Lecture1 BE Semiconducting Materials

8/2/2019 Lecture1 BE Semiconducting Materials

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Agenda

• Horario de Clase: Lunes 6:30pm – 9:30pm

• Cursos pasados (Background)

• El Programa y las Reglas de Juego.

• La Importancia del Inglés en este curso y en la

vida misma

05/03/2012 Basic Electronics 1

A Lumped Element

-Quantum Mechanics-Statistical Mechanics

-Solid State

-

-…

01/03/2012 Fundamentals of Electronics - Lecture 1 2

The behavior at the terminals is more important than the details of the behavior

Internal to the black box

Introduction to SemiconductorPh sics

05/03/2012 3Basic Electronics

Introduction

• Materials can be described according to their

properties.

• We are interested in the electrical properties.

• T ere are t ree in s o materia s

– Conductor

– Semiconductor

– Insulator




Material

Conductor Insulator Semiconductor

A conductor is a material

that easily conducts

electrical current.

Single-element materials:

-copper, silver, gold, and

aluminum

It does not conduct

electrical current under

normal conditions.

Most good insulators are

compounds rather than

-

A semiconductor in its

pure (intrinsic) state is

neither a good conductornor a good insulator.

The most common

-


Atoms with only one

valence electron very

loosely bound to the atom

Therefore, a conductive

material has many free

electrons that when moving in

the same direction, make up

the current.

-rubber, plastics, glass,

mica, and quartzsemiconductors:

-silicon, germanium, and

carbon.

We can also find

compounds

Valence electrons are

tightly bound to the

atoms;

there are very few free

electrons in an insulator.


Resistivity Values


Material

Conductor Insulator Semiconductor

A conductor is a material

that easily conducts

electrical current.

Single-element materials:-copper, silver, gold, and

aluminum

It does not conduct

electrical current under

normal conditions.

Most good insulators are

compounds rather than

sin le-element materials.

A semiconductor in its

pure (intrinsic) state is

neither a good conductor

nor a good insulator.

The most common

-


Atoms with only one

valence electron very

loosely bound to the atom

Therefore, a conductive material

has many free electrons that.

when moving in the same

direction, make up the current.

.

semiconductors are

silicon, germanium, and

carbon.

We can also find

compounds.



Morphology


Solar Cells … $$$$$$

Unit Cells


Unit Cell: Si, Ge


Important Semiconducting Materials




Quantum Phenomena


Quantum Phenomena I

Photons

Wave-particle duality


Discretization – Car Speed?

Mathematics changes

Quantum Phenomena II:

Wave-particle duality


Quantum Phenomena III:

Uncertainly Principle

• It is impossible to determine simultaneously both the position

and velocity of an electron with any degree of accuracy or

certainty (Heisenberg).• The result of this principle produces a concept of the atom

,

of where electrons in an atom are most likely to be located.

Bohr Model.




Atoms – Generalities

Electronic Distribution and Shells


Shell 1

Shell 2

05/03/2012 18Shell 3

Atoms – Example: Si

Shell 3


Let’s consider carefully this picture!

Shell 1

Take care! Bohr Model




Bohr Model VS Quantum Model


-The atomic number = the number

of protons in the nucleus (it is the

same as the number of electrons).

-The positive charges cancel the

negative charges, and the atom has

a net charge

-Electrons that are in orbits farther from the nucleus have higher

energy and are less tightly bound to the atom than those closer


to the nucleus.

-Electrons with the highest energy -> outermost shell of an atom

and are relatively loosely bound to the atom.

-This outermost shell is known as the valence shell and electrons

in this shell are called valence electrons.

-Valence electrons: contribute to chemical reactions and bonding

within the structure of a material and determine its electrical

properties.

• Each discrete distance (orbit) from the nucleuscorresponds to a certain energy level.

• In an atom, the orbits are grouped into energybands known as shells.

• A given atom has a fixed number of shells. Eachshell has a fixed maximum number of electronsat permissible energy levels (orbits).

• The differences in energy levels within a shell aremuch smaller than the difference in energybetween shells.

• The shells are designated 1, 2, 3 and so on, with1 being closest to the nucleus. Some references

designate shells by the letters K, L M and so on.


• The maximum number of electrons (Ne) that can exist in each shell of an

atom is a fact of nature and can be calculated by the formula,




Ionization


Ionization

• Ionization:

– When an atom absorbs energy (from a heat source or fromlight) the energies of the electrons are raised.

– The valence electrons can easily jump to higher orbitswithin the valence shell when external ener is absorbed.

– The departure of a valence electron leaves a previouslyneutral atom with an excess of positive charge (moreprotons than electrons).

– The process of losing a valence electron is known as

ionization, and the resulting positively charged atom iscalled a positive ion.


Atoms – Example: Si

Shell 3


Let’s consider carefully this picture!

Shell 1

Ionization

• Ionization:

– When an atom absorbs energy (from a heat source or from

light) the energies of the electrons are raised. – The valence electrons can easily jump to higher orbits

within the valence shell when external ener is absorbed.

– The departure of a valence electron leaves a previouslyneutral atom with an excess of positive charge (moreprotons than electrons).

– The process of losing a valence electron is known asionization, and the resulting positively charged atom is

called a positive ion.




05/03/2012 Basic Electronics 29 05/03/2012 Basic Electronics 30




From Atoms to Solids (Crystals)


More on Atoms: Si Vs Cu


The valence electron in copper has more energy than the valence electron in silicon.

This means that it is easier for valence electrons in copper to acquire enough additional

energy to escape from their atoms and become free electrons in the conduction band

than it is in silicon.

More on Atoms: Si Vs GeThe valence electrons in germanium are in the fourth shell while

those in silicon are in the third shell


This means that the germanium valence electrons are at higher energy levels than those in

silicon and, therefore, require a smaller amount of additional energy to escape from the

atom.

This property makes germanium more unstable at high temperatures, and this is a basic

reason why silicon is the most widely used semiconductive material.

Covalent Bonds I

-An intrinsic crystal is one that has no

impurities.

-Covalent bonding for germanium is

similar because it also has four valence

electrons.


An intrinsic crystal is one that has no impurities.



Covalent Bonds II


In a Si crystal, there are billions of Si atoms, each with eight valence electrons.

These valence electrons are covalent bonds that hold the crystal together – that give it solidity.

It produces chemical stability.

Si is almost an insulator @ Troom – That’s why we have to dope☺ !!!

Important Definition: eV

(Let me introduce this concept now)


Electronvolt [EV]

• One volt is the potential difference between two points in an

electric circuit when the energy required to move 1 coulomb

of charge (electrons) from one point to the other is 1 joule.

W

Energy [J]

• Applying to an individual electron:


Q=ec r ca o en a

Charge [C]

C

W V

1910602.1

1−

×

=

( ( ) eV J V C W 110602.1110602.11919

=×=×=−−

Bands




-Each shell around the nucleus corresponds

to a certain energy band.

-Each shell is separated from adjacent shells

by energy gaps, in which no electrons can

exist.

At a temperature of absolute 0 Kelvin.

An intrinsic (pure) silicon crystal at room


temperature has sufficient heat (thermal

energy) for some valence electrons to

jump the gap from the valence band into

the conduction band, becoming free

electrons. Free electrons are also called

conduction electrons.

☺ Bands come from put toghether

millions of atoms (solid)






Conduction in Semiconductors

Electrons and Holes


Holes

• @ Troom, the heat energy in air causes the atoms in a Si crystal vibrate.

• Temperature: Vibration of atoms!!!

• These vibrations can disloge an electron from the valence orbit.

• Electron gains energy from vibrations and it behaves as a free-electron.

• The departure of the electron creates a vacancy called hole.

• ☺ Holes makes the difference (compared to conductors).

• Generation of holes and free electrons is low because of temperature

(dope it!! )


Recombination and Lifetime




Processes in a Crystal


conduction band by external energy, there

is one hole left in the valence band,

creating what is called an electron-hole

pair.

Recombination occurs when a conduction-

band electron loses energy and falls back

into a hole in the valence band.

-This movement of free electrons is one

type of current in a semiconductive material

and is called electron current.

-Electrons remainin in the valence band

-Another type of current occurs in the

valence band, where the holes created by

the free electrons exist.


are still attached to their atoms and are not

free to move randomly in the crystal

structure as are the free electrons.

However, a valence electron can move into

a nearby hole with little change in its energy

level, thus leaving another hole where it

came from.

Transport

• @ Troom silicon is an insulator because it has

only few electrons and holes produced by

thermal energy.




Transport






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Lecture1 BE Semiconducting Materials