C11082 Analog and Digital
Electronics for Chemist- Analog
Electronics
Hiran H E Jayaweera
hiran.jayaweera@gmail.com
1
Course Content
• Introduction to P &N type semiconductors, P-N junction diode and
its action under forward-bias and reverse-bias conditions
• Diode as a circuit element, Diode models, Rectifier circuits, Zener
diodes,
• Voltage regulation and low voltage DC power supply, Limiting and
clamping circuits, Special diode types (LED, Photo diode etc), Seven
segment and other display devices and their applications.
• Bipolar transistors, Operation of an npn transistor in the active
mode, Transistor biasing and transistor as an amplifier,
• Designing of a common emitter amplifier, Voltage gain, Transistor
as a switch,
• Introduction to field effect transistors, JFETs and MOSFETs,
• Operational amplifiers, Inverting and non-inverting amplifiers,
Summing amplifiers, Op-amp based electronic ammeters and
voltmeters,
• Semiconductor device applications in chemical industry
2
What is Electronics?
3
What is Electronics?
• Electronics is a branch of Physics that studies
the flow of electrons through various
materials, and devices
4
ELECTRICITY
• Everything is made of atoms
• An atom consists of electrons, protons and
neutrons
5
Resistivity
• AKA Electrical resistivity or specific electrical
resistance
• Insulators  > 105  cm (diamond  = 1016 )
• Semiconductors 10-3 <  < 105  cm
• Conductors  < 10-3  cm (copper  = 10-6 )
6
Atomic structure
• Electrons with the highest energy levels exist
in the outermost shell (called valence
electrons)
• The term valence is used to indicate the
potential required to removed any one of
these electrons
7
Band theory of solids
• An isolated atom: the electrons in each orbit have
definite energy associated with it
• In solids (atoms are close to each other): energy levels
of outermost orbit electrons are affected by the
neighboring atoms
• So the electrons in same orbit exhibits different energy
levels called energy band
• Energy levels of inner orbit electrons are not much
affected by the presence of neighboring atoms
8
9
Band theory of solids
10
Badgap
• Bandgap is an energy range in a solid where
no electron states can exist. It refers to the
energy difference between the top of the
valence band and the bottom of the
conduction band in insulators and
semiconductors
11
At room temperature
12
Silicon Covalent Bond Model (cont.)
Silicon atom
13
Silicon Covalent Bond Model (cont.)
Covalent bond
Silicon atom
Silicon atom
14
Silicon Covalent Bond Model (cont.)
Silicon atom
Covalent bonds in silicon
15
16
Silicon Covalent Bond Model (cont.)
•
•
•
•
What happens as the temperature
increases?
Near absolute zero, all bonds are complete
Each Si atom contributes one electron to
each of the four bond pairs
The outer shell is full, no free electrons,
silicon crystal is an insulator
17
Silicon Covalent Bond Model (cont.)
•
•
•
Near absolute zero, all bonds are complete
Each Si atom contributes one electron to
each of the four bond pairs
The outer shell is full, no free electrons,
silicon crystal is an insulator
•
•
•
Chap 2 - 18
Increasing temperature adds energy to the
system and breaks bonds in the lattice,
generating electron-hole pairs.
The pairs move within the matter forming
semiconductor
Some of the electrons can fall into the
holes – recombination.
Semiconductor Doping
• The interesting properties of semiconductors emerges
when impurities are introduced.
19
Semiconductor Doping
• The interesting properties of semiconductors emerges
when impurities are introduced.
• Doping is the process of adding very small well controlled
amounts of impurities into a semiconductor.
20
Semiconductor Doping
• The interesting properties of semiconductors emerges
when impurities are introduced.
• Doping is the process of adding very small well controlled
amounts of impurities into a semiconductor.
• Doping enables the control of the resistivity and other
properties over a wide range of values.
21
Semiconductor Doping
• The interesting properties of semiconductors emerges
when impurities are introduced.
• Doping is the process of adding very small well controlled
amounts of impurities into a semiconductor.
• Doping enables the control of the resistivity and other
properties over a wide range of values.
• For silicon, impurities are from columns III and V of the
periodic table.
22
Donor Impurities in Silicon
•
•
•
Phosphorous (or other column V
element) atom replaces silicon atom
in crystal lattice.
Since phosphorous has five outer
shell electrons, there is now an
‘extra’ electron in the structure.
Material is still charge neutral, but
very little energy is required to free
the electron for conduction since it is
not participating in a bond.
Chap 2 - 23
Donor Impurities in Silicon
q
•
•
•
Phosphorous (or other column V
element) atom replaces silicon atom
in crystal lattice.
Since phosphorous has five outer
shell electrons, there is now an
‘extra’ electron in the structure.
Material is still charge neutral, but
very little energy is required to free
the electron for conduction since it is
not participating in a bond.
q
e
A silicon crystal doped by a pentavalent element
(f. i. phosphorus). Each dopant atom donates a
free electron and is thus called a donor. The
doped semiconductor becomes n type.
Chap 2 - 24
Acceptor Impurities in Silicon
•
•
•
•
Boron (column III element) has been
added to silicon.
There is now an incomplete bond
pair, creating a vacancy for an
electron.
Little energy is required to move a
nearby electron into the vacancy.
As the ‘hole’ propagates, charge is
moved across the silicon.
Chap 2 - 25
Acceptor Impurities in Silicon
•
•
•
•
Boron (column III element) has been
added to silicon.
There is now an incomplete bond
pair, creating a vacancy for an
electron.
Little energy is required to move a
nearby electron into the vacancy.
As the ‘hole’ propagates, charge is
moved across the silicon.
q
e
q
Vacancy
A silicon crystal doped with a trivalent impurity (f.i.
boron). Each dopant atom gives rise to a hole, and
the semiconductor becomes p type.
Chap 2 - 26
Acceptor Impurities – Hole
propagation
Hole is propagating through the silicon.
27
Acceptor Impurities – Hole
propagation
e
Hole
Hole is propagating through the silicon.
28
Acceptor Impurities – Hole
propagation
Hole
Hole is propagating through the silicon.
29
Acceptor Impurities – Hole propagation
e
Hole is propagating through the silicon.
30
Doped Silicon Carrier Concentrations
• In doped material, the electron and hole
concentrations are no longer equal.
• If n > p, the material is n-type.
If p > n, the material is p-type.
• The carrier with the largest concentration is the
majority carrier, the smaller is the minority carrier.
31
Electronic Properties of Si
• Silicon is a semiconductor material.
–
Pure Si has a relatively high electrical resistivity at room temperature.
• There are 2 types of mobile charge-carriers in Si:
–
–
Conduction electrons are negatively charged;
Holes are positively charged.
• The concentration
(#/cm3) of conduction electrons & holes in a
semiconductor can be modulated in several ways:
1.
2.
3.
4.
by adding special impurity atoms ( dopants )
by applying an electric field
by changing the temperature
by irradiation
32
Terminology
donor: impurity atom that increases n
acceptor: impurity atom that increases p
N-type material: contains more electrons than holes
P-type material: contains more holes than electrons
majority carrier: the most abundant carrier
minority carrier: the least abundant carrier
intrinsic semiconductor: n = p = ni
extrinsic semiconductor: doped semiconductor
33
Semiconductor usage in electronics
• Ge was widely used in the early days of
semiconductor development for transistors
and diodes
• Si is now used for the majority of rectifiers,
transistors and integrated circuits
• C in crystalline form is diamond and at room
temperature it is an insulator
• Diamonds are used in high temperature
application (400 C)
34
Possible Semiconductor Materials
Carbon
Silicon
1. Very Expensive
2. Band Gap Large: 6eV
3. Difficult to produce without high contamination
C
6
Si
1. Cheap
2. Ultra High Purity
14
3. Oxide is amazingly perfect for IC applications
(insulator)
1. High Mobility
Germanium Ge 32 2. High Purity Material
3. Oxide is porous to water/hydrogen (problematic)
35
Drift and diffusion
Drift is due to the application of electric field. Higher the filed the faster charges move
Diffusion is from higher charge density to lower charge density.
37
PN junction
38
PN junction
• As free electrons and holes diffuse across the
junction, a region of fixed ions is left behind.
This region is known as the “depletion region.”
39
Barrier potential
I drift , p  I diff , p
I drift ,n  I diff ,n
• The fixed ions in depletion region create an electric field that
results in a drift current.
• At equilibrium, the drift current flowing in one direction cancels
out the diffusion current flowing in the opposite direction,
creating a net current of zero.
40
Zero biased mode
• At zero bias (vD=0),
very few electrons or
holes can overcome
this built-in voltage
barrier of ~ 0.7 volts
(and exactly
balanced by
diffusion)

iD = 0
41
Reverse biased mode
When the N-type region of a
diode is connected to a higher
potential than the P-type
region, the diode is under
reverse bias, which results in
wider depletion region and
larger built-in electric field
across the junction.
Is
vD
42
Voltage dependant capacitor
The PN junction can be viewed as a capacitor. By varying VR, the
depletion width changes, changing its capacitance value; therefore,
the PN junction is actually a voltage-dependent capacitor.
Cj = eA/W, where W depends on the bias voltage
43
Forward biased mode
When the N-type region of a
diode is at a lower potential
than the P-type region, the
diode is in forward bias.
The depletion width is
shortened and the built-in
vD
electric field decreased.
44
Forward biased mode
• In forward bias, there
are large diffusion
currents of minority
carriers through the
junction. However, as
we go deep into the P
and N regions,
recombination currents
from the majority
carriers dominate.
These two currents add
up to a constant value.
45
IV Characteristic of PN Junction
VD
I D  I S (exp  1)
VT
• The current and voltage relationship of a PN
junction is exponential in forward bias region, and
relatively constant in reverse bias region.
46
Diode applications
51
Ideal diode
i
Anode
Cathode
Reverse bias
Forward bias-0
V
Diode symbol
i-v characteristic
52
Diode and mechanical switch
i
i
Compare and contrast the
electrical characteristic of an ideal
diode operation and mechanical
switch
+
-
v
(v < 0  i = 0)
Equivalent circuit in the reverse
direction
+
-
v
( i > 0  v = 0)
Equivalent circuit in the forward
direction
53
i=0
i
v=0
v
Opened
switch
i
Closed
switch
v
i – v characteristic
What is PN junction ….?
The junction diode
• The pn junction diode produces i-v
characteristic nearly same as that of an ideal
diode
• Silicon and germanium are the common
semiconductor materials used to fabricate
junction diodes.
56
The junction diode – CONT.
Diode current (mA)
12
Si
Ge
10
8
6
4
2
0.2
0.4
0.6
0.8
1.0
1.2
Applied voltage (V)
Barrier potential at room temperature -
0.2 V Ge
0.7 V Si
57
Find the current I and the voltage
+5 V
I
V
2.5 k
2.5k
V
I
-5 V
(a)
(b)
58
Find the current I and the voltage V
+3 V
+2 V
+1 V
I
1 k
V
59
Discover yourself
1. Study how junction diodes can be used for
signal rectifying purposes
60
Battery backup
Vout
230 V
main
supply
DC
15 V
Regulat
or
15 V to 10
V
electronic
clock
12 V Battery
61
Zener diode
Diod
e
curre
nt ( I
)
-V
Z0
-VZ
- VZK
0
I
Slope =
- IZK
V
=
1
rz
Applied voltage (V)
Q
-IZT (Test current)
V
I
Zener diodes are fabricated with breakdown voltages
(also known as zener voltages) in the range of a few
volts to a few hundred volts.
62
Zener diode as a regulator
Vunreg.
VRs
VZ
t
VRs
Iin
t
RS
To the
rectifier
C
Vunregulated
+
VO (= VZ)
RL
-
V0
VZ
t
63
Vunreg.
VRs
VZ
t
VRs
Iin
To the
rectifier
C
Vunregulated
t
RS
+
VO (= VZ)
RL
-
V0
VZ
t
64
RS
12 V
RL
A 7.5 V zener diode is used in the circuit shown below,
and the load current is to vary from 10 mA to 100 mA.
Find the value of RS required to maintain this load
current if the supply voltage is 12 V. Take the knee
current of the zener diode as 10 mA.
65
Peak detector
V
vP
C
vP
t
•VP of a signal is very much greater the diode will be forward
biased
•Capacitor will be charged to the peak value of the pulse
•When peak has passed, the diode will be reverse biased
and the charge on the capacitor will be trapped
66
Voltage doubler
D1
v
+
C1
vP
t
2vP
C2
vP
_
D2
•During positive half cycle, the diode D1 conducts
•Capacitor C1 will charge up to the peak voltage vp
•The diode D1 becomes reverse- bias and the charge on C1 will
get trapped.
•In the similar manner, the diode D2 will be forward biased
during the second half cycle, and C2 will receive a voltage of vp,
and therefore the output voltage will become 2vP .
67
Voltage clipper
R
vs
vo
D1
t
V1
D2
V2
t
The diode D1 conducts when its anode voltage is
greater than V1, and D2 conducts when its cathode
voltage is less than V2, limiting the output voltages to
the respective power supply voltages.
68
Clamping a waveform at a positive voltage
C
v0
vi
0
v0
t
V1
V1
0
t
69
(1)The zener diode in the circuit shown below has following
specifications.
•Zener voltage = 5 V
•Minimum zener current that must be maintained in order to
ensure proper operation of the diode = 2 mA.
•Maximum allowable power dissipation of the diode = 1W.
RS
12 V
RL
Find the maximum current that can be sent safely
through the zener diode.
(a)Calculate the smallest possible value for RL.
(b)If RL is a variable resistor, resistance of which can
be increased up to infinity, calculate a suitable value for
RS.
?????????????????
• Design a power supply capable of producing
5 V DC signal from the 230 V AC source.
LED (Light Emitting Diode)
• Indicators
– High speed response
– Low power dissipation
Tri-Colour LED
LED Displays
a
a
a
b
c
d
e
f
g
f
b
g
c
e
Cathode
terminals
d
Common anode
[To be connected
to a positive
polarity of a
battery (5 V)]
Displaying more digits
Other displays – 16/14 segments
a2
a1
f
k
g1
e
h l
g2
m j
d1
n
d2
b
c
7 x 5 dot-matrix
R1
R2
R3
R4
R5
R6
R7
C1 C2 C3 C4 C5
Photodiode
p
n
Circuit symbol
+
_
• energy of the incident photon  atoms
photo-electrons
• Photo Electric Effect
• hf= +KE
Photovoltaic and Photoconductive
•
Additional carriers  increase in the
conductivity (Photoconductive)
Reverse biased
•
A voltage is generated across a cell when
light shines upon it.  voltage cause
current to flow (Photovoltaic)
Forward biased
Phot
odio
de
curr
ent
Reverse bias
Forward bias
I=0
I = 100 W/cm2
I = 200 W/cm2
-5V
- 200 A
- 400 A
500 mV
Photodiode voltage