Download Chapter-5: Introduction to Semi

Chapter-5: Introduction to Semi-Conductors
and Diode Circuits
Conduction band
3p
Energy gap
Valence band
3s
2p
2s
1s
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.
1
Energy Bands
Electron Energy
Electron Energy
Electron Energy
Conduction
Band
Valence Band
Overlapping bands
- little energy is
needed for
conduction
Energy Gap
Energy Gap
Conduction
Band
Conduction
Band
Valence Band
Valence Band
Insulator
Conductor
Semiconductor
2
IA
VIIIA
2 4.002
1
1.008
H
Hydroge
n
3 6.939
Li
IIA
4
Be
Berylliu
Lithiu
m
m
11 22.98 12 24.312
9
Mg
K
Ca
IVA
VA
VIA
VIIA
B
C
N
O
F
6
Heliu
m
5 10.81 6 12.01 7 14.00 8 15.99 9 18.99 10 20.18
1
1
7
9
8
3
9.01
2
Na
IIIA
He
Ne
Florin
Boron
Carbon Nitroge Oxygen
Neon
n
e
26.98
28.08
30.97
32.06
35.45
13
14
15
16
17
18 39.94
1
6
4
4
3
8
Transition Metals
Al
Si
P
S
Cl
Ar
Ga
Ge
As
Se
Br
Kr
IVB VB
VIB VIIB
VIIIB
IB IIB Aluminum Silicon Phosphorus Sulfur Chlorin Argon
Sodiu Magnesium IIIB
e
m
19 39.10 20 40.08 21 44.95 22 47.90 23 50.94 24 51.99 25 54.93 26 55.84 27 58.93 28 58.71 29 63.54 30 65.37 31 69.72 32 72.59 33 74.92 34 78.96 35 79.90 36 83.80
2
6
2
6
8
7
3
2
9
Sc
Ti
V
Cr
Mn
Fe
Co
Cu
Ni
Zn
Potassiu Calcium Scandium Titaniu Vanadium Chromiu Manganese Iron
Nickel
Zinc
Cobalt
Coppe
Galliu Germaniu Arseni
Seleniu Bromin Krypton
m
m
m
m
r
m
c
m
e
37 85.47 38 87.62 39 88.90 40 91.22 41 92.90 42 95.94 43 99 44 101.0 45 102.91 46 106.4 47 107.8 48 112.4 49 114.8 50 118.6 51 121.7 52 127.6 53126.90 54 131.3
5
6
7
7
0
9
5
0
4
0
2
Rb
Sr
Y
Zr
Nb
Tc
Mo
Ru
Pd
Rh
Ag
Cd
Sn
In
Sb
Te
I
Xe
Rubidiu Strontium Yttrium Zirconium Niobium Molybde- TechnitiumRuthenium Rhodiu Palladium Silver Cadmium Indium
Tin
Antimony Tellurium Iodine
Xenon
num
m
m
1137.34 57138.91 72178.49 73180.95 74183.85 75 186.2 76 190.2 77 192.2 78195.09 79196.967 80200.59 81204.37 82207.19 83208.98 84 210 85 210 86 222
55132.90 56
Cs
Ba
La
Hf
Ta
Cesium Barium Lanthanum Hafniu
m
87 223 88 226 89 227 104
Fr
Ra
Francium Radium
Ac
Re
W
Ir
Os
Tantalum Tungsten Rhenium Osmium
105
Rf
106
Ha
107
Sg
108
Uns
Pt
Iridium Platinum
109
Uno
Hg
A
Gold
u
Tl
Pb
Mercury Thallium
At
Bismuth Poloniu
m
Lead
110
Une
Po
Bi
Rn
Astatine
Radon
Nonmetals
Uun
Actiniu
m
Metalloids
(semimetals)
58 140.1 59140.91 60 144.2 61 147 62 150.3 63 151.9 64 157.2 65 158.9 66 162.5 67 164.9 68 167.2 69 168.9 70 173.0 71 174.9
Lanthanides
Ce
2
Cerium
Th
Nd
4
Praseodym- Neodym
ium
-ium
90232.04 91
Actinides
Pr
Pm
Pa
U
5
Eu
6
Gd
5
Prome- Samarium Europium Gadolinium
thium
231 92 238.0 93
3
ProcatThorium inium
Sm
Tb
Terbium
237 94 242 95 243 96 247 97
Np
Pu
Am
Cm
2
Dy
0
Ho
3
Er
6
Tm
3
Yb
4
Lu
7
Dyspro- Holmium Erbium
sium
Thuliu
Ytterbiu Lutetium
m
m
24 98 249 99 254 100 253 101 256 102 253 103 257
7
Bk
Cf
CaliforUranium Neptunium Plutoniu Americium Curium Berkelium nium
m
Es
Einsteinium
Fm
Md
Mendelev
Fermium
-ium
No
Nobeliu
m
Lr
Lawrencium
3
Group IVA Elemental Semiconductors
Carbon
Silicon
Germanium
Tin
Lead
C
Si
Ge
Sn
Pb
6
• Very Expensive diamond
• Band Gap Large: 6V
• Difficult to produce without high contamination
14
• Cheap
• Ultra High Purity
• Oxide is amazingly perfect for IC applications
32
• High Mobility (good)
• High Purity Material (good)
• Oxide is porous to water/hydrogen (disasterous!)
50
82
• Only “White Tin” is semiconductor
• Converts to metallic form under moderate heat
• Only “White Lead” is semiconductor
• Converts to metallic form under moderate heat
4
Covalent Bonding of Pure Silicon
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.
5
Electrons in N-Type Silicon with Phosphorus Dopant
Si
Si
Si
Si
Si
Si
Si
Si
P
Si
Si
P
Si
Si
Si
Si
Si
Si
P
Si
Si
Si
Si
Si
Si
Excess electron (-)
Phosphorus atom
serves as n-type
dopant
Donor atoms provide excess electrons
to form n-type silicon.
6
Holes in p-Type Silicon with Boron Dopant
Si
Si
Si
Si
Si
Si
Si
Si
B
Si
+ Hole
Si
B
Si
Si
Si
Boron atom serves
as p-type dopant
Si
Si
Si
B
Si
Si
Si
Si
Si
Si
Acceptor atoms provide a deficiency
of electrons to form p-type silicon.
7
Silicon Dopants
Acceptor Impurities
Semiconductor
Donor Impurities
Group III (p-type)
Group IV
Group V (n-type)
Boron
Carbon
6
Silicon
14
Germanium
32
5
Aluminum 13
Nitrogen
7
Phosphorus 15
Gallium
31
Arsenic
33
Indium
49
Antimony
51
8
Conduction in n and p-Type Silicon
Positive terminal from
voltage supply
Negative terminal
from voltage supply
ow
e Fl
l
o
H
low
F
n
tro
Elec
-Electrons flow toward
positive terminal
+Holes flow toward
negative terminal
9
Silicon Resistivity Versus Dopant Concentration
1021
Dopant Concentration (atoms/cm3)
1020
1019
1018
1017
n-type
1016
p-type
1015
1014
1013
10-3
10-2
10-1
100
101
102
103
Electrical Resistivity (ohm-cm)
10
n- and p-Type Silicon
Important Facts:
The dominant
Not shown are the silicon
charge carrier
atoms, which are present in
in n-type Si is
vastly greater numbers than
the electron.
either arsenic or boron.
Typical doping concentrations
are 1016 arsenic or boron
3.
atoms
per
cm
The dominant
The concentration of silicon
charge carrier
22
in p-type Si is atoms is about 10 atoms per
cm3.
the hole
For every "dopant" atom,
there are about a million
silicon atoms.
11
Open-Circuit Condition of a p-n Junction Diode
p-type Si
}
Depletion
Region !x
n-type Si
Anode
Cathode
Diffusing electrons and
holes create an electric field
Eo and potential step at the
junction.
Eo
Charge distribution
of barrier voltage
across a pn Junction.
Metal contact
Barrier
voltage
0
Potential
step
12
Forward-Biased PN Junction Diode
p
Eapp
n
Eo
Hole flow
Electron flow
V
Lamp
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.
13
Reverse-Biased PN Junction Diode
Eapp
p
3V
Open-circuit condition
(high resistance)
n
Lamp
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.
14
I-V Characteristic Curve
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)
Reverse current exaggerated; typical
reverse saturation current: IS ~10-(6-12)A.
I = I S (e+ eVD /kT − 1) ~ I S e+ eVD /kT I S = reverse bias saturation current
VD = diode voltage kT = 0.025eV at 300 o K
15
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 er min e the forward diode current VD = 1 V T = 300 0 K and I S = 10 pA.
I = 10 pA exp(1eV / 0.025eV ) = 2.4e9A !!
16
Solving Diode Equation by Iteration
ε − Vd − IR = 0 Kirchhoff ' s law ε − Vd
I=
= I S ( eeVd /nkT − 1) n ≡ diode realty factor n ~ 1.5
R
⎛ I
⎞
eVd
= ln ⎜ + 1⎟ nkT
⎝ IS
⎠
ε
I
⎛ ε − Vd
⎞
⎛ n⎞
Vd = ⎜ ⎟ kT ln ⎜
+ 1⎟ Solve by iteration
⎝ e⎠
⎝ IS R
⎠
DIODE EQ. ITERATIVE SOLUTION
1.40
1.20
Is
emf
R
n
2.00E-06
10
1000
1
Amps
Volts
Ohms
1.00
Vd
(1)
Vd
(2)
⎛ ε − Vd (0)
⎞
⎛ n⎞
= ⎜ ⎟ kT ln ⎜
+ 1⎟
⎝ e⎠
⎝ IS R
⎠
⎛ ε − Vd (1)
⎞
⎛ n⎞
= ⎜ ⎟ kT ln ⎜
+ 1⎟
⎝ e⎠
⎝ IS R
⎠
Vd
.
0.80
Stop when Vd ( N +1) ~ Vd ( N )
0.60
0.40
http://en.wikipedia.org/wiki/Diode_modelling#Shockley_diode_model
0.20
0.00 0
5
10
Iteration Number
15
20
17
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.
18
Basic Symbol and Structure of the pn Junction
Diode
pn junction diode
Cathode
Anode
Heavily doped p region
Heavily doped n region
Metal contact
p- Substrate
19
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.
- - - -
hf
Activation Energy
Fermi level
+
Activation Voltage
Uo
Fermi level
+ + + +
Forbidden
zone
E=hc/λ = 1240 eV-nm / λ(nm)
Red light of 650nm would correspond to Uo = 1240/650 eV = 1.9 eV
20
Simple Diode Circuit
~0.6V
zd
R Vout
V
Vdiode ! 0.6V
R
1
)V = (
)V ! (1− r / R) V ! V − Vdiode r+R
1+ r / R
R
= (
)V = 0
∞+ R
V > Vdiode zd = r and VOUT = (
V < Vdiode zd = ∞ and VOUT
zd
V
R
Vout
~0.6V
21
Half Wave Rectifier - AC-to-DC Conversion
zd
Vs
Vout
C
R
Vout
AC - DC Converter
Capacitor ch arg es to peak voltage VOUT ~ VS − 0.6V
Voltage decays on down − cycle VOUT = VS e−t /RC
VOUT (t) = VS (1− t / RC + ... ) t / RC << 1 VS t
i
ΔV = VOUT (t) − VS = =
ripplevoltage
R C f C
r = ΔV / VOUT ripple fa ct or
VOUT ≅ VS − 0.6V We have produced a DC voltage from AC input. 22
Full Wave Rectifier - AC-to-DC Conversion
V2= (N2/N1)V1
120VAC
CR
Vout = 1 2 V2
1 i
r=
f C
2
(
)
Vout
- 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:
23
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
24
Voltage Doubler Circuit
D2
V=2Vo
C
Q
C
Q
D1
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)!
25
Zener Diode / Voltage regulator
• 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.
Zener breakdown
Voltage ~ -VZ
R
a
V
-V
Vz
0.6 V
RL Vout
+V
Choose Rto limit the current thru the Zener diode (IZmax ≈ 50ma)
I L = VZ / RL
I = (V − VZ ) / R
I Z = I − I L < I Z max
(V − VZ ) / R − VZ / RL < I Z max
(V − VZ ) / R < I Z max + VZ / RL
(V − VZ )RL
(10V − 5V )1000Ω
R>
>
= 91Ω
I Z max RL + VZ
0.005A ⋅1000Ω + 5V
26
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.
27
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!
a
id
i
28
Load Protection with Diodes
•  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.
+
-
-
+
29