Post on 04-Jul-2018
1
Introduction to Semi-Conductors and DiodeCircuits
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
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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.
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}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. (1.5-2.0 V LED)
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!
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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!!!
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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
Forbiddenzone
Fermi level
Fermi level
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
+ +
++ ++