bruce mayer, pe licensed electrical & mechanical engineer [email protected]

[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt1

Bruce Mayer, PE Engineering-45: Materials of Engineering

Bruce Mayer, PELicensed Electrical & Mechanical Engineer

[email protected]

Engineering 45

ElectricalProperties

-2



Learning Goals – Electrical Props How Are Electrical Conductance And

Resistance Characterized What Are The Physical Phenomena That

Distinguish Conductors, Semiconductors, and Insulators?

For Metals, How Is Conductivity Affected By Imperfections, Temp, And Deformation?

For Semiconductors, How is Conductivity Affected By Impurities (Doping) And Temp?



SemiConductivity Materials of

Valence 4 (Grp IVA in the Periodic Table) Exhibit the property of Semiconductivity• Si, Ge in

Particular• C, Sn to a

Lesser Extent Also Observed in

Compounds

• III-V → GaAs• II-VI → InP

Semiconductivity Characterized by• Insulative Behavior at

Room Temperature– 106-1012 times LESS

conductive than metals• INCREASING

Conductivity with Increasing Temp– Opposite of

Metal Behavior



Carriers in Semiconductors

At non-zero temperatures, electrons are thermally excited from the valence band to the conduction band.

The activated “free electrons” and the remaining “holes” left behind act as two “ideal gases”!!

Certain types of impurities that are grown or implanted into the SemiConductor crystal produce extra free electrons or holes.

Energy gap, Eg

Conduction band (at T = 0 K unpopulated with electrons)

Valence band (at T = 0 K totally filled with electrons)



Intrinsic (Pure) Semiconductors σ Data for

Pure Silicon• Note σ↑ as T↑

Why This Temp Behavior?• Semiconductor e−

Band StructureSi electrical conductivity, σ

(S

/m)

50 100 100010-210-1100101102103104

pure (undoped)

T(K)

Energy

filled band

filled valence band

empty band

fille

d st

ates

GAP?

– Thermal Energy Can Allow the e- to jump the “Forbidden” Gap between the “Valence” Band and the “Conduction” Band



Intrinsic (Pure) Carrier Concen Recall

Conductivity Eqn from the Metals Dicussion

Note the Exponential Increase in the Intrinsic carrier Concentration, ni or

nq kTE

igen

Since µ Does Not change nearly as much as ni with T

kTE ge



Some BandGapsInSb 0.17 eVGe 0.67 eVInN 0.7 eVHgCdTe 0.0 - 1.5 eVInGaAs 0.4 - 1.4 eVSilicon 1.14 eVInP 1.34 eVGaAs 1.42 eVCdTe 1.56 eVAlGaAs 1.42 – 2.16 eVInGaP2 1.8 eV

GaAsP 1.42-2.26eVInGaN 0.7 - 3.4 eVAlAs 2.16 eVGaP 2.26 eVAlGaInP 1.91 - 2.52 eVZnSe 2.7 eVSiC 6H 3.03 eVSiC 4H 3.28 eVGaN 3.37 eVDiamond 5.46 - 6.4 eV

http://www.answers.com/topic/indium-iii-antimonide

http://www.answers.com/topic/electronvolt

http://www.answers.com/topic/germanium


http://www.answers.com/topic/indium-iii-nitride


http://www.answers.com/topic/mercury-ii-cadmium-ii-telluride


http://www.answers.com/topic/indium-gallium-arsenide


http://www.answers.com/topic/silicon


http://www.answers.com/topic/indium-iii-phosphide


http://www.answers.com/topic/gallium-iii-arsenide


http://www.answers.com/topic/cadmium-telluride


http://www.answers.com/topic/aluminium-gallium-arsenide


http://www.answers.com/topic/indium-gallium-phosphide

http://www.answers.com/topic/indium-gallium-phosphide


http://www.answers.com/topic/gallium-arsenide-phosphide


http://www.answers.com/topic/indium-gallium-nitride


http://www.answers.com/topic/aluminium-arsenide


http://www.answers.com/topic/gallium-iii-phosphide


http://www.answers.com/topic/aluminium-gallium-indium-phosphide


http://www.answers.com/topic/zinc-selenide


http://www.answers.com/topic/silicon-carbide


http://www.answers.com/topic/silicon-carbide


http://www.answers.com/topic/gallium-iii-nitride


http://www.answers.com/topic/diamond




Conduction by e− & h+ Migration Concept of Electrons (e-) & Holes (h+)

• When e- moves to the Conduction Band it leaves Its Parent Atom Core, and Moves Freely

• This Leaves behind an electron “HOLE” Which Results in a POSITIVELY Charged Atom/Ion Core

• This Positive Charge can Attract an e- from an ADJACENT Atom, Thus the hole, h+, can move Left↔Right or Up↔Down– This Transfers the POSITIVE Charge-Center to the

Adjacent Atom-Core– From an electrical current perspective, the Step-by-Step

movement of the hole appears as the movement of a POSITIVELY Charged Particle; some Analogies A bubble in a Liquid moves to the high side of a sealed tube One open Spot in A parking Lots Moves Further from the Bldg

as the cars move into the Close spot in Step-By-Step Fashion



e− & h+ Electrical Conduction Schematically

- +

electron hole pair creation

- +

no applied electric field

applied electric field

valence electron Si atom

applied electric field

electron hole pair migration

- + - +

In Metals, only e− Participate in Electrical Conduction (e− “sea”), But in Semiconductors HOLES also aid conduction



SemiConductor Conductivity With the

Participation of Electrons and Holes• Where

– q electronic charge, 1.6x10-19 Coulomb per e- or h+

– n electron concentration, e-/m3

– p hole concentration, h+/m3

– µe electron mobility, m2/V-s

– µh hole mobility, m2/V-s

hesemi pqnq Qty Si GaAs CdTe InPµe(m2/V-s)

0.19 0.88 0.105 0.470

µh(m2/V-s)

0.05 0.04 0.008 0.018

µe (4-30) times Greater Than µh• Why?

– Parking Garage Analogy



h+ & e-Parking Garage

Analogy n-Type

Semiconductor illustrated in (a) & (c)

p-Type Semiconductor illustrated in (b) & (d)

Thus µe >µh



INtrinsic vs. EXtrinsic Conduction INtrinsic SemiConductors → n = p

• Case for “pure” Semiconductors; e.g., Si EXtrinsic SemiConductors → n p

• occurs when impurities are added with a different no. of valence e−’s than the host (e.g., Si atoms)

N-type EXtrinsic: (n>>p) P-type EXtrinsic: (p>>n)


5+4+ 4+ 4+ 4+

4+4+4+4+4+

4+ 4+

Phosphorus atom


Boron atom

valence electron

Si atom

conduction electron

hole

3+4+ 4+ 4+ 4+

4+4+4+4+4+

4+ 4+enq hpq



Doped SemiConductors: vs T increases w/ Doping

Reason: imperfection sites lower the activation energy needed to produce mobile e- or h+

N-Type Si, n vs T

doped 0.0013at%B

0.0052at%B

elec

trica

l con

duct

ivity

, σ

(S

/m)

50 100 100010-210-1100101102103104

pure (undoped)

T(K)– FreezeOut → Not Sufficient

Thermal Energy to ionize either Dopants or Si

– Extrinsic → n = doping– Instrinsic → ni > doping

nd = 1021/m3



FreezeOut etc. Recall Reln for ni

The similar Reln for (N-Type) dopant Concentrations

– FreezeOut → kT << [Eg or Ed] Neither Si or Dopants are Ionized

– Extrinsic → Ed < kT < Eg Only Dopants are (Singly)

ionized and nd >> ni

– Intrinsic kT>> [Ed or Eg] nd fixed at dopant at%,

ni continues to Rise

kTEi

gen

kTEd

den Impurity

Donor Ed Acceptor Ea

P 0.044

As 0.049

Sb 0.039

B 0.045

Al 0.057

Egap 1.1 1.1

Si D

opan

t Ion

izat

ion

(eV

)nd = 1015/cc



p-n Junction Physics P and N Type

Semi Matls Brought Together to form a METALLURICAL (seamless) Junction

The HUGE MisMatch in Carrier Concentrations Results in e- & h+ DIFFUSION• Remember that?

Carrier Diffusion• e- Diffuse in to the

P-Type Material• h+ Diffuse in to the

N-Type Material



p-n Junction Physics cont. In a p-n Jcn

Carrier Cross-Diffusion is SELF-LIMITING• The e-/h+ Diffusion

leaves Behind IONIZED Atom Cores of the OPPOSITE Charge

• The Ion Cores set up an ELECTRIC FIELD that COUNTERS the Diffusion Gradient

For Si the Field-Filled Depletion Region• E-Field 1 MV/m• Depl Reg Width,

xd = 1-10 µm• E-fld•dx 0.6-0.7 V

– “built-in” Potential

E-Field



p-n Junction Rectifier A Rectifier is a

“Check Valve” for Current flow• Current Allowed

in ONE Direction but NOT the other

Side Issue → “Bias” Voltage• A “Bias” Voltage

is just Another name for EXTERNALLY APPLIED Voltage

E-Field



p-n Junction Rectifier cont p-n junction

Rectification• A small “Forward

Bias” Voltage results in Large currents

• Any level of “Reverse” Bias results in almost NO current flow

Class Q:• For Fwd Bias,

Which End is +; P or N???

A: the P end • The Applied Voltage

REDUCES the internal E-Field; This “Biases” The Junction in Favor of DIFFUSION

E-Field



p-n Junction Rectifier cont.2 p-n junction

No Applied Voltage

• Internal Field ENHANCED– Carriers Pulled AWAY

from Jcn; xd grows Forward Bias

• Diffusion & E-Field in Balance, No Current Flows

Reverse Biased• Internal Field

REDUCED– Carriers PUSHED and

Diffuse to the Jcn where they are “injected” into the other side; xd Contracts

Xd



Properties of Rectifying Junction

ForwardReverse IN914 PN Diode

• IF = 75 000 µA• IR = 0.025-50 µA



Transistors Transistors are

“Transfer Resistors”

Xsistors Have Three Connections • Input• Output• CONTROL

In Electronic Applications Transistors have TWO Basic Fcns

• Amplification – Both Current & Voltage

• On/Off Switching Two Main Types

• BiPolar Junction Transistor (BJT)– Good Amps

• Field Effect Transistor (FET)– Depletion Mode

Good Amps– Enhancement Mode

Good Switches



BJT The Classic pnp

or npn configurations• Basically Two pn

jcns Back-to-Back

npn In “Forward-Active” mode• b-e pn jcn

FORWARD Biased• b-c pn jcn

REVERSE Biased Very Little “base”

Current Large emitter &

collector currents• Good Current-Driving

Amplifiere

b

ce

b

c



Depletion Mode - JFET JFETs are

“Normally On” Transistors

Reverse Bias on the “gate” expands the NonConducting depletion region Until the channel is “Pinched Off” and no longer conducts• Gate is Reverse

Biased → little Control-Current

• Good Depl Region modulation → good I/V amp

OPEN “Channel” Between the “source” and “drain”

http://www.st-and.ac.uk/~www_pa/Scots_Guide/info/comp/active/jfet/jfet2.gif



Enhancement Mode - IGFET Insulated Gate

Field Effect Transistors are Normally-Off devices

Applying a Positive Voltage to the Gate will attract e− to the Channel• This will eventually

“invert” a thin region below the gate to N-type, creating a conducting channel between S & D

IGFETs are Great Switches• Used in almost all

digital IC’s

Back-to-Back pn Jcns Between “source” & “drain”



Ionic Materials In Metals and

Semiconductors, the atomic Ion-cores are fixed in the crystal Lattice• Although they

have the same charge as a “hole” they have almost NO “Mobility”– Thus They do

NOT contribute to Electrical Conduction

Some Small Atomic Radii impurities can be CHARGED (ionic) and MOBILE within another material• e.g., Na+ can move

fairly easily thru GLASS (SiO2)

The Total σ for Ionic Materials

ioniceletronic tot



Ionic Mobility • Diffusion• E-Field

Combine These two effects into Mobility

• Where– nI Ion Valence– DI Ion Mass

Diffusion Coeff, m2/s– q, k, T as Before

• Exercise → Find units for nI

As in the Electronic case

IIionic qN

kTqDn II

I • Where– NI Ion Concen,

Ions/m3

– q electronic Charge– µI Ionic Mobility,

m2/V-s

Two Forces move The Ions



Ceramics Most Ceramics have

WIDE BandGaps• SiO2 9 eV• Si3N4 4.7eV

Thus Ceramics Tend to be VeryGood Electrical INSULATORS

But as with SemiConductors. for Ceramics nintrinsic Increases with Temperature• Thus Insulative Capacity DEGRADES at Hi-T

– e.g; mullite = 3Al2O3•2SiO2

ρ(25°C) 1012 Ω-m; ρ(500°C) 106 Ω-m

Energy

filled band

filled Valence band

empty band

fille

d st

ates

GAPConduction

Band



Polymers Most “Standard” Plastics are Good Insulators

• c.f. Their use as insulation on metal WIRES• Conduction Mechanism Not well understood

– Believed to be More Electronic than Ionic

A Few Polymers are Good Conductors, with σ 107 S/m• About 2X HIGHER than Cu for Conductivity/lb • Mechanism appears to be SemiConductor-like

with a doping Requirement• Discovery of these “synthetic metals” Resulted in

the 2000 Chemistry Nobel Prize for Heeger, MacDiarmid and Shirakawa

http

://w

ebpa

ges.

char

ter.

net/

dmar

in/c

oat/

#his

tory



PiezoElectric Materials Piezoelectricity application of force

pressure produces Electrical Potential

at rest compression induces voltage

applied voltage induces

expansion



WhiteBoard Work Problem 18.30

• Antimony DopedGermanium

• EXtrinsic form– All Sb Ionized

• The μ’s:– μe = 0.1 m2/V·s– μh = 0.05 m2/V·s

bruce mayer, pe licensed electrical & mechanical engineer [email protected]

Documents

pe engineering

egconduction band

electronsvalence band

metalsincreasing conductivity

n s j o h n s o n c

extra free electrons

activated free electrons

electrical conductance