chapter-5: introduction to semi-conductors and diode...
TRANSCRIPT
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Chapter-5: Introduction to Semi-Conductors and Diode Circuits
Valence band
Conduction band
bound electrons
When atoms form a crystalline structure (metals, semiconductors, ) the valence electrons loose their attraction to a local atom and form an band of ~equivalent charges- valence band. The conduction band lies within or above the valence band.
1s
2s
2p
3s
3p Energy gap
2
Energy Bands
Semiconductor
Conduction Band
Valence Band
Energy Gap
Electron Energy
Conduction Band
Valence Band
Insulator
Energy Gap
Electron Energy
Conduction Band
Valence Band
Overlapping bands - little energy is needed for conduction
Conductor
Electron Energy
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Rf 104
Ha 105
Sg 106
Uns 107
Uno 108
Une 109
IA
IIA
IIIB IVB VB VIB VIIB IB IIB
IIIA IVA VA VIA VIIA
VIIIA
VIIIB
Hydrogen
H 1
1.008
Beryllium
Be 4 9.01
2
Na 11
Sodium
22.989
Li 3
Lithium
6.939
12 24.312
Mg Magnesium
19
K Potassiu
m
39.102
Ca 20 40.08
Calcium
Sc 21
Scandium
44.956
Ti 22
Titanium
47.90
V 23
Vanadium
50.942
Manganese Mn
25 54.938
Fe 26
Iron
55.847
Co 27
Cobalt
58.933
Ni 28
Nickel
58.71
Rh 45
Rhodium
102.91
Zn 30
Zinc
65.37
As 33
Arsenic
74.922
Se 34
Selenium
78.96
Br 35
Bromine
79.909
Kr 36
Krypton
83.80
Al 13
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
B 5
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
He 2
Helium
4.0026
Rb 37
Rubidium
85.47
Sr 38
Strontium
87.62
Y 39
Yttrium
88.905
Zr 40
Zirconium
91.22
Nb 41
Niobium
92.906
Molybde- num
Mo 42 95.94
Cr 24
Chromium
51.996
Technitium
Tc 43 99
Ru 44
Ruthenium
101.07
Cd 48
Cadmium
112.40
Cu 29
Copper
63.54
Palladium
Pd 46 106.4
Silver Ag
47 107.87
Sm 62
Samarium
150.35
Ga 31
Gallium
69.72
In 49
Indium
114.82
Ge 32 72.59
Germanium
Sn 50
Tin
118.69
Sb 51
Antimony
121.75
Te 52
Tellurium
127.60
I 53
Iodine
126.904
Xe 54
Xenon
131.30
Cs 55
Cesium
132.90
Ba 56
Barium
1137.34
La 57
Lanthanum
138.91
Hf 72
Hafnium
178.49
Ta 73
Tantalum
180.95
W 74
Tungsten
183.85
Re 75
Rhenium
186.2
Os 76
Osmium
190.2
Ir 77
Iridium
192.2
Pt 78
Platinum
195.09
Au
79
Gold
196.967
Hg 80
Mercury
200.59
Tl 81
Thallium
204.37
Pb 82
Lead
207.19
Bi 83
Bismuth
208.98
Po 84
Polonium
210
At 85
Astatine
210
Rn 86
Radon
222
Uun 110
Fr 87
Francium
223
Ra 88
Radium
226
Ac 89 227
Actinium
Ce 58
Cerium
140.12 Pr
59
Praseodym-ium
140.91 60
Nd Neodym
-ium
144.24
Pm 61
Prome-thium
147
Europium
Eu 63 151.9
6 Gd 64
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
Lu Lutetium
174.97
Th 90
Thorium
232.04
Pa 91
Procat- inium
231
U 92
Uranium
238.03 Np
93
Neptunium
237
Pu 94
Plutonium
242
Americium Am
95 243
Cm 96
Curium
247
Berkelium
Bk 97 24
7 Cf 98
Califor-nium
249
Es 99
Einstein-ium
254
Fm 100
Fermium
253
Md 101
Mendelev-ium
256 102
No Nobeliu
m
253
Lr 103
Lawren-cium
257
Transition Metals
Nonmetals
Metalloids (semimetals)
Lanthanides
Actinides
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Group IVA Elemental Semiconductors
Carbon C 6 • Very Expensive • Band Gap Large: 6V • Difficult to produce without high contamination
Silicon Si 14 • Cheap • Ultra High Purity • Oxide is amazingly perfect for IC applications
Germanium Ge 32 • High Mobility (good) • High Purity Material (good) • Oxide is porous to water/hydrogen (disasterous!)
Tin Sn 50 • Only “White Tin” is semiconductor • Converts to metallic form under moderate heat
Lead Pb 82 • Only “White Lead” is semiconductor • Converts to metallic form under moderate heat
diamond
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Si
Si Si Si
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 electrons to form insulator-like bonds.
Covalent Bonding of Pure Silicon
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Donor atoms provide excess electrons to form n-type silicon.
Phosphorus atom serves as n-type dopant
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
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Acceptor atoms provide a deficiency of electrons to form p-type silicon.
+ Hole
Boron atom serves as p-type dopant
Si
Si Si
Si
Si
Si
Si Si
Si
Si
Si
Si
Si
Si
Si Si
B Si
Si Si
Si
Si
Si
B
B
Holes in p-Type Silicon with Boron Dopant
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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 Impurities Semiconductor
Silicon Dopants
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Conduction in n and p-Type Silicon
Electron Flow
Hole Flow
Positive terminal from voltage supply
Negative terminal from voltage supply
+Holes flow toward negative terminal
-Electrons flow toward positive 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
1013 10-3 10-2 10-1 100 101 102 103
n-type p-type
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n- and p-Type Silicon
The dominant charge carrier in n-type Si is the electron.
The dominant charge carrier in p-type Si is the hole
Important Facts: Not shown are the silicon atoms, which are present in vastly greater numbers than either arsenic or boron. Typical doping concentrations are 1016 arsenic or boron atoms per cm3. The concentration of silicon atoms is about 1022 atoms per cm3. For every "dopant" atom, there are about a million silicon atoms.
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} p-type Si n-type Si
Depletion Region !x
Cathode Anode
Metal contact
Potential step
Charge distribution of barrier voltage
across a pn Junction.
Open-Circuit Condition of a p-n Junction Diode
Diffusing electrons and holes create an electric field Eo and potential step at the junction.
0
Eo
Barrier voltage
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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 charges begin flowing across the junction when Eapp>Eo or Vapp>~0.6V.
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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 zone until all charge carries are near the contacts - fully depleted.
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I-V Characteristic Curve
Reverse current exaggerated; typical reverse saturation current: IS ~10-(6-12)A.
This is the "characteristic" curve of a pn junction diode. It shows the slow, then abrupt, rise of current as the voltage is raised. Under reverse bias, even very large voltages will cause only very small currents, essentially constant 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 varies according to the doping concentration, diode type. Notice the sharp rise in current near the turn-on voltage. This behavior will be exploited later in the construction of the transistor amplifier.
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
ε −Vd − IR = 0 Kirchhoff ' s law
I = ε −VdR
= IS eeVd /nkT −1( ) n ≡ diode realty factor n ~ 1.5eVdnkT
= ln IIS
+1⎛⎝⎜
⎞⎠⎟
Vd =ne
⎛⎝⎜
⎞⎠⎟ kT ln
ε −VdISR
+1⎛⎝⎜
⎞⎠⎟ Solve by iteration
Vd(1) = n
e⎛⎝⎜
⎞⎠⎟ kT ln
ε −Vd(0)
ISR+1
⎛⎝⎜
⎞⎠⎟
Vd(2) = n
e⎛⎝⎜
⎞⎠⎟ kT ln
ε −Vd(1)
ISR+1
⎛⎝⎜
⎞⎠⎟
.
Stop when Vd(N+1) ~ Vd
(N )
I
http://en.wikipedia.org/wiki/Diode_modelling#Shockley_diode_model
DIODE EQ. ITERATIVE SOLUTION
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
0 5 10 15 20
Iteration Number
Vd
Is 2.00E-06 Ampsemf 10 VoltsR 1000 Ohmsn 1
ε
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A PN Junction Diode and Its Symbol
Black band corresponds to the "point" in the diode symbol on the right.
It points as "point" is spelled, with "p" first, and"n" later.
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p- Substrate
Cathode Anode
pn junction diode
Metal contact
Heavily doped p region Heavily doped n region
Basic Symbol and Structure of the pn Junction Diode
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Light Emitting Diodes As electrons and holes pass thru the forward biased junction, some annihilate 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 Voltage Uo
+ + + +
- - - -
- +
hf
Activation Energy
Forbidden zone
Fermi level
Fermi level
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Vdiode ! 0.6V
V >Vdiode zd = r and VOUT = (R
r + R)V = ( 1
1+ r / R)V ! (1− r / R) V !V −Vdiode
V <Vdiode zd = ∞ and VOUT = (R
∞ + R)V = 0
zd
R
~0.6V
V Vout
Vout
Simple Diode Circuit
~0.6V
zd
R V
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AC-DC ConverterCapacitor charges to peak voltage VOUT ~VS − 0.6VVoltage decays on down − cycle VOUT =VS e
− t /RC
VOUT (t) =VS (1− t / RC + ... ) t / RC <<1
ΔV = VOUT (t)−VS = VSRtC
= if C
ripplevoltage
r = ΔV /VOUT ripple factorVOUT ≅ VS − 0.6V We have produced a DC voltage from AC input.
Half Wave Rectifier - AC-to-DC Conversion zd
R Vs C
Vout
Vout
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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 power wasted and a reduced ripple factor:
C R
V2= (N2/N1)V1 Vout = 12 V2
r =12
if C( )
Vout
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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
+ +
++ ++
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Voltage Doubler Circuit
V=2Vo
C
C
D1
D2 Q
Q
Vo
• Capacitors are charged to Q by Vo sin(wt) after a few cycles.
• Charge can not leak off due to diodes. T
• Total Voltage sum is V=2Vo (doubler)!
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Zener Diode / Voltage regulator
Zener breakdown Voltage ~ -VZ
0.6 V +V -V
• A Zener works in reverse bias mode. It is be designed to break down at the precise voltage called VZener ~10-100V eg. • Point a in the circuit is fixed to the Zener voltage when Vin > Vz. • The Zener diode is used for tight DC voltage regulation.
V Vout
R
RL
Vz a
ChooseRto limit the current thru the Zener diode (IZmax ≈ 50ma)
IL =VZ / RL
I = (V −VZ ) / RIZ = I − IL < IZmax
(V −VZ ) / R −VZ / RL < IZmax
(V −VZ ) / R< IZmax +VZ / RL
R > (V −VZ )RL
IZmaxRL +VZ> (10V − 5V )1000Ω
0.005A ⋅1000Ω + 5V= 91Ω
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Input/Output Protection with Diodes
A diode placed in series with the positive side of the power supply is called a reverse protection diode. It ensures that current can only flow in the positive direction, and the power supply only
applies a positive voltage to your circuit.
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Input/Output Protection with Diodes
• The 470Ω resistor in series with the LED limits the forward bias current flowing through the LED typically Ilimit< 20ma. • 9V - 470 I – 0.6V = 0 so I = 8.4V/470Ω =18ma (safe).
• Id = 0 in most cases where point a is maintained with a positive voltage.
• If someone attached the battery in the reverse directions the LED would blow out w/o the protection diode. The diode allows the current to bypass the LED!
i
id a
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• Consider a circuit with a power supply and an inductive load (e.g. motor) on it. From the instant the switch is closed, the inductive load will accumulate stored energy. When the switch is opened this energy will generate a high reverse voltage and arc across the contacts of the switch. This could damage the switch, load and other circuit components.
• A power diode placed across the inductive load will provide a path for the release
of energy stored in the inductor while the inductive load voltage drops to zero.
Load Protection with Diodes
- +
- +