons are required, a hole is formed with each trivalent atom

+4+4+4+4

+3+3

+4+4

+4+4+4+4+4+4

+4+4+4+4

P-Type semiconductor material

Boron (B) Valence Electrons

SiSi

SiSi

B Hole

Conduction in P-type Semiconductor

The number of holes due to doping can be controlled by the number of impurity atoms added to silicon

A hole created by doping is not accompanied by a conduction (free) electron

If the P-type semiconductor is heated, electrons in the valence band will gain energy and move to conduction band and leave holes in the valence band

Conduction in P-type Semiconductor

Conduction Band

Valence Band

Holes from Doping

Thermally generated Electrons

Thermally generated Holes

For P-type semiconductor Holes are the Majority carriers and Electrons are the Minority carries

The PN Junction (steady state)

__

PP nn

- - - - - -- - - - - -- - - - - -- - - - - -- - - - - -- - - - - -- - - - - -- - - - - -- - - - - -- - - - - -

+ + + + + + + + + + ++

+ + + + + + + + + + ++

+ + + + + + + + + + ++

+ + + + + + + + + + ++

+ + + + + + + + + + ++

NaNa NdNdMetallurgical JunctionMetallurgical Junction

Space Charge Space Charge RegionRegionionized ionized

acceptorsacceptorsionized ionized donorsdonors

E-FieldE-Field

++++ __

h+ drifth+ drift h+ diffusionh+ diffusion e- diffusione- diffusion e- drifte- drift== ==

The PN Junction Diode

The basic use of P-type and N-type semiconductor materials is in diodes

The PN junction is made of two regions

P-type silicon

N-type silicon

P-type N-type

pn-junction

Thermal HolesThermal ElectronsDoping Electrons

Doping Holes

The PN Junction Diode

The n-region has free electrons (majority carriers) and few thermally generated holes (minority carriers)

The p-region has many holes (majority carries) and few thermally generated electrons (minority carriers)

The junction diode is a continuous crystal, free electrons can move across the junction and fill the holes

For each electron that crosses the junction and recombines with the hole, a pentavalent atom is left with a net of positive charge (Positive Ion)

Also when an electron combines with a hole is the p-region, a trivalent atom acquires a net negative charge (Negative Ion)

The PN Junction (steady state)

Steady StateWhen no external source is connected to the pn junction, diffusion and drift balance each other out for both the holes and electrons

Space Charge Region: Also called the depletion region. This region includes the net positively and negatively charged regions. The space charge region does not have any free carriers. The width of the space charge region is denoted by W in pn junction formula’s.Metallurgical Junction: The interface where the p- and n-type materials meet.Na & Nd: Represent the amount of negative and positive doping in number of carriers per centimeter cubed. Usually in the range of 1015 to 1020.

PP nn

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

+ + + + ++ + + + ++ + + + ++ + + + ++ + + + + + + + + + + + + + ++ + + + +

NaNa NdNdMetallurgical Metallurgical JunctionJunction

Space Charge Space Charge RegionRegionionized ionized

acceptorsacceptorsionized ionized donorsdonors

E-FieldE-Field

++++ __ __

h+ drifth+ drift h+ diffusionh+ diffusion e- diffusione- diffusion e- drifte- drift== ==== ==

Depletion Layers

The width of these two layers increases until they reach an equilibrium condition in which just enough electrons are on the left side to repel any more electrons spilling over.

Because this is a semiconductor instead of a good conductor, these layers* stay in place at the two sides of the interface. (In a metal, the extra electrons would move quickly away from the interface and go all over the surface of piece of metal.)

This double layer of two opposite net electric charges (+ and -) is also called a “dipole” layer or “sandwich”

*Called depletion layers, although only one of them is actually “depleted” below the normal number of electrons.

The PN Junction Diode

Depletion Layer

Depletion region has no carriers therefore it is an insulator

Ions

VBp n

The PN Junction Diode

The ions form a charge that prevents more electrons from crossing the junction

The positive and negative ions on opposite sides of the junction creates a Barrier Voltage (VB) across the depletion layer

At 25OC Barrier voltage is: ≈ 0.7 V for Silicon ≈ 0.3 V for Germanium