ELECTRONIC DEVICES AND CIRCUITS
(ENGI27907)
Lecture 1
Introduction to
Semiconductors
Textbook (Thomas Floyd): chapter 1
1
Outline
READINGS
This Session Lecture:
Textbook (Thomas Floyd): chapter 1
QUIZ 1
Week 3 during scheduled lecture
Make sure your laptop work well
Arrive on time
HOMEWORK 1
Do not submit
but the earlier you complete the more benefit for your Lab and Quiz
Due @the BEGINNING of next Lecture
Introduction to Semiconductors
Watch the video, answer the following questions:
• What is semiconductor?
• Why semiconductor is important?
• What is the difference among conductor, insulator and
semiconductor?
https://www.semiconductors.org/semicon
ductors-101/what-is-a-semiconductor
2:46
Conductors - Insulators - Semiconductors
https://www.youtube.com/watch?v=0NBTvJF6ghQ
3
List of Topices
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Properties of insulators, conductors, and semiconductors
Energy diagrams
Charge Carriers: Electrons and Holes
Electron Hole Pair Generation
Electron Hole Pair Recombination
n-type and p-type Semiconductor
The pn Junction
Introduction of Diodes
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Conductor Materials
• The valence shell (electrons) determines the ability of the material to conduct current.
When a valence electron gains sufficient energy from an external source, it can break free
from its atom, and become free electron.
• Conductor materials can conduct electricity easily since they have many free electrons.
• Most metals are good conductors. For example: Copper has one valence electron which is
easy to escape.
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Insulator Materials
• Insulators have few free electrons for
conduction; therefore, they do not
conduct electricity current, and have
very high resistivity.
• Glass, air, paper, plastic and rubber are
excellent insulators that are widely used
in electronics.
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Semiconductor Materials
• Conductivity of semiconductor is between conductors and insulators.
• Silicon is the most commonly used semiconductor
• Silicon has 4 valence electrons which are shared among the
neighboring silicon atoms to make strong covalent bonds.
• Each silicon atom share valence electrons through covalent bonds
with 4 adjacent silicon atoms to form a silicon crystal.
Bonding diagram
silicon crystal
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Energy diagrams
• The ability to conduct current for
the three types of materials can be
explained by energy diagrams
• The valence shell of an atom
represents a band of energy levels,
call valence band, valence
electrons are confined to valence
band.
• When an electron acquires enough
additional energy, it can leave the
valence shell, become a free
electron, and exist in what is known
as the conduction band.
• The difference in energy between
the valence band and the
conduction band is called band
gap.
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Insulator has very big band gap
Conductors has overlapped band gap at room
temperatures
This course focuses the study on semiconductor
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Charge Carriers: Electrons and Holes
• Free electrons in conduction band can move and contribute in the current flow. Electrons
are negative charge carriers.
• Electrons in valence band can also move between atoms.
• Effectively free holes move in opposite direction and contribute in the current flow like
positive charge carriers.
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Electron Hole Pair Generation
• Electrons can acquire energy and move from valence band to conduction band.
This process generates one free electron in conduction band and one hole in
valence band.
• Current in a semiconductor is composed of electron current and hole current
• Application: variable conduction with light (photocell).
• Application: converting sunlight to electrical power (solar cells).
• Issue: unwanted effect of temperature on electronic devices.
Semiconductor has relatively narrow band gap
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Electron Hole Pair Recombination
• Electrons (in the conduction band) and holes (in the valence band) can
recombine and release energy in the form of heat or light (photons). Free
electrons and holes disappear in this process  recombination
• Application: Light Emitting Diodes
• Issue: Electronic devices get warmer while operating.
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n-type and p-type Semiconductor
Conductivity of semiconductor can be increased by the addition of impurities
• Doping: adding pentavalent impurity atoms
such as Arsenic (As), Phosphorus (P), and
Antimony (Sb).
• These atoms share 4 valence electrons with
neighboring Silicon atoms and easily release
their 5th electron (donor impurity).
• Results:
increases the number of free electrons
Creates positive impurity ions
N-type
• Doping: adding trivalent impurity atoms
such as Boron (B), Aluminum (Al).
• These atoms share 3 valence electrons with
neighboring silicon atoms and should accept
(acquire) the 4th electron to complete the
bonds (acceptor impurity)
• Results:
Increases the number of free holes
Creates negative impurity ions
P-type
The pn Junction
After watch the video, answer the following questions:
 What is pn junction?
 How depletion region forms?
PN Junction
https://www.youtube.com/w
atch?v=ar7xDMR4P_U
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The pn Junction
• n-side: free electrons diffuse toward p-side, recombine with free holes and leave
positive ions.
• p-side: free holes diffuse toward n-side, recombine with free electrons and leave
negative ions.
• A depletion region and a potential barrier is formed, depends on type of
semiconductive material, the amount of doping, and the temperature. Typically 0.7 V
for silicon, 0.3 V for germanium
• The charge carriers’ transfer continues until the internal potential barrier becomes
strong enough.
• The pn junction is the basis for diodes, certain transistors, solar cells…
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PN junction energy levels
To an electron trying to diffuse across the junction, the path it must travel looks
like an energy hill. It must receive the extra energy from an outside source
(power supply) to climb the energy hill.
Diode
• A diode is a pn junction with metal contacts and leads.
• Diodes control the direction of current-flow, conduct in one
direction only.
• Diode’s terminals are polarized
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Diode Types
signal diodes usually have a
medium-high forward voltage
drop and a low maximum current
rating. E.g 1N4148.
Through-hole LEDs. From left to
right: yellow 3mm, blue 5mm, green
10mm, super-bright red 5mm, RGB
5mm and blue 7-segment LED.
Zener Diodes are usually
used to intentionally
conduct reverse current
surface-mount diode
Schottky diode has smaller
forward voltage drop (0.15V
and 0.45V), very large
breakdown voltage, useful in
limiting losses, when voltage
must be spared.
A rectifier or power diode has
much higher maximum current
rating, with the cost of a larger
forward voltage. E.g 1N4001
https://learn.sparkfun.com/tutorials/diodes/all
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Typical diode packages with terminal identification
• Through-hole diodes and surface mount diodes
• Diodes come in a variety of sizes and shapes. The design and structure is
determined by what type of circuit they will be used in
• A for anode and K for cathode
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Diode connection
Forward bias
• Positive voltage is applied to the
anode.
• The applied voltage is close to or
greater than the potential barrier
diode is on (conducting) + diode
current would be significant.
• When diode is forward biased:
Depletion region become narrow.
Potential barrier height is 0.7V for
silicon diode.
The current can be controlled by
the applied voltage.
the flow of majority carriers
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Diode connection
Reverse Bias Diode
• Positive voltage is applied to the cathode.
• The diode is off (not conducting); diode current
is very small and can be negligible.
• When diode is reverse biased:
Depletion region become wide (application:
variable capacitor with voltage)
Potential barrier height increases.
Current is almost zero (very small temperature
dependent current).
All voltage is dropped across the reverse diode.
the minority carriers from thermally
generated electron-hole pairs
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Testing Diode with DMM
• With the diode check function
a good diode will show
approximately .7 V or .3 V
when forward biased.
• When checking in reverse
bias the full applied testing
voltage will be seen on the
display.
K A
A K
Troubleshooting Diodes
Open Diode:
 Diode check function: no current flows in either direction
display full checking voltage for both forward and reverse
connections.
 Ohmmeter: high resistance in both forward and reverse
connections.
Shorted Diode:
 Diode check function : maximum current flows  0 V
 Ohmmeter: low resistance for both forward and reverse
connections.
Summary
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Diodes, transistors, and integrated circuits are all made of
semiconductor material.
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P-materials are doped with trivalent impurities
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N-materials are doped with pentavalent impurities
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P and N type materials are joined together to form a PN
junction.
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A diode is nothing more than a PN junction.
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A depletion region is formed at PN junction. It creates a voltage
barrier of .3 V for a Germanium and .7 V for Silicon for
conduction to take place.
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A diode conducts when forward biased and does not conduct
when reverse biased