1

Chapter-5: Introduction to Semi-Conductorsand Diode Circuits

Valence band

Conduction band

boundelectrons

When atoms form a crystalline structure (metals, semiconductors, )the valence electrons loose their attraction to a local atom and form anband of ~equivalent charges- valence band. The conduction bandlies within or above the valence band.

1s

2s

2p

3s

3pEnergy gap

2

Energy Bands

Semiconductor

ConductionBand

Valence Band

Energy Gap

Electron Energy

ConductionBand

Valence Band

Insulator

Energy Gap

Electron Energy

ConductionBand

Valence Band

Overlapping bands- little energy isneeded forconduction

Conductor

Electron Energy

3

Rf104

Ha105

Sg106

Uns107

Uno108

Une109

IA

IIA

IIIB IVB VB VIB VIIB IB IIB

IIIA IVA VA VIA VIIA

VIIIA

VIIIB

Hydrogen

H1

1.008

Beryllium

Be4 9.01

2

Na11

Sodium

22.989

Li3

Lithium

6.939

12 24.312

MgMagnesium

19

KPotassiu

m

39.102

Ca20 40.08

Calcium

Sc21

Scandium

44.956 Ti

22

Titanium

47.90

V23

Vanadium

50.942

Manganese

Mn25 54.93

8 Fe26

Iron

55.847 Co

27

Cobalt

58.933

Ni28

Nickel

58.71

Rh45

Rhodium

102.91

Zn30

Zinc

65.37

As33

Arsenic

74.922 Se

34

Selenium

78.96

Br35

Bromine

79.909 Kr

36

Krypton

83.80

Al13

Aluminum

26.981 Si

14

Silicon

28.086 P

15

Phosphorus

30.974 S

16

Sulfur

32.064 Cl

17

Chlorine

35.453 Ar

18

Argon

39.948

B5

Boron

10.811 C

6

Carbon

12.011 N

7

Nitrogen

14.007 O

8

Oxygen

15.999 F

9

Florine

18.998 Ne

10

Neon

20.183

He2

Helium

4.0026

Rb37

Rubidium

85.47

Sr38

Strontium

87.62

Y39

Yttrium

88.905

Zr40

Zirconium

91.22

Nb41

Niobium

92.906

Molybde-num

Mo42 95.94

Cr24

Chromium

51.996

Technitium

Tc43 99

Ru44

Ruthenium

101.07

Cd48

Cadmium

112.40

Cu29

Copper

63.54

Palladium

Pd46 106.4

SilverAg

47107.87

Sm62

Samarium

150.35

Ga31

Gallium

69.72

In49

Indium

114.82

Ge32 72.59

Germanium

Sn50

Tin

118.69

Sb51

Antimony

121.75

Te52

Tellurium

127.60

I53

Iodine

126.904

Xe54

Xenon

131.30

Cs55

Cesium

132.90

Ba56

Barium

1137.34

La57

Lanthanum

138.91

Hf72

Hafnium

178.49

Ta73

Tantalum

180.95

W74

Tungsten

183.85

Re75

Rhenium

186.2

Os76

Osmium

190.2

Ir77

Iridium

192.2

Pt78

Platinum

195.09

Au

79

Gold

196.967

Hg80

Mercury

200.59

Tl81

Thallium

204.37

Pb82

Lead

207.19

Bi83

Bismuth

208.98

Po84

Polonium

210

At85

Astatine

210 Rn

86

Radon

222

Uun110

Fr87

Francium

223

Ra88

Radium

226

Ac89 227

Actinium

Ce58

Cerium

140.12 Pr

59

Praseodym-ium

140.91 60

NdNeodym

-ium

144.24

Pm61

Prome-thium

147

Europium

Eu63 151.9

6 Gd64

Gadolin-ium

157.25 Tb

65

Terbium

158.92 Dy

66

Dyspro-sium

162.50 Ho

67

Holmium

164.93 Er

68

Erbium

167.26 Tm

69

Thulium

168.93 Yb

70

Ytterbium

173.04

71

LuLutetium

174.97

Th90

Thorium

232.04

Pa91

Procat-inium

231

U92

Uranium

238.03 Np

93

Neptunium

237

Pu94

Plutonium

242

Americium

Am95 243

Cm96

Curium

247

Berkelium

Bk97 24

7 Cf98

Califor-nium

249

Es99

Einstein-ium

254

Fm100

Fermium

253

Md101

Mendelev-ium

256 102

NoNobeliu

m

253

Lr103

Lawren-cium

257

Transition Metals

Nonmetals

Metalloids(semimetals)

Lanthanides

Actinides

4

Group IVA Elemental Semiconductors

•Only “White Lead” is semiconductor•Converts to metallic form under moderate heat82PbLead

•Only “White Tin” is semiconductor•Converts to metallic form under moderate heat50SnTin

•High Mobility (good)•High Purity Material (good)•Oxide is porous to water/hydrogen (disasterous!)

32GeGermanium

•Cheap•Ultra High Purity•Oxide is amazingly perfect for IC applications

14SiSilicon

•Very Expensive•Band Gap Large: 6V•Difficult to produce without high contamination

6CCarbondiamond

5

Si

Si SiSi

Si

Si

Si Si

Si Si

Si

Si

Si

Si

Si

Si

Si

Si Si

Si Si

Si

Si

Si

Si

Silicon atoms share valence electronsto form insulator-like bonds.

Covalent Bonding of Pure Silicon

6

Donor atoms provide excess electronsto form n-type silicon.

Phosphorus atomserves as n-typedopant

Excess electron (-)

Si

Si Si

Si

Si

Si

Si Si

Si

Si

Si

Si

Si

Si

Si

Si Si

Si

Si

Si

Si

Si

P

P

P

Electrons in N-Type Silicon with Phosphorus Dopant

7

Acceptor atoms provide a deficiencyof electrons to form p-type silicon.

+ Hole

Boron atom servesas p-type dopant

Si

Si Si

Si

Si

Si

Si Si

Si

Si

Si

Si

Si

Si

Si Si

B Si

SiSi

Si

Si

Si

B

B

Holes in p-Type Silicon with Boron Dopant

8

Group III (p-type)

Boron 5

Aluminum 13

Gallium 31

Indium 49

Group IV

Carbon 6

Silicon 14

Germanium 32

Group V (n-type)

Nitrogen 7

Phosphorus 15

Arsenic 33

Antimony 51

Acceptor Impurities Donor ImpuritiesSemiconductor

Silicon Dopants

9

Conduction in n and p-Type Silicon

Electron Flow

Hole Flow

Positive terminal fromvoltage supply

Negative terminalfrom voltage supply

+Holes flow towardnegative terminal

-Electrons flow towardpositive terminal

10

Silicon Resistivity Versus Dopant Concentration

Dop

ant C

once

ntra

tion

(ato

ms/

cm3 )

Electrical Resistivity (ohm-cm)

1021

1020

1019

1018

1017

1016

1015

1014

101310-3 10-2 10-1 100 101 102 103

n-type p-type

11

n- and p-Type Silicon

The dominantcharge carrierin n-type Si isthe electron.

The dominantcharge carrier in p-type Si isthe hole

Important Facts:Not shown are the siliconatoms, which are present invastly greater numbers thaneither arsenic or boron.Typical dopingconcentrations are 1016

arsenic or boron atoms percm3.The concentration of siliconatoms is about 1022 atoms percm3.For every "dopant" atom,there are about a millionsilicon atoms.

12

}p-type Si n-type Si

DepletionRegion Δx

CathodeAnode

Metal contact

Potentialstep

Charge distribution of barrier voltage

across a pn Junction.

Open-Circuit Condition of a p-n Junction Diode

Diffusing electrons andholes create an electric fieldEo and potential step at thejunction.

0

Eo

Barriervoltage

13

Forward-Biased PN Junction Diode

p n

V

Hole flow Electron flow

Lamp

Eapp

Eo

In forward bias the applied E-field cancels internal electric field and chargesbegin flowing across the junction when Eapp>Eo or Vapp>~0.6V.

14

p n

3 V Lamp

Open-circuit condition(high resistance)

Reverse-Biased PN Junction Diode

Eapp

In the reverse bias the external E-field increases the size of the depletion zoneuntil all charge carries are near the contacts - fully depleted.

15

I-V Characteristic Curve

Reverse current exaggerated; typicalreverse saturation current: IS ~10-(6-12)A.

This is the "characteristic" curveof a pn junction diode. It showsthe slow, then abrupt, rise ofcurrent as the voltage is raised.

Under reverse bias, even verylarge voltages will cause onlyvery small currents, essentiallyconstant reverse bias currents.

Shockley!Diode!Law!(Ideal)I = IS (e

+eVD /kT !1)!~!IS !e+eVD /kT !!!

IS = reverse!bias!saturation!currentVD =!diode!voltage!!!!!!!!kT = 0.025eV !at !300o K!

16

Turn-On Voltage

A pn junction diode "turns on" at about 0.6 V, but that variesaccording to the doping concentration, diode type.

Notice the sharp rise in currentnear the turn-on voltage. This behavior will be exploited laterin the construction of the transistoramplifier.

Use!the!Shockley!Equation!to!det ermin e!the! forward !diode!current !VD =!1!V !T = 3000K !and !IS = 10pA.I = 10pA!exp(1eV / 0.025eV ) =!2.4e9A!!!

17

Solving Diode Equation by Iteration

! "Vd " IR = 0!!!!!Kirchhoff ' s!law!!!!!!!!!

I = ! "VdR

= IS eeVd /nkT "1( )!!!!!!!n # diode!realty! factor !n ~ 1.5eVdnkT

=!ln IIS

+1$%&

'()!!!

Vd !=ne

$%&

'() kT ln

! "VdISR

+1$%&

'()!!!!!Solve!by!iteration

!!

Vd(1) != n

e$%&

'() kT ln

! "Vd(0)

ISR+1

$%&

'()

Vd(2) != n

e$%&

'() kT ln

! "Vd(1)

ISR+1

$%&

'()

.

Stop!when!Vd(N+1) ~ Vd

(N )

I

http://en.wikipedia.org/wiki/Diode_modelling#Shockley_diode_model

Is 2.00E-06 Ampsemf 10 VoltsR 1000 Ohmsn 1

ε

18

A PN Junction Diode and Its Symbol

Black band correspondsto the "point" in the diodesymbol on the right.

It points as "point" is spelled, with "p" first, and"n" later.

19

p- Substrate

Cathode Anode

pn junction diode

Metal contact

Heavily doped p region Heavily doped n region

Basic Symbol and Structure of the pn JunctionDiode

20

Light Emitting DiodesAs electrons and holes pass thru the forward biased junction, someannihilate and release hf = eUo of activation energy in to light.

E=hc/λ=1240 eV-nm / λ(nm)

Red light of 650nm would correspond to Uo = 1240/650 eV = 1.9 eV

Activation VoltageUo

+ + + +

- - - -

-+

hf

Activation Energy

21

V >Vdiode !!!!!!zd =!r !!and !VOUT =!( Rr + R

)V !!V !Vdiode !

V <Vdiode !!!!zd = "!and !!VOUT =!( R" + R

)V = 0

zd

R

~0.6V

V

Simple Diode Circuit

~0.6V

zd

RV

22

AC -DC!ConverterCapacitor !charges!to! peak !voltage!VOUT ~ VS ! 0.6VVoltage!decays!on!down ! cycle!!VOUT = VS !e

! t /RC

!!!!!!!VOUT == VS !(1! t / RC + ...!!!!!)!!!!!!t / RC << 1!

!!!!!!!!"V =!VOUT (t) !VS =!VSRtC

=if !C

r = "V /VOUT !!!!ripple!! factorVOUT !#!VS !!0.6V !!!We!have! produced !a!DC !!voltage! from!!AC !input.!

Half Wave Rectifier - AC-to-DC Conversion

zd

RVS C

Vout

23

Full Wave Rectifier - AC-to-DC Conversion

120VAC

- In a Full Wave Rectifier two diodes act to rectify both positive and negative cycles.

- A transformer is generally used to isolate the 120VAC (20A) from the secondary.

- Less wasted power and reduced ripple factor:

C R

V2= (N2/N1)V1Vout = 12!V2

r =12

if !C( )

Vout

24

Cockroft -Walton Voltage Multipliers

4-stage Multiplier

1. On the positive halfcycle C1,C2,C,C4 charge to +Us thru D2 and D4. D1 and D3 are blocking.

2. On the negative halfcycle C1,C3, charge to -Us, but C2 and C4 can not discharge.

3. On the 2nd cycle C1,C2,C,C4 charge again to +Us thru D2 and D4. and C2 and C4 coming each chargeing to 2 Us.

4. The total potential difference across the output C2+C4 is Vout= +4Us.

5. Practically speaking Vout < +4Urms.

6. You can hear your digital camera charge up the capacitor banks for the flash. The cockroft walton concept can only supply a short burst of charge and must be modified for high DC current use.

http://www.blazelabs.com/e-exp15.asp

+ +

++ ++

25

Voltage Doubler Circuit

+VC1

C2

D1

D2

26

Zener Diode

Zener breakdownVoltage ~ -VZ

0.6 V +V-V

•A Zener works in reverse bias mode. It is be designed to break downat the precise voltage called VZener ~10-100V eg.

Vin Vout

R

RL

Vz

VIN ! iR