AWIM Series
Semiconductors
2016-2017
Lawndale High School Agenda
• Resistors as sensors
• What are semi-conductors?
• The Junction & The Diode - The basis for all modern
electronics
• Types of Diodes
• Experiment #6 - Electrical circuits using:
Light Emitting Diode, LED and
Light Dependent Resistor, LDR
• Explore the change in LED light intensity with changing
light conditions on the LDR.
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Lawndale High School
• Resistors as sensors
• Resistors that change with Strain
• Strain Gauges/Force Sensing
Resistors (FSRs)
Lawndale High School
• Resistors as sensors
• Resistors that change with Strain
• Strain Gauges
• Resistors that change with Temperature
• Thermistors
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Lawndale High School
• Resistors as sensors
• Resistors that change with Strain/Force
• Strain Gauges/Force Sensing resistors (FSR)
• Resistors that change with Temperature
• Thermistors
• Resistors that change with Light
• Light Dependent Resistor (LDR)
Light Dependent Resistor (LDR)
An example of photocell construction commonly used in industrial
and home applications.
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• Semiconductors & the PN Junction
• CONDUCTORS and INSULATORS
• Conductors and Insulators
•The valence band
• Atomic size vs conduction
• Resistance in materials
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Materials Discussed to Date and New Materials
Insulators
Used in Modern
Semi-conductors
Conductors
Dry Air
Wood
Paper
Plastic
Silicon, Si
Titanium, Ti
Lead, Pb
Iron, Fe
Nickel, Ni
Zinc, Zn
Tungsten, W
Aluminum, Al
Gold, Au
Copper, Cu
Silver, Ag
Low Conductivity
High Resistance
Insulator in pure form
High Conductivity
Low Resistance
The best conductor
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In Earth's crust, silicon is the second most abundant element
after oxygen, making up 27.7% of the crust by mass.
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Making semiconductors
• A semiconductor is a manufactured material with electrical conductivity
between that of a conductor and an insulator. It is the foundation of
modern electronics.
• We make a semiconductor by adding impurities to Silicon (Si), which is
normally an insulator. This process is called “Doping”.
• If we add arsenic to Si, we get a material with Excess Electrons. This is
called “N” type Silicon.
• If we add Boron or Aluminum to Si, we get a material with a deficiency
of Electrons. This is called “P” type Silicon.
Semiconductors are materials that are engineered
to allow a controlled flow of electrons.
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What is a Diode?
• A diode consists of two doped Si materials, (one N and one P type), joined
together forming a junction. This junction gives the Diode its unique
properties, allowing it to function as a conductor in one direction and an
insulator in the opposite direction.
• In modern diodes, the materials are joined by depositing one material onto
the other. This is usually done in a vacuum to prevent contamination.
P
Anode
N
Cathode
Diode layout
Diode Symbol
A diode is a engineered semiconductor.
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How Diodes Work
O Diodes are formed by the
junction of two different
semiconductor materials, or one
material modified as follows:
O One side is ‘Doped’ with a
material having Excess
Electrons, this forms the n-Type
side (cathode).
O The other side is ‘Doped’ with
a material having a shortage of
electrons (‘Holes’), this forms
the p-Type side (Anode).
O Where the two materials meet a P-N junction is formed. At the
junction, the electrons and ‘Holes’ combine to form a thin,
non-charged boundary called the Depletion layer. As this layer
has a neutral charge, which forms a barrier, thus no electrons
flow from one side to the other.
How Diodes Work
O If we connect a Voltage source
as shown, the electrons in the nType material are attracted to the
positive (+) battery terminal, and
the Holes are attracted to the
negative (-) terminal. If the
Voltage provides enough energy
to overcome the Depletion layer,
the electrons will flow across the
junction. This voltage is about
0.6v for Silicon devices, and the
diode is said to be “Forward
Biased’ or ‘conducting’.
O Once the Diode starts conducting, its resistance becomes
very low and small changes in voltage cause large changes in
the current flow.
How Diodes Work
O If we reverse the Voltage
source as shown, the electrons
in the n-Type material are
attracted to the positive (+)
battery terminal, and the Holes
are attracted to the negative (-)
terminal. The Depletion layer
becomes larger, and no
electrons will flow across the
junction. The diode is now said
to be “Reverse-Biased’ or ‘nonconducting’.
O In the non-conducting state, its resistance becomes very high
and changes in voltage have almost no effect on the current
flow. Refer to the diagram on the following page.
How Diodes Work
O This diagram shows the
characteristics of a typical diode.
In the ‘Forward-Biased’ region
(to the right of the current line),
no current flows until the
‘Threshold’ Voltage is reached
(0.6v for Silicon). Once the diode
starts conducting, it looks like a
low resistance to the circuit, and
small voltage changes cause
large current changes.
O In the ‘Reverse-Biased’ direction (left side of current line),
only a very small leakage current flows for all voltage levels (up
to the breakdown of the diode).
Diode Packages
Some Types of Diodes
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Small Signal Diodes
Tunnel Diodes – High Frequency applications
Zener Diodes – (Regulate Voltages)
PhotoDiodes – Light sensors
Unijunction diodes – Generate pulses
Variable Capacitance – ‘Tuning Circuits’
Temperature sensors
Varistors – Protection Circuits
Thyristors (SCRs) – Power Control
Triacs – AC power switching/control
Schottky diodes – High Frequency
Laser Diodes - pointers
Light Emitting Diodes (LED) – Indication/Display
Light Emitting Diode (LED)
•
The LED is a special kind of diode. When a LED is forward biased,
electrons are able to release energy in the form of light. This effect is
called electroluminescence
•
LEDs are formed using Gallium-Arsenide material (not Silicon).
•
The GaAs material is ‘Doped’ (like a diode) to form N-Type and P-type
material.
•
The N and P type materials are put together to form a Junction, (again
like a standard diode).
•
When Voltage is applied to Forward Bias the Junction, current flows
and the diode emits light from the P type material (see figure 1).
•
The type and amount of doping determine the color of the light
emitted.
Electroluminescence (EL) is an optical and electrical phenomenon in which a
material lights up as the electric current passed through it.
LED Construction
Light-Emitting Diode
Invented in 1962 by:
Nick Holonyak Jr.
Electronic Symbol
+
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Experiment – LED, LDR
Light Dimmer Electric Circuit Diagram
R1
LED
220 ohms
+
_
6VDC
Battery
LDR
-
A
+
An engineer would draw such a circuit diagram to help choose
components to implement light dimmer design.
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Explore How Light Can Affect Electric Current
With Light Dependent Resistor, LDR
Connections:
Battery (+) to R1 at +Bus
R1 = 220 Ohms; +Bus-J15
Led (+) to G15
Led (-) to E15
LDR to A15
LDR to -Bus
Battery (-) to LDR at -Bus
Needed Parts:
LED; R1, LDR;
RED and BLACK alligators (1 ea.)
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Light Dimmer Experiment
Circuit Checkout
With RED alligator connected between Power Supply + and R1
Clip the BLACK alligator between Power Supply (-) and LDR
LED should be bright;
If NOT check out the circuit:
Is the resistor correct?
Are they connected to the pins as indicated?
Is LED connected to G15?
Is LED polarity correct (longer lead to R1 at G15 (+)?
Is LDR connected to e15
Repeat the test!
If OK, continue experiment after covering LDR face with your finger
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Does LED light Dim? If yes, go to the light intensity test.
Experiment – LDR Operation
LED
Battery Pack
Alligator Lead
Alligator Lead
- 6v
+6v
Long Lead
03.5
BB=Blue Bus
BB-J15
BB-e15
Red DMM Lead
BB=A15
Black DMM Lead
Important! Set Meter to
20ma Current Range
Before connecting!
Alligator Lead
LDR
BB-Red Bus
220 Ohms
Name ____________________
Date____________
Light Intensity Experiment with LED and LDR
Set the Multimeter dial to the 20 mA range to measure
electric current .
1. Test: 1
Clip the Multimeter (-) black end to the Power Supply (-)
Clip the RED probe tip to LDR (Blue bus) on the
breadboard
Note the current reading and the LED brightness below:
Current=
? mA
Brightness=___ ? (0 to 10)
2. Test: 2
Block the light to the LDR with your finger or the palm of
your hand .
Note the current reading and the LED brightness below:
Current=
? mA
Brightness=___ ? (0 to 10)
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Name ___________________
Date ____________
Observations
•What determines the current in Test no. 1?
Total resistance in the circuit is composed of:
LDR resistance plus resistor R1 in series (R//tot. = 220 + LDR Ohms)
Their total resistance establishes the current flowing through the LED.
This current sets the LED brightness.
•How has LED brightness changed in Test no. 2?
Explain why!
LDR is a light dependent resistor. Its resistance changes with light
conditions.
LDR resistance increases when the light is blocked. This increases
total resistance in the LED circuit and reduces LED brightness
by reducing its electric current.
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New Terminology
Semiconductor- Semiconductors are materials that can be engineered with impurities to
allow flow of electrons in one direction only.
Silicon- Silicon is the second most abundant element after oxygen, making up 27.7% of the Earth
crust by mass.
LED- Light emitting diode is a semiconductor that emits light when electric current flows through it.
LDR- Light Dependent Resistor
whose resistance decreases with increasing incident light intensity.
Diode-is an engineered conductor in one direction and an insulator in the opposite direction; a
semiconductor.
P-N Junction- It is an electronic junction of two semiconductor materials with added impurities
that allows current to flow in a preferred direction.
Electroluminescence- Electroluminescence is an optical and electrical phenomenon in
which a material emits light in response to an electric current that is passing through it.
Circuit Drawing- Engineers use electronic components symbols to draw circuit diagrams to
help them design the actual circuits.
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Appendix - A
Semiconductors form the heart of modern electronics.
Metals tend to be good conductors of electricity because they usually have "free electrons" that
can move easily between atoms, and electricity involves the flow of electrons. While silicon
crystals look metallic, they are not, in fact, metals. All of the outer electrons in a silicon crystal are
involved in perfect covalent bonds, so they can't move around. A pure silicon crystal is
nearly an insulator -- very little electricity will flow through it.
A modern semiconductor diode is made of a crystal like silicon that has impurities added to it
to create a region on one side that contains negative charge carriers (electrons), called n-type
semiconductor, and a region on the other side that contains positive charge carriers (holes),
called p-type semiconductor. The diode's terminals are attached to each of these regions. The
boundary within the crystal between these two regions, called a PN junction, is where the action
of the diode takes place. The crystal conducts a current of electrons in a direction from the Ntype side (called the cathode) to the P-type side (called the anode), but not in the opposite
direction; that is, a conventional current flows from anode to cathode (opposite to the electron
flow, since electrons have negative charge).
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