ELECTRONICS
SMJP2143
CHAPTER 3
Semiconductor Diode
Dr. Nurshafinaz Mohd Maruai
nurshafinaz@utm.my
Recap from previous chapter
1. A semiconductor is a material that has a conductivity level between conductor an insulator.
2. A bonding of atoms, strengthened by the sharing of electrons between neighboring atoms, is
called covalent bonding.
3. Increasing temperatures can cause a significant increase in the number of free electrons in a
semiconductor material.
4. Intrinsic materials are those pure semiconductors whereas extrinsic materials are
semiconductors that have been exposed to a doping process .
6. A p-type material is formed by adding acceptor atoms with three valence electrons to
establish a high level of holes in the material. In a p-type material, the hole is the majority
carrier and the electron is the minority carrier.
7. The region near the junction of a diode that has very few carriers is called the depletion
region.
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5. An n-type material is formed by adding donor atoms that have five valence electrons to
establish a high level of relatively free electrons. In an n-type material, the electron is the
majority carrier and the hole is the minority carrier.
Semiconductor Diode
CLO1 : Apply the fundamental principles of electronic device and circuit.
PN Junction
Forward and Reverse Bias
PN Junction
No Applied Bias
Zener Diode and Solar Cells
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Data Sheet
Chapter Learning Outcomes
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At the end of this class, you should be able to :
1. Develop a clear understanding of the basic operation and characteristic
of diode in the no-bias, forward-bias and reverse-bias region
2. Understand the impact of an equivalent circuit – ideal and practical
3. Become familiar with the operation and characteristic of Zener diode and
Solar Cells.
PN Junction
Depletion Region
n-type
region
E
-
+
+
+
+
+
+
+
+
+
depletion region
•
The accumulation of electric charge that are in
different poles in two separate region has caused the
formation of electric field between the two regions –
potential difference/ barrier potential (knee voltage,
Vk).
• The potential difference/ barrier potential determines
the drift current through the pn junction.
• The pn junction is useful when the width of depletion
region can be controlled
→ pn resistance can be controlled and therefore,
the current is also can be controlled.
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p-type
region
• The formation of depletion layer occurs through
diffusion of excess electrons/holes from n/p type
material.
PN Junction
No bias
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In the absence of an applied bias across
a semiconductor diode, the net flow of
charge in one direction is zero, ID = 0.
PN Junction
Forward Bias
A
forward-bias
or “on”
established by applying :• p-type : positive voltage
• n-type : negative voltage
condition
is
The application of a forward-bias potential VD
“pressure” electrons in the n -type material
and holes in the p -type material to recombine
with the ions near the boundary and reduce
the width of the depletion region  narrow.
The electric field by ions at depletion region
becomes weak.
• Holes in p-type flow into n-type
• Electrons in n-type flow into p-type
As a result, current flows
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*Bias is a direct current or voltage which
are used to operate a device by outside
source.
PN Junction
Forward Bias
p-type
Electron
•
The reduced depletion region width of the
depletion region has resulted in a heavy
majority flow across the junction.
•
The majority carriers see the reduced barrier
at the junction as a strong attraction for the
opposite polarity terminal.
•
As the applied bias increases in magnitude,
the depletion region will continue to decrease
in width until a flood of electrons can pass
through the junction, resulting the current
flows.
Hole
Electron
Hole
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n-type
PN Junction
Reverse Bias
A reverse bias is established by applying :• n-type : positive voltage
• p-type : negative voltage
The net effect of this occurrence is the
widening of depletion region  great barrier
for the majority carriers to overcome.
As a result, majority carriers is reducing to
zero Imajority ≈ 0
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The number of uncovered positive ions in the
depletion region of the n-type material will
increase due to the large number of free
electrons drawn to the positive terminal –
similar occurrence in p-type.
PN Junction
Reverse Bias
p-type
n-type
•
Due to the opposite bias applied to the pn
junction, the depletion region is widen.
• For current flows, electron should be
supported from the p-type to the n-type
and hole should be supported from the ntype to the p-type.
Hole
• Now, the due to the widening of depletion
region, it increases the barrier potential
and thus barricades the current to flow.
As a result, very small to none current is
flowing  reverse saturation current, IS
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Electron
What is a Semiconductor Diode ?
When a junction is formed between n-type
and p-type semiconductor material, the
resulting device is called diode – two
terminal device.
Anode
Cathode
Symbol representation of a diode
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It offers an extremely low resistance to
current flow in one direction and extremely
high resistance to current flow in the other
•
It can be applied when a circuit is
required to behave according to the
direction of current flowing in it.
What is a Semiconductor Diode ?
The simplest semiconductor device that having a characteristic closely
match those of a simple switch.
The operation of switch :
Characteristic of an ideal diode
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Pass an infinite current in one direction
and no current in other direction.
Current-Voltage Characteristic
Ideal and Real Diode
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• Ideally it conducts current in only one direction
• Acts like an open in the opposite direction
Current-Voltage Characteristic
Ideal and Real Diode
FORWARD BIAS ON A DIODE IS EQUIVALENT TO SWITCH ON
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REVERSE BIAS ON A DIODE IS EQUIVALENT TO SWITCH OFF
Current-Voltage Characteristic
Ideal and Real Diode
Look at the vertical line!!
In the non-conduction region,
ideally:
• All of the voltage is
across the diode
• The current is 0A
• The reverse resistance,
RR = VR/IR
• The diode acts like an
open circuit
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Look at the horizontal line!!
In the conduction region,
ideally:
• The voltage across the
diode is 0V
• The current is ∞
• The forward resistance,
RF = VF/IF
• The diode acts like a
short circuit
Current-Voltage Characteristic
Ideal and Real Diode
ID
ID
Real Diode
Ideal Diode
Forward Biased
_
+
_
+
Reversed Biased
VD
0.7 V _
Forward Biased
VD
_
0.7 V +
Reversed Biased
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+
Current-Voltage Characteristic
Shockley diode equation
−1
ID : diode current, A
Is : reverse saturation current (leakage current), A
e : natural number (2.71828)
VD : bias voltage across diode, V
VT : Temperature voltage = kB T/q
kB : Boltzmann's constant (1.381 x 10-23 J/K)
T : temperature, K
q : electron charge (1.602 x 10 -19 C)
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끫歸끫歮 = 끫歸끫毀
끫殒끫歮
끫殤 끫殶끫殒끫殎
Current-Voltage Characteristic
Operating Condition
Since the diode is a two-terminal device, the application of a voltage across
its terminals leaves three possibilities:
1. No bias: No external voltage is applied and no current is flowing
3. Reverse bias: VD < 0V
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2. Forward bias: VD > 0V
Current-Voltage Characteristic
No Bias
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No external voltage is applied (VD = 0V) and no current is flowing (ID = 0A)
Current-Voltage Characteristic
Forward Bias Diode
• External voltage is applied across the
p-n junction in the same polarity of the
p- and n-type materials (VD > 0V)
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• The positive terminal is connected to
the anode and the negative terminal is
connected to the cathode
• The diode current will increase
exponentially with increase in voltage
across the diode
Current-Voltage Characteristic
Forward Bias Diode
•
The point at which the diode changes from No Bias
condition to Forward Bias condition happens when the
electron 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 current, IF remains low until the forward
bias voltage exceeds the knee voltage, beyond this
point, IF increases almost linearly with the increase of
VF
•
The forward bias voltage required for a:
 Germanium diode, VF > VT = 0.3V
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 Silicon diode, VF > VT = 0.7V
Current-Voltage Characteristic
Reverse Bias Diode
• The negative terminal is connected
to the anode and the positive
terminal is connected to the
cathode
• The current that exist under
reverse-bias condition is called
reverse saturation current (Is)
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• External voltage is applied across
the p-n junction in the opposite
polarity of the p- and n-type
materials (VD < 0V)
Current-Voltage Characteristic
• The point at which the diode changes from No
Bias condition to Reverse Bias condition
happens due to the reverse polarity of applied
voltage - electron and holes will move towards
the terminal opposite to its chargers and widen
the depletion region- increase voltage barrier.
• A small current flows with a reverse bias
applied due to the minority carriers.
• High reverse bias voltage creates high reverse
current – diode breaks down.
• Ge is more sensitive to the temperature – more
leakage current.
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Reverse Bias Diode
Current-Voltage Characteristic
•
As the voltage across the diode increases in
the reverse-bias region, the velocity of the
minority carriers responsible for the reverse
saturation current Is will also increase until it
reaches the avalanche breakdown (VBV)
•
Zener breakdown - contribute to the sharp
change in the characteristic. It occurs
because there is a strong electric field in the
region of the junction that can disrupt the
bonding forces within the atom and
“generate” carriers.
•
The maximum reverse-bias potential that
can be applied before entering the
breakdown region is called the peak inverse
voltage (PIV rating).
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Reverse Bias Diode
Current-Voltage Characteristic
Commercial Semiconductor Diode Characteristic
Knee Voltage of each diode
Semiconductor
VK(V)
Ge
0.3
Si
0.7
GaAs
1.2
a. Determine the voltage across each
diode at a current of 1 mA.
b. Repeat for a current of 4 mA.
c. Repeat for a current of 30 mA.
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Quick Assessment
Current-Voltage Characteristic
•
As temperature increases it
adds energy to the diode
•
It reduces the required
Forward Bias Voltage in
Forward Bias condition
•
It increases the amount of
Reverse Current in Reverse
Bias condition
•
Germanium diodes are more
sensitive
to
temperature
variations than Silicon Diodes
Variation in diode characteristics
with temperature change
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Temperature Effects
Data Sheet
Data on specific semiconductor devices are normally provided by the manufacturer in one of two
forms.
5.
6.
7.
8.
The forward voltage VF (at a specified current and temperature)
The maximum forward current IF (at a specified temperature)
The reverse saturation current IR (at a specified voltage and temperature)
The reverse-voltage rating [PIV or PRV or V(BR), where BR comes from the term “breakdown”
(at a specified temperature)]
The maximum power dissipation level at a particular temperature
Capacitance levels
Reverse recovery time trr
Operating temperature range
Depending on the type of diode, an additional data could also be provided such as frequency
range, noise level, switching time, thermal resistance levels and peak repetitive values.
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1.
2.
3.
4.
Data Sheet
A. Minimum reverse bias voltage of 125 at
specified reverse bias current
B. Operational temperature range
C. The maximum power dissipation 0.5 W
끫殆끫歮 = 끫殒끫歮 끫歸끫歮
D. The maximum sustainable current 500mA
E. Forward bias voltage for each current level.
F. Reverse saturation current
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Temperature and applied reverse bias are
very important factors in designs sensitive to
the reverse saturation current.
G. Capacitance at reverse bias voltage
H. Reverse recovery time
Electrical characteristics of a high voltage,
low-leakage device
Zener Diode and Solar Cells
Zener Diode
• A special diode – to operate in reversebiased mode (voltage that exceeds the
breakdown voltage)
• It provides a constant voltage to the load
from a source whose voltage may vary
over a sufficient range.
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Zener diode are heavily doped silicon
diodes which, exhibit abrupt reverse
breakdown at relatively low voltages.
Zener Diode and Solar Cells
Zener Diode
Zener Diode is a diode operated in reverse bias at
the Zener Voltage (Vz) and commonly used in a
voltage regulation circuit - to fix the output voltage
at a constant value across an electronic circuit.
1. The input voltage has to be higher than the
Zener voltage ( Vin > Vz )
2. The resistor value must be chosen such that
there is always current flowing through the
Zener.
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There are a couple of requirements to consider for
Zener diode to act as voltage regulator :
Zener Diode and Solar Cells
Zener diode characteristics with the equivalent model for each region
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Zener Diode
Zener Diode and Solar Cells
Light-Emitting Diode (LED)
• In any forward-biased (usually 2-3V), electrons from the n-type material
will cross the p-n junction and recombine with holes in the p-type material
• The recombining electrons and holes release energy in the form of heat
and light
• LED displays are available in many different sizes and shapes
• They are presently available in red, green, yellow, orange, white and white
with blue
• Applications : Commonly used as a digital displays such as in calculators,
watches and all forms of instrumentation
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• Light-Emitting Diode or LED is a diode that
will give off visible light when it is energized
Zener Diode and Solar Cells
Light-Emitting Diode (LED)
Example 2 : Seven segment display a) Face with
pin identification b) Pin function c) Displaying
the numeral 5
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Example 1 : Interactive LED dining table circuit
Zener Diode and Solar Cells
Photodiode
Vλ
Iλ
• It will conduct when light of a particular wavelength is applied to the junction
• The higher the intensity of the light, the higher the conduction through the
photodiode
• When there is no light is applied, the reverse current is almost negligible
and it is called the dark current
• Applications: In instrumentation circuits as a sensor such as in alarm
system and detection of objects on a conveyor belt
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Photodiode is a semiconductor p-n
junction device whose region of operation
is limited to the reverse-bias region.
Zener Diode and Solar Cells
Photodiode
• A photodiode is a semiconductor p-n
junction device that converts light into an
electrical current.
• The current is generated when photons
are absorbed in the photodiode.
•
Photodiodes may contain optical filters,
built-in lenses, and may have large or
small surface areas.
• The common, traditional solar cell used to
generate electric solar power is a large
area photodiode.
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• Photodiodes usually have a slower
response time as their surface area
increases.
Conclusion
1. The characteristics of an ideal diode are a close match with those of a simple switch
except for the important fact that an ideal diode can conduct in only one direction .
2. The ideal diode is a short circuit –conduction region and an open circuit in the region
of nonconduction.
3. In the absence of any externally applied bias, the diode current is zero (VD = 0, ID = 0).
4. In the forward-bias region the diode current increases exponentially with increase in
voltage across the diode. In the reverse-bias region the diode current is the very small
reverse saturation current until Zener breakdown is reached and current will flow in the
opposite direction through the diode.
5. The threshold voltage is about 0.7V for Si and 0.3V for Ge.
7. Light emitting diodes (LEDs) emit light under forward-bias conditions but require 2V to
4 V for good emission.
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6. The direction of conduction for a Zener diode is opposite to that of the arrow in the
symbol, and the Zener voltage has a polarity opposite to that of a forward-biased diode.
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ELECTRONICS
SMJP2143
CHAPTER 1
Fundamental of Electronics
Dr. Nurshafinaz Mohd Maruai
nurshafinaz@utm.my
Branches of
Electrical Engineering
Active and
Passive Components
Fundamental of Electronics
CLO1 : Apply the fundamental principles of electronic device and circuit.
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Discrete and
Integrated Circuits
TELECOMUNICATION
MECHATRONICS
ELECTRONICS
Branches of
Electrical Engineering
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POWER
POWER ENGINEERING
Power engineering is a subfield of electrical engineering that deals with the
generation, transmission, distribution, and utilization of electric power, and
the electrical apparatus connected to such systems.
TRANSMISSION NETWORKS
Generates electricity
Transport electricity over long distances
DISTRIBUTION LINES
Transport electricity to its final destination
SUBSTATION TRANSFORMER
SUBSTATION TRANSFORMER
Raises the voltage of electricity for
efficient transportation
Lowers the voltage of electricity ready
to deliver for everyday use
HOMES AND BUSINESSES
Electricity is used to power
our everyday life
(appliances, lighting and heating)
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POWER STATION
TELECOMMUNICATION
Telecommunications engineering focuses on the transmission of information
across a communication channel such as a coax cable, twisted cable, optical
fiber or free space.
Transmitter
Transmission
Channel
Output signal
r(t)= vm(t) + n(t)
Receiver
Noise
n(t)
Block diagram of communication system
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Input signal
vm(t)
MECHATRONICS
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Mechatronics engineering is an interdisciplinary branch of engineering that
focuses on the integration of mechanical, electronic and electrical engineering
systems, and also includes a combination of robotics, electronics, computer
science, telecommunications, systems, control, and product engineering.
ELECTRONICS
• The field of electronics is a branch of physics and electrical engineering
that deals with the emission, behavior and effects of electrons using
electronic devices.
• Electronic devices are used in/for :
1. Communication method to connect far places
2. Calculation and computing data in digital device.
3. Precise measuring technologies – high accuracy and adaptability.
4. Controlling Machines
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• Electronics
uses
active
devices
to
control
electron
flow
by amplification and rectification, which distinguishes it from
classical electrical engineering, which only uses passive effects such
as resistance, capacitance and inductance to control electric current flow.
ELECTRONICS
Application of Electronic Devices – Data Acquisition System (DAS)
Data Acquisition System
(DAS) is an information
system that collects, stores
and distributes information
on a digital device from
electrical
signals
or
environmental conditions.
Subsonic wind tunnel
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Wind tunnel testing usually involves with the measurement of aerodynamic
forces, pressure and motion in the form of analogue electrical signals.
ELECTRONICS
Application of Electronic Devices – Data Acquisition System (DAS)
• Analog : Quantity changes to continuously. All physical
quantities in nature are analog. (length, mass, temperature,
voltage etc.)
• Digital : Quantity is expressed with discreet value. Numerical
numbers are digital. Concepts in brain are also digital.
Example. Idea of “Yes” or “No” is digital.
Analog signal
• Analog data are easily suffered by “noise”. However digital
data are safe for noise disturbance. Therefore, digital
technologies have been developed rapidly.
Digital signal
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• Existing physical quantity is analog, but measured value is
digital. For example, we measured a voltage to be V=1.501.
However, true value must be in the range from V=1.5010 to
1.5019.
ELECTRONICS
Application of Electronic Devices – Data Acquisition System (DAS)
Analog
Signal
Temperature
Pressure
Motion
Flow
Transducer
/ Sensor
Signal
Conditioning
A/D
Converter
8 bit
Resolution
Noisy
Electrical
Signal
Filtered
And
Amplified
Signal
16 Samples
Per Cycle
Computer
00110010
01010101
10100100
11010100
00101010
01010010
11000101
10010101
01010101
11000101
11001010
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Physical
system
Digital
Signal
ELECTRIC
ELECTRONIC
CIRCUITS
Consists of passive circuit
elements such as resistor,
capacitor and inductor.
Absorb/store power from
others
Passive
Electronic circuit
Consists of not only but also
active circuit elements or
functional devices such as diode,
transistor, integrated circuit
Signal is generated and
amplified to supply.
Active
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Electric circuit
ELECTRIC QUANTITIES
Charge is an electrical
property of the atomic
particles of which matter
consists
• Positive charge – proton
• Negative charge – electron
Electrostatic Forces
Voltage, V
Voltage is the energy in an
electric circuit that creates
current flow in the circuit.
Due to electrostatic force
from
opposite
charges,
there is a potential different
in terms of energy between
them  voltage
끫殔
끫殒 =
끫殈
Current, I
Electric current is a flow
of electric charge.
Electrons gained energy
from applied voltage
potential enables the
electrons to flow in the
circuit.
끫殈
끫歸 =
끫毂
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Charge, Q
ELECTRIC ELEMENTS
RESISTOR
• Means of controlling the currents and voltage in a circuit.
• To limit current, divide voltage and to generate heat.
• Acts as a load to stimulate the presence of a circuit during
testing.
• Symbols
끫毆 = 끫殊끫殊
• The consuming (dissipating) power of a resistor :
끫毆 2
2
끫殆 = 끫毆끫殊 =
끫殊
= 끫殊끫殊
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• R : Resistance [Ω] - measures how much the flow of this electric
charge is restricted within the circuit.
• Ohm’s Law :
ELECTRIC ELEMENTS
INDUCTOR
• Means of storing electrical energy in the form of a magnetic field.
• Symbols
끫殢끫殊
끫毆 = 끫歾
끫殢끫毂
• The stored magnetic energy by inductor :
1 2
끫毈 = 끫歾끫殊
2
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• L : Inductance [H]
• The voltage induced across the terminal of an inductor:
ELECTRIC ELEMENTS
CAPACITOR
• Means of storing electric charge, which can be deposited and extracted later.
• Symbols
• C : Capacitance [F]
• The capacitance of the capacitor:
끫毂
끫殈
1
끫歬 = = ∫0 끫殊 끫欞 끫殢끫欞
끫毆
끫毆
끫殢끫毆
끫殊 = 끫歬
끫殢끫毂
Q : amount of the charge at the electrode [C]
• The stored electric energy by capacitor :
1 2
끫毈 = 끫歬끫毆
2
More on capacitors : https://youtu.be/X4EUwTwZ110
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• The current flowing in capacitor
+ 끫殈(0)
ELECTRIC ELEMENTS
CAPACITOR
Charging capacitor:
When a capacitor is connected to voltage
supply, electrons will move out from plate
A and the same number of electrons will
be kept in plate B through connecting wire
and power supply
Electron motion will stop once the voltage
across capacitor is equal to the voltage
supply.
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Hence, the plate A is stacked with protons
and plate B will be stacked with electrons.
ELECTRONIC ELEMENTS
DIODE
• Device that results from the formation of junction between n-type and p-type
semiconductor material.
• Offers a low resistance to current flow in one direction
• Symbol :
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• Diode is used as a switch and rectifier to convert AC voltage into DC voltage.
Rectifiers are found in all dc power supplies that operate from an ac voltage
source.
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ELECTRONIC ELEMENTS
DIODE
ELECTRONIC ELEMENTS
TRANSISTOR
• Three elements, two junction device used to control electron flow
• Fundamental active circuits elements in electronics :
• Types of transistors (symbols).
pnp bipolar
n channel MOSFET
• Transistor is a semiconductor device that controls current/ voltage between two
terminals based on the current or voltage at a third terminal and is used for the
amplification or switching of electrical signals.
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npn bipolar
Discreet Circuit
Circuit construction, in
which the components are
manufactured separatelyconnected by conducting
wires, breadboard or a
printed circuit board.
Integrated Circuit
Huge
number
of
transistors and diodes
are fabricated on a
semiconductor wafer.
The
advantages
of
an
integrated circuit (IC) :
1. Smaller
2. Cheaper
3. Better reliability
4. Lower operational power
5. More complex circuits can
be built into smaller circuit
board
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DISCREET AND INTEGRATED CIRCUIT
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utmofficial
ELECTRONICS
SMJP2143
CHAPTER 2
Introduction to Semiconductor
Dr. Nurshafinaz Mohd Maruai
nurshafinaz@utm.my
Introduction to Semiconductor
CLO1 : Apply the fundamental principles of electronic device and circuit.
Drift & Diffusion
Currents
Minority & Majority
Carrier
n-type, p-type
Semiconductor
Covalent Bond
Conductor, Semiconductor
and Insulator
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Atom of
Semiconductor
Intrinsic & Extrinsic
Semiconductors
Conductor, Semiconductor and Insulator
Insulator
Conductor
• Materials that permit current flow easily.
• Contain a large number of free electrons
• Resistance increases with temperature
Example : Au, Ag, Al, Cu
Semiconductor
•
Materials that have characteristic
between insulators and conductors.
• Sensitive to temperature, magnetic
force and light
• Conductivity becomes better with heat
• We can control value of resistance by
voltage application and specially
designed structure.
Example : IV group (C, Si, Ge)
III-V Compound ground
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•
• Materials that prevent the flow of electricity
Electric conductivity in this material is very small
• Resistance decreases with temperature
Example : Stone, Glass (SiO2), Plastics
What is an atom ?
Atom is the smallest particle of an element.
• Every atom has a nucleus.
Nucleus is located at the center
of the atom – contains positive
charged carriers (protons) and
uncharged carriers (neutrons).
ELECTRON
PROTON
+
NEUTRON
ORBIT
NUCLEUS
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• Negative
charged
carriers
(electrons) orbit around the
nucleus.
Valence shell
-
-
•
The electrons orbit in a concentric circle about the nucleus.
•
Each orbit is called a shell. The outer shell is called
valence shell and the number of electrons it contains is the
valence.
•
a.
The farther the valence shell is from the nucleus, the
less attraction the nucleus has on each valence electron.
Therefore, the potential for the atom to gain or lose
electrons increases if the valence shell is not full and
located far enough away from the nucleus.
•
b.
Valence is an indication of the atom’s ability to gain or
lose electrons and determines the electrical and chemical
properties of the atom.
-
-
-
-
-
-
For example: an atom having seven electrons in the
valence shell is less conductive than an atom having
three electrons in the valence shell.
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What is an atom ?
Electron Configuration
Electron configuration is the distribution of electrons in an atom. In order for
the electron configuration to be stable, an atom must have
a full outer octet, or 8 electrons in it's outermost shell.
• An atom that has the same number of electrons and
protons – electrically balanced.
• A balanced atom that loss one (more) electrons –
positive ion.
Shell
No. of electrons
K
2
L
8
M
16
N
32
O
32
P
18
Q
8
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• A balanced atom that receives one (more) electrons
– negative ion.
Conductor, Semiconductor and Insulator
Energy Level
• The energy of an electron in a shell consists of two parts, namely the
kinetic energy due to its motions surrounds the nucleus and the potential
energy due to proton electrostatic charge that presents in the nucleus.
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• The energy level in an atom is measured in electron volt (eV) – sum of
kinetic energy that is required to move one electron through one volt of
potential difference is 1.602 x 10-9 Joule.
Conductor, Semiconductor and Insulator
• Electron can move from one shell to another
when receives energy from outside or losing its
own energy.
• Energy can be given to an electron in the form of
light and heat. The energy level of electron will
increase and when it is sufficiently high, the
electron can become a free electron - ionization
process.
Attract
+
-
+
+
-
-
Repel
Electrostatic Forces
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• The electron that is far from nucleus is less
influenced by the nucleus electrostatic force – it
has more potential energy.
Ionization Process
Conductor, Semiconductor and Insulator
Energy Band
Band gap
Overlap
Fermi level
Valence band
Conductor
Semiconductor
-
-
-
- -
-
Insulator
-
-
-
-
- -
-
-
-
-
-
- -
-
-
-
• Semiconductors and insulators
have a greater and greater
energetic difference between the
valence band and the conduction
band, requiring a larger applied
voltage in order for electrons to
flow.
• Semiconductor has the band gap
that is small enough that thermal
energy can bridge the gap for a
small fraction of the electrons
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Electron energy
Conduction band
• A conductor has its valence band
and conduction bands overlapping,
allowing excited electrons to flow
through the empty band with little
force (voltage).
Atom of a Semiconductor
NUCLEUS
• 14 protons
• 14 neutrons
-
-
-
-
Si
-
-
-
-
-
14 electrons
in orbit
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• Made from materials that have four (4)
valence electrons in their outer orbits
• Germanium and silicon are the most common
semiconductor materials used in solid-state
devices.
• Silicon is preferred due to its ability to
withstand heat.
• When refined into pure form, the molecules
arrange themselves into a structure called a
lattice structure.
• A pure semiconductor material such as
silicon or germanium has no special
properties and will make poor conductive
material.
Covalent Bond
• Process of sharing valence electrons, resulting in the formation of crystals
• To gain stability – the atom must acquire electrons in its valence shell.
Example of covalent bonding in silicon :
Si
-
-
Si
-
-
-
Si
Si
Si
-
Simplified silicon atom
-
Si
Crystal of silicone with covalent bond
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-
Covalent Bond
As the temperature rises, the valence electrons became agitated and some of
it will break the covalent bonds and drift towards once atom to the next →
electron-hole pair
• The amount of current flow is determined by the number of electron-hole pair.
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• Temperature ↑, Current ↑
A hole is created when electron breaks (temp↑)
Intrinsic & Extrinsic Semiconductors
Intrinsic semiconductor
• pure semiconductor elements are carbon (C), germanium (Ge) and silicon (Si)
• can only supports small numbers of electron-hole pairs at room temperature
• allows for conduction of very little current.
Extrinsic semiconductor
• Conductivity is increased due to doping agent.
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• composed of chemical elements (doping agent) of at least two different
species. For example: Gallium arsenide (GaAs), Gallium Nitride (GaN)
Intrinsic & Extrinsic Semiconductors
Extrinsic semiconductor
Example : By adding certain impurities to pure (intrinsic) silicon, more holes or electrons are
produced within the crystals.
III
IV
V
5
6
B
Boron
C
Carbon
13
14
15
Al
Aluminum
Si
Silicon
P
Phosphorus
31
32
33
Ga
Gallium
Ge
Germanium
As
Arsenic
49
51
In
Indium
Sb
Antimony
To increase the number of
conduction band electrons,
pentavalent impurities are added
forming a n-type semiconductor.
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To increase the number of holes,
trivalent impurities are added
forming a p-type semiconductor.
n-type semiconductor
Doped with pentavalent
(V group/electron donor atoms)
p-type semiconductor
Doped with trivalent
(III group/electron acceptor atoms)
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n-type & p-type Semiconductors
n-type semiconductor
In an n-type material the electron is
called the majority carrier and the
hole the minority carrier.
p-type semiconductor
In a p-type material the hole is the
majority carrier and the electron
are the minority carrier.
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n-type & p-type Semiconductors
Electric Current
Electric current is the movement of electrons ( negative charge carriers)
When Electromotive force is applied to the
free electrons, the force acts upon the
electrons
to move them in particular
direction.
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The motion of the free electrons is
normally very haphazard - it is random as many electrons move in one direction
as in another and as a result there is no
overall movement of charge.
Electric Current
Electron flow:
The electron flow is from negative to positive
terminal. Electrons are negatively charged and
are therefore attracted to the positive terminal as
unlike charges attract.
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Conventional current flow:
The conventional current flow is from positive to the
negative terminal and indicates the direction that
positive charges would flow.
Drift and Diffusion Currents
Drift Current
Drift current arises from the movement of
carriers in response to an applied electric
field.
The net motion of charged particles generates
a drift current that is in the same direction as
the applied electric field.
Current flow in semiconductor
• Electron towards (+) terminal
• Hole towards (-) terminal
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• Positive carriers (holes) move in the same
direction as the electric field while;
• Negative carriers (electrons) move in the
opposite direction.
Drift and Diffusion Currents
Diffusion Current
Flow of charge carriers within a semiconductor travels from a higher
concentration region to a lower concentration region. It occurs when a
semiconductor is doped non-uniformly.
-
-
-
-
-
Diffusion of electrons
-
-
-
-
-
-
-
-
-
High concentration area
-
-
Low concentration area
Further reference
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-
-
PN Junction
The presence of an n-type or p-type semiconductor
material alone will not be useful for active devices.
If a block of p-type is connected to n-type , a very useful
structure can be created – widely used in electronic
devices construction.
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It is called PN Junction.
PN Junction
• When n-type and p-type semiconductors are connected, electrons in n-type
semiconductor migrate into the p-type semiconductor around the connected
area.
• Similarly, holes in p-type semiconductor
semiconductor around the connected area.
migrate
into
the
n-type
• PN junction will not be formed by just placing both materials beside each
other – factorization process.
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• This connection is called pn junction.
PN Junction
Depletion Region
p-type region
n-type region
+
+
+
+
+
• Similarly, the holes diffuse to the n-type region
until the n-type repels the diffusion of holes.
+
+
+
+
depletion region
hole
-
negative ion
from filled hole
electron
+
positive ion
from removed
electron
• The combining of electrons and holes
depletes the holes in the p-region and the
electrons in the n-region near the junction.
• Thus after the formation of the depletion layer,
no free electrons can pass the depletion
region (vice versa).
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-
• The electrons diffuse to the p-type region, the
negative charge increase inside the p-type
after some diffusion, the negatively charged
region in the P-type repels the diffusion of
electrons.
PN Junction
Depletion Region
• The accumulation of electric charge that are in different poles in two
separate region has caused the formation of electric field between the two
regions.
E
-
+
+
+
+
+
+
+
+
+
• The width of depletion region depends on the level of doping p and n
materials.
• The depletion region also determines the potential difference or barrier
potential, which allows the current to flow through the junction.
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depletion region
PN Junction
Depletion Region
• The accumulation of electric charge that are in different poles in two separate region has
caused the formation of electric field between the two regions.
E
-
+
+
+
+
+
+
+
+
+
• The width of depletion region depends on the level of doping p and n materials.
• The depletion region also determines the potential difference or barrier potential, which
allows the current to flow through the junction → barricades the drift current.
• The value of this barrier potential depends on the doping level in p and n regions, type of
material and temperature.
• The barrier potential for germanium, Ge is 0.3V and silicon, Si is 0.7V.
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depletion region
PN Junction
Depletion Region
• The pn junction is useful when the width of depletion region can be controlled
→ pn resistance can be controlled and therefore, the current is also can be controlled.
Width of depletion
region
Junction resistance/
Potential difference
Junction current
Minimum
Minimum
Maximum
Maximum
Maximum
Minimum
So far…..
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No bias PN Junction
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