EE2 Lecture 1

EE2 Physics for Electrical Engineers

Lecture 1

Introduction to Physical Electronics

Course Instructor: Marko Sokolich

28 September, 2015

© 2015 Marko Sokolich All Rights Reserved

2© 2015 Marko Sokolich All Rights Reserved

Posted on the CCLE Website

• Survey of Previous Coursework • EE2 Sylabus

– Contact Information– Class Schedule/Office Hours– Grading Policy/Exam Schedule– Teaching Assistant/Discussion Sections– Textbook/References– Course Outline


Course Philosophy

• Solid foundation in fundamental principles• Qualitative Understanding first, Quantitative Skill second• Conceptual foundations of Quantum Mechanics• Mental and numerical models of physical processes• Thorough treatment of the physics of pn Junctions• Introduction to a wide variety of 2-terminal semiconductor devices

– Each illustrates a physical principle– Connect each to applications– Be able to understand operation of novel devices

• Mastery of material requires considerable self-study– Not all concepts will be covered in lectures– Homework is an important component of success– Homework will require reading ahead of the materials in lecture


The Importance of Homework

Quote of the Day...

"I hear and I forget. I see and I remember.I do and I understand."

-- attributed to Kong Fuzi…Confucius (551 – 479 bce)

The importance of Homework has been known for at least 2500 years !

Lecture

“I hear and I forget

Your Notes,Slides andText

I see and I remember

Homework

I do and I understand”


Why study quantum mechanics?

• Classical, Newtonian, physics breaks down at atomic dimensions.

• Forget atoms! Simple, macroscopic things can’t be explained

– Why are quartz and diamond transparent?– Why does conductivity sometimes increase with T?– How can current flow be asymmetric?

• The behavior of semiconductors cannot be explained without quantum mechanics.


Fine, but then why study semiconductors?


Semiconductor Devices

Where are they?• Computers• Watches• Smart Phones• Cameras• CD Players• Satellites• Undersea Cable• Automobiles• Airplanes• Lights• The Cloud• The Internet• Humans/Dogs/Whales etc.• Interstellar space


Semiconductors: Historical Perspective

Where are they? What did they replace? When? Computers Mechanical Calculators,

Tubes, Magnetic cores 1960s

Watches Mechanical Movements 1970s Cell Phones [Made them possible] 1980s CD Players Mechanical stylus, Tubes 1980 Satellites [Made them possible] 1957 Radio, TV Vacuum Tubes 1950s

1970s Automobiles Mechanical timing, Maps 1980s Airplanes Crew Members (Navigator) 1980s Traffic Lights Light Bulbs 1990s Smart Phones Ambient Lighting

Cell Phones Incandescent and Fluorescent

2000 2010


Imagine a day without silicon: 1977

• First Star Wars movie is playing in theaters• Apple computer launches their first commercial success (Apple II)• Voyager 1,2 spacecraft have just been launched

– With an incredible 40 kilobits of on-board memory (80,000 transistors)

• A day without silicon– Leave your calculator at home– Don’t go to the computer lab– Drive wherever– Watch TV (at parent’s house… 10 year old set)– Play your LP records all day and night (tube amp, turntable)– Go to the Star Wars movie!


Imagine a day without silicon 2015

• Yet another Star Wars Movie!• Voyager 1 and 2 have reached interstellar space (in 2013)• One Apple iPhone 6s processor (A8) has 2 Billion transistors

– The equivalent of 2 Million Apple II* computers from 1977*In its first 4 years Apple sold “over 300,000” units!

• A day without silicon:– No driving– No smartphone, laptop, calculator, FitBit, watch– Wooden center – Bball and Weights only, no elliptical/treadmill!– No music, no TV, no gaming– No movie – digital projector– Go to the beach– Hike in the Santa Monica Mountains– Stay away from civilization


Semiconductors: The Future (my predictions 2002)

• Ambient Lighting– Incandescent Light bulbs will be as hard to find by 2015 as vacuum tubes

are today?• Pervasive Computing

– Microprocessors will be imbedded in many previously low-tech systems.– Segway (personal scooter) uses 3 microprocessors just to maintain

balance. These will become pervasive.– Cars won’t crash.

• Communications– Cell phones will be the size of a watch and operate without batteries

(scavenging power from the environment)– Fiber to the home will become indispensable.

• Personal Robots will become commonplace by 2010 for limited functions.• Microprocessor and memory industries will become mature and cease to

grow in dollar volume. All semiconductor sales growth will be in sensors, ambient lighting and communications.

“The future ain’t what it used to be”Yogi Berra (1925-2015)


Semiconductors: The Future (2015 Predictions)

• Pilotless full size aircraft (cargo 2020, passenger ?)• Consumer priced floor to ceiling video screen (2020)• Silicon minds (Siri on steroids by 2025)• All-body monitors (complete personal health monitoring by 2025)• 50% of cars sold are self-driving (2025)• Driverless Pizza delivery (2025)• 90% of cars sold are self-driving (2030)• Biometric PIN credit/debit cards (2030)• Cash not accepted at 50% of all businesses (2030)• 50% of California electricity from solar (2030)• Robot soccer team beats human world cup team (2050)• The singularity [a machine superior to a human] (2070+)

“It's tough to make predictions, especially about the future.”Yogi Berra (1925-2015)


Semiconductors: The Future (Your Prediction)

• Provide one prediction about the future of technology (somehow involving semiconductors)

– At least 20 years in the future (2025 or later)– Falsifiable… precise enough for a definitive “yes” or “no”– Example of a good prediction: “10% of Americans will own

personal vehicles capable of flight by 2027”… easy to check– Your choice of “anonymous” or with attribution (anonymous

unless you sign your name) – Make a prediction about something you know better than your

peers (a hobby, an interest outside of school, a unique work experience)


Solid state electronics in a nutshell(A pictorial outline of the syllabus)

– Experimental Observations– Allowed Energy States in Atoms– Semiconductor Crystals– Energy Bands in Solids– Metals, Insulators and Semiconductors– Electrons and Holes– Counting charge carriers– Generation and Recombination– Electrical Conductivity– Junctions– Solids and Light

Next: A whirlwind review of the entire course in 20 slides!


Early semiconductor observations

• In 1833 Michael Faraday observed that the conductivity of silver sulfide increased with increasing temperature… no metal behaved this way.

• In 1874, 24-year old Ferdinand Braun noted that current flowed only one way when he probed a lead sulfide crystal with a thin metal wire… the first observation of “diode” behavior.

• Neither effect can be explained without Quantum Mechanics


Experimental Observations: Crystals

Max Von Laue won the 1914Nobel Prize in Physics for observingthe interference effects of X-rays(Roentgen Rays) with crystalsin 1912.

Atoms in some materials are arranged in a uniform lattice with long-range order


Experimental Observations: Photoelectric Effect

0

vacuum

Battery

- +

Ammeter

EmitterCollector Blue Light Blue Light - current flows

Red Light - no current

0

Red Light

Light behaves as particles. Electrons can interact only with a single light particle (photon).

Electrons are bound to the metal with energy eVo

Observations by Lenard in 1902Explanation by Einstein in 1905Einstein won Nobel Prize in 1921

Frequency of Light

Bat

tery

Vol

tage

eV = h - eVo

Red Green Blue


Experimental Observations: Spectral Lines

Unlike light from thesun which containsall the colors of the rainbow, elements emita distinct signatureof spectral lines.

The simplest elementsH and He have the fewestspectral lines.


Potential Well of a Nucleus

F = eE 1/r2

E = d/dr 1/r

Nucleus looks roughly like a “box” potential

er

Potential energy of an electron is e

An electron in the attractivepotential of a nucleus canonly have one of a set of discrete or quantizedenergy levels.

The lowest of these levels iscalled the ground state.

Transitions between states result in the absorption oremission of a quanta of energy(a photon)




er





F = eE 1/r2

E = d/dr 1/r



F = eE 1/r2

E = d/dr 1/r


er






Crystal Lattices

Some elements array themselves in highly regular crystals with very long range order. Silicon is an example.

The distance between adjacent atoms is a few Angstroms (tenths of nanometers) so that electrons are shared between adjacent atoms.

The order imposes restrictions on the wave functions of electrons…they must have properties similar to the symmetry of the lattice.

The high purity with which such crystals can be grown results in the ability to change their properties with miniscule addition of impurities.


Atoms in Proximity: Splitting of Quantum States

E0 E0

E0 +

E0

E0 +

E0

Atoms far apart do not have electronwave functions that overlap. Theyhave the same ground state energy for all electrons.

As atoms are moved closer together the energy levels split

The closer they get the greater the splitting


Periodic Array of Potential Wells: Energy Bands

E0 +

E0

As many states as there are atoms in the solid.So close together that they form into “bands”.

If each atom contributed one electron then in the ground (lowest energy) state the lower (valence)band of the solid is filled with electrons and the higher (conduction) band is empty.

In between there is an energy gap Eg.

Eg.


Insulators, Metals and Semiconductors

Conduction band

Valence band

Empty

Filled

Filled

Empty

Eg ~ 1 V

Semiconductor

Metal

Insulator

Conduction band

Valence band

Eg ~ 5 V

Filled

EmptyConduction band

Valence band


Semiconductors

Large substrates available in Si, GaAs, Ge, InP

Carbon is an insulator and Tin (Sn) is a conductor

III IV V


Crystal Engineering

Semiconductors can be “engineered” by growing different materials on top of each other.

This can be done if the lattice constant (the spacing between unit cells) is the same or similar.

Because semiconductors want to array themselves in a crystalline form this is relatively straightforward

5.4310

Any compound along this line can be grown on an InP wafer


Electrons and Holes: Thermal Excitation

+4+4

+4

+4

+4 +4+4

+4

+4

+4

+4+4

+4

+3

+5

CSiGeSn

BAlGaIn

NP

AsSb

Hole

Free Electron

E


Electrons and Holes: Doping

+4+4

+4

+4

+4 +4+4

+4

+4

+4

+4+4

+4

+3

+5

CSiGeSn

BAlGaIn

NP

AsSb

Free Electron

+5

Hole

+3


Semiconductor Statistics

U0

0,ε

Two state system in thermal*equilibrium with a reservoir

kTPP exp

)0()(

Ratio of the probability that the system has energy ε to the probability that it has energy 0

* Systems can exchange energy


Semiconductor Statistics

U0

0,ε

Two state system in thermal and diffusive* equilibrium with a reservoir

kTPP F exp

)0,0(),1(

The system can be in a state with 0 particles and 0 energy or in a state with 1 particle and energy ε

1exp

1),1(

kT

PF

Normalize1)0,0(),1( PP

* Systems can exchange energy andparticles.

εF is called the Fermi energy


Electrons and Holes: Optical Excitation

+4+4

+4

+4

+4 +4+4

+4

+4

+4

+4+4

+4

+3

+5

CSiGeSn

BAlGaIn

NP

AsSb

Free Electron

+4

photon

A photon of sufficient energy strikingthe semiconductor creates an electron-holepair.


t = 0

Excess Carriers: Diffusion, Recombination

t1

semiconductor

Illumination creates electrons and holes (carriers)

t2

As time passes the carriers diffuseand recombine


Excess Carriers: Drift

semiconductor

Illumination creates electrons and holes (carriers)

t = 0

t1

t2

As time passes the carriers drift, diffuse

and recombine

V

Apply Voltage Across the sample


•Free space electrons move with constant acceleration in a constant electric field: F= eE = ma

• In a solid electrons do not move freely…they periodically collide with the atoms in the lattice:

• Velocity is proportional to field for low fields < ~ 1V/m

• Proportionality constant is called mobility (): v =

• Nearly constant velocity at high fields

• Peak velocity at intermediate fields

Peak velocity up to 0.3 m/picosecond (ps)

Saturated velocity 0.1 to 0.2 m/ps

InP

Si

InGaAs

GaAs

1V/m0.1V/m

Motion of Electrons: Mobility and Velocity


Electrical Conductivity

• Recall from your Electromagnetics course that: J = E or J = E (the macroscopic analog is V = IR) where is the conductivity and is the resistivity (=1/ )

• Current density J (Amperes/cm2 = Coulombs/sec/cm2) is proportional to the number of charge carriers, their individual charge and their velocity: J = nev = neμE so that the conductivity is given by: = neμ

• In a semiconductor we can manipulate the density of charge carriers in various ways…thus we can change the conductivity.


Conductivity

Semiconductor 1

I

V

1

R

I

V

1

R

LVAneEAneI

LVELEV

AJJAreaIEnenevJ

AreaL


Junctions

Semiconductor 1Metal,

Insulator or Semiconductor 2

I

V

+V

pn DiodesMS Diodestunneling


Ideal Diode IV Characteristic

0.0E+00

5.0E-04

1.0E-03

1.5E-03

2.0E-03

2.5E-03

0 0.2 0.4 0.6 0.8

10

kTeV

eII

Amperes104 150

I

Diode “turns-on” to mA level at about 0.65V

I(A)

V


Solar Cells: Optically Generated Current

• If there is uniform EHP generation in the sample (Gop) then the optically generated current is given by:

p n

-xp xn-xp-Ln xn+Lp

E

AWLLeGI pnopop

WIop


Solar Cells: Power Delivered to a Resistive Load

• The power from a solar cell must be delivered into a resistive load.• The load can be chosen to maximize the power.

0

20

40

60

80

100

120

140

0 0.1 0.2 0.3 0.4 0.5 0.6Voltage, V

Cur

rent

, mA

Full Sun

Half Sun

0.13 Sun

RLIop

+

-

V

The equivalent circuit of the solar cell is a diode in parallel with an independent current source IopThe resistor can be depicted on the IV characteristic as shown.

RL

Vm

Im

The power delivered to RL is ImVm.


Light Emitting Diodes

• LED and Lasers require a population inversion of carriers. • This means that an excess of electrons exists in the conduction

band and and excess of holes exists in the valence band in the same space location.

• One way to do this is with a heavily doped p+n+ junction in forward bias.

Fp

Fn

The wavelength of the emitted light is related to the bandgap as: gEh


LEDs Semiconductor Choices

LEDs are primarily used as visible light sources.

As a result, the visible spectrum is our best guide

to LED materials.

For many years a blue LED was not available because

there were no suitable semiconductors in the

blue.

Now GaN is used for bright blue LEDs


Summary of Course Content

• Physics of Semiconductors– QM of Solids: Energy Bands– Charge Carriers– Drift and Diffusion– Interaction with Light– Generation and Recombination

• Semiconductor Junctions– Metal-semiconductor– pn junction– Circuit (macroscopic) Models

• 2 Terminal devices (diodes) [a very brief intro]– Junction diode– Tunnel diode– Solar cell– Light emitting diode

Documents

EE2 Lecture 1