Download PED 2023 CHAP 01 Semiconductor Theory

Chapter 1:
Semiconductor Theory
Semiconductor Materials: Ge, Si, and GaAs
Semiconductors are a special class of elements having a
conductivity between that of a good conductor and
that of an insulator.
• They fall into two classes : single crystal and compound
• Single crystal : Germanium (Ge) and silicon (Si).
• Compound : gallium arsenide (GaAs),
cadmium sulfide (CdS),
gallium nitride (GaN),
gallium arsenide phosphide (GaAsP)
The three semiconductors used most frequently in the
construction of electronic devices are Ge, Si, and GaAs.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
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Group →
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
↓ Period
1
1
H
2
He
2
3
Li
4
Be
5
B
6
C
7
N
8
O
9
F
10
Ne
3
11
N
a
12
M
g
13
Al
14
Si
15
P
16
S
17
Cl
18
Ar
4
19
K
20
Ca
21
S
c
22
Ti
23
V
24
Cr
25
Mn
26
Fe
27
Co
28
Ni
29
Cu
30
Zn
31
Ga
32
Ge
33
As
34
Se
35
Br
36
Kr
5
37
R
b
38
Sr
39
Y
40
Zr
41
Nb
42
Mo
43
Tc
44
Ru
45
Rh
46
Pd
47
Ag
48
Cd
49
In
50
Sn
51
Sb
52
Te
53
I
54
Xe
6
55
Cs
56
Ba
*
72
Hf
73
Ta
74
W
75
Re
76
Os
77
Ir
78
Pt
79
Au
80
Hg
81
Tl
82
Pb
83
Bi
84
Po
85
At
86
Rn
7
87
Fr
88
Ra
**
104
Rf
105
Db
106
Sg
107
Bh
108
Hs
109
Mt
110
Ds
111
Rg
112
Uub
113
Uut
114
Uuq
115
Uup
116
Uuh
117
Uus
118
Uuo
* Lanthanides
57
La
58
Ce
59
Pr
60
Nd
61
Pm
62
Sm
63
Eu
64
Gd
65
Tb
66
Dy
67
Ho
68
Er
69
Tm
70
Yb
71
Lu
** Actinides
89
Ac
90
Th
91
Pa
92
U
93
Np
94
Pu
95
Am
96
Cm
97
Bk
98
Cf
99
Es
100
Fm
101
Md
102
No
103
Lr
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
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History
• Diode , in 1939 was using Ge
• Transistor, in 1947 was using Ge
• In1954 Si was used in Transistor because Si is less
temperature sensitive and abundantly available.
• High speed transistor was using GaAs in 1970 (which is 5
times faster compared to Si)
• Si, Ge and GaAs are the semiconductor of choice
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
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Atomic Structure
Valence shell (4 valence electrons)
Valence shell (4 valence electrons)
Valence
electron
shells
+
+
Valence
electron
Nucleus
orbiting
electrons
orbiting
electrons
Germanium
Silicon
32 orbiting electrons
(tetravalent)
14 orbiting electrons
(Tetravalent)
• Valence electrons: electrons in the outermost shell.
• Atoms with four valence electrons are called tetravalent.
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Atomic Structure
Valence shell (3 valence electrons)
Valence shell (5 valence electrons)
Valence
electron
shells
shells
+
Nucleus
Valence
electron
+
orbiting
electrons
Nucleus
Gallium
orbiting
electrons
Arsenic
31 orbiting electrons
(trivalent)
33 orbiting electrons
(pentavalent)
• Atoms with three valence electrons are called trivalent, and
those with five are called pentavalent.
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Covalent Bonding
Covalent bonding of Si crystal
This bonding of atoms, strengthened by the sharing of electrons,
is called covalent bonding
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Covalent Bonding
There is sharing of
electrons, five electrons
provided by As atom and
three by the Ga atom.
Covalent bonding of GaAs crystal
Electronic Devices and Circuit Theory, 10/e
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
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Energy Levels
The farther an electron is from the nucleus, the higher is the
energy state.
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Energy Levels
An electron in the valence band of silicon must absorb more energy than
one in the valence band of germanium to become a free carrier. [free
carriers are free electrons due only to external causes such as applied
electric fields established by voltage sources or potential difference.
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n-Type and p-Type materials
n-Type Material
n-Type materials are created by
adding elements with five valence
electrons such as antimony, arsenic,
and phosphorous.
There is a fifth electron due to
the (Sb) atom that is relatively free
to move in the n-Type material.
The atoms (in this case is
antimony (Sb)) are called donor
atoms.
Doping with Sb, (antimony)
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n-Type and p-Type materials
n-Type Material
The free electrons due to the added atoms have higher energy
levels and require less energy to move to conduction band.
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n-Type and p-Type materials
p-Type Material
p-Type materials are created by
adding atoms with three valence
electrons such as boron, gallium, and
indium.
In this case, an insufficient
number of electrons to complete the
covalent bonds.
The resulting vacancy is called a
“hole” represented by small circle or
plus sign indicating absence of a
negative charge.
Boron (B)
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The atoms (in this case boron(B))
are called acceptor atoms.
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Majority and Minority carriers
Two currents through a diode:
Majority Carriers
•The majority carriers in n-type materials are electrons.
•The majority carriers in p-type materials are holes.
Minority Carriers
•The minority carriers in n-type materials are holes.
•The minority carriers in p-type materials are electrons.
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p-n Junctions
One end of a silicon or germanium crystal can be doped as a ptype material and the other end as an n-type material.
The result is a p-n junction.
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p-n Junctions
At the p-n junction, the
conduction-band electrons
n-type side are attracted
valence-band holes on the
side.
excess
on the
to the
p-type
The electrons in the n-type
material migrate across the
junction to the p-type material
(electron flow).
The result is the formation of
a depletion region around
the junction.
The electron migration results in a
negative charge on the p-type side
of the junction and a positive
charge on the n-type side of the
junction.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
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Diodes
The diode is a 2-terminal device.
A diode ideally conducts in only
one direction.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
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Diode Operating Conditions
Reverse bias
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
• No bias
• Forward bias
• Reverse bias
Forward bias
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Diode Operating Conditions
No Bias
•
•
•
No external voltage is applied: VD = 0 V
No current is flowing: ID = 0 A
Only a modest depletion region exists
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Robert L. Boylestad and Louis Nashelsky
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Diode Operating Conditions
Reverse Bias
External voltage is applied across the p-n junction in
the opposite polarity of the p- and n-type materials.
•
•
•
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The reverse voltage causes the
depletion region to widen.
The electrons in the n-type material
are attracted toward the positive
terminal of the voltage source.
The holes in the p-type material are
attracted toward the negative
terminal of the voltage source.
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Diode Operating Conditions
Forward Bias
External voltage is applied across the p-n junction in
the same polarity as the p- and n-type materials.
•
•
•
Electronic Devices and Circuit Theory, 10/e
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The forward voltage causes the
depletion region to narrow.
The electrons and holes are pushed
toward the p-n junction.
The electrons and holes have
sufficient energy to cross the p-n
junction.
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Actual Diode Characteristics
Note the regions for no
bias, reverse bias, and
forward bias conditions.
Carefully note the scale
for each of these
conditions.
The reverse saturation
current is seldom more
than a few microamperes.
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Robert L. Boylestad and Louis Nashelsky
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Diode equation
where
VT : is called the thermal voltage.
Is : is the reverse saturation current.
VD : is the applied forward-bias voltage across the diode.
n : is a factor function of operation conditions and physical
construction. It has range between 1 and 2. assume n=1 unless
otherwise noted.
K : is Boltzman’s constant =1.38 x 10-23
T: is temperature in kelvins = 273+temperature in C.
q : is the magnitude of electron charge = 1.6 x 10-19 C.
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Zener Region
The Zener region is in the diode’s reverse-bias region.
At some point the reverse bias voltage is so large the
diode breaks down and the reverse current increases
dramatically.
• The maximum reverse voltage
that won’t take a diode into the
zener region is called the peak
inverse voltage (PIV) or peak
reverse voltage (PRV).
• The voltage that causes a diode
to enter the zener region of
operation is called the zener
voltage (VZ).
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
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Forward Bias Voltage
The point at which the diode changes from no-bias condition to
forward-bias condition occurs when the electrons and holes are
given sufficient energy to cross the p-n junction. This energy
comes from the external voltage applied across the diode.
The forward bias voltage required for a:
• gallium arsenide diode  1.2 V
• silicon diode  0.7 V
• germanium diode  0.3 V
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Comparison Ge, Si, GaAs
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Temperature Effects
As temperature increases it adds energy to the diode.
• It reduces the required forward bias voltage for forward-bias
conduction.
• It increases the amount of reverse current in the reverse-bias
condition.
Germanium diodes are more sensitive to temperature variations
than silicon or gallium arsenide diodes.
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Temperature Effects
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Resistance Levels
Semiconductors react differently to DC and AC currents.
There are three types of resistance:
• DC (static) resistance
• AC (dynamic) resistance
• Average AC resistance
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Robert L. Boylestad and Louis Nashelsky
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DC (Static) Resistance
For a specific applied DC voltage
V D, the diode has a specific current
I D, and a specific resistance R D.
VD
RD 
ID
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AC (Dynamic) Resistance
The dynamic resistance is the resistance
offered by the diode to the AC signal. Is is
equal to the slope of the VI characteristics
(dV/dI or ΔV/ ΔI ) ,
change in voltage
dV V
rD 


resulting change in current dI
I
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AC (Dynamic) Resistance

sin ce I D  I s e
V D / nV T

I s V D / nVT
dI D
-1 ,

e
dV D nV T
dI D
dI D
ID
1


 I D  I s  , sin ce I D  I s ,
dV D nV T
dV D nV T
dV D
nV T
 rD 
dI D
ID
for n=1, and at room temperature of 27o C, T=273+27=300K
KT
VT 
q
1.38 10 


 26mV
23
1.6 1019
26mV
rD 
ID
Electronic Devices and Circuit Theory, 10/e
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AC (Dynamic) Resistance
rd 
In the forward bias region:
26 mV
 rB
ID
• The resistance depends on the amount of current (I D) in the diode.
• r D = 26 mV/ID is the resistance of the p-n junction and does not
include the resistance of the semiconductor material itself (the body
resistance).
• r B is added to account for body resistance and it ranges from a
typical 0.1  to 2 .
In the reverse bias region:
rd  
The resistance is effectively infinite. The diode acts like an open.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
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Average AC Resistance
rav
ΔVd

ΔId
pt. to pt.
AC resistance can be
calculated using the current
and voltage values for two
points on the diode
characteristic curve.
Electronic Devices and Circuit Theory, 10/e
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Diode Equivalent Circuit
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Diode Specification Sheets
Data about a diode is presented uniformly for many different diodes.
1. Forward Voltage (VF) at a specified current and temperature
2. Maximum forward current (IF) at a specified temperature
3. Reverse saturation current (IR) at a specified voltage and
temperature
4. Reverse voltage rating, PIV or PRV or V(BR), at a specified
temperature
5. Maximum power dissipation at a specified temperature
6. Capacitance levels
7. Reverse recovery time, t rr (is the time required for a diode to stop
conducting once it is switched from forward bias to reverse bias)
8. Operating temperature range
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Diode
Specification
Sheets
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Diode Symbol and Packaging
The anode is abbreviated A
The cathode is abbreviated K
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Diode Testing - Ohmmeter
An ohmmeter set on a low Ohms scale can be used
to test a diode. The diode should be tested out of
circuit.
Electronic Devices and Circuit Theory, 10/e
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Other Types of Diodes
Zener diode
Light-emitting diode
Diode arrays
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Robert L. Boylestad and Louis Nashelsky
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Zener Diode
•A Zener diode is a type of diode that permits current
not only in the forward direction like a normal diode,
but also in the reverse direction if the voltage is larger
than the breakdown voltage known as "Zener voltage“
(VZ).
•Common Zener voltages are between 1.8 V and 200 V.
•Zener diode is used as regulator (circuits will be shown
in chapter 2).
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
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Light-Emitting Diode (LED)
•An LED emits photons when it is forward biased.
•These can be in the infrared or visible spectrum.
•The forward bias voltage is usually in the range of 2 V to 5 V.
Light-Emitting Diodes
Color
Construction
Typical Forward
Voltage (V)
Amber
AlInGaP
2.1
Blue
GaN
5.0
Green
GaP
2.2
Orange
GaAsP
2.0
Red
GaAsP
1.8
White
GaN
4.1
Yellow
AlInGaP
2.1
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Diode Arrays
Multiple diodes can be
packaged together in an
integrated circuit (IC).
A variety of combinations
exist.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common Anode
Common Cathode
21
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