Download Polymer Materials 4471

Special Topic In Polymer Materials (KU 4471)
Lecture #12
Part 1
Professor Kwok Wai Lem
Department of Materials Chemistry and Engineering
Konkuk University
May 18, 2010
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 1
Special Topic In Polymer Materials (KU 4471)- Outline
Grade (100%)
1. Midterm Exam:
25%
2. Final Exam:
30%
3. Homework:
10%
4. Team Project:
25%
5. *Class Participation:
10%
*Engagement in brainstorming sessions, discussion for project and solution to the homework
assignments.
You Own This!!!
Schedule
Lecture #
1.
2.
3.
4.
5.
6.
7.
8.
9.
10-12
13.
14-15
16.
Contents
Introduction - Principles of Functional Polymer Materials and Devices
Project Introduction/Dr. Lem's Expectations (Lecture #1)
Fundamental of Materials and Value Chain Concept
Project Selection Finalize and Team Identify (Lecture #2)
Polymer Structural Hierarchy/Properties
Polymer and Hybrid Availability
Effect of Shape and Size on Properties
Polymer Functional Properties
Design Tools/Criteria
Team Project Interim Report
Midterm
Devices Design Criteria/Tools
Processing of Devices
Devices Structure/Properties
Market Driven Applications
Team Project Final Report Presentation
Final Examination
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 2
Final Schedule
1. Lecture 12 – May 18
2. Lecture 13 – May 25
3. Lecture 14 - June 8
4. Final Presentation – June 15
5. Examination - June 15
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 3
Finals
• Final Project Presentation (15 min)
• 3:00 – 3:20 pm June 15
• Final Exam (2. 5 hours – Open Book)
• 3:30 – 6:30 pm June 15
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 4
Homework #1 (HW L-12-1 - Due Lecture #13)
Reading Assignments from Organic Electronics (2007)
1. “Organic Materials for Thin Film Transistors” by Z. Bao, pages 4-6
2. “New Conducting And Semiconducting Polymers For Plastic Electronics” by
Luebben and Sapp, pages 12-14
3. “Fullerene-Based n-Type Semiconductors in Organic Electronics,” by
Kronholm and Hummelenpage, pages 16-20
4. “Achieving High Efficiency In Organic Light-Emitting Devices,” by
Polikarpov and Thompson, pages 21-23
5. “Light-Emitting Polymers,” by QB Pei, Pages 26-28
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 5
Special Topic in Polymer Materials (KU 4471) - Project
1. The project aims to challenge and assess your ability to critically evaluate a plastic device that
is being sold currently in the marketplace. You are asked to guide the class through a reengineering approach (material tear down, how it was made, etc.) of the material engineered
device being explored. This is an exercise that you will encounter over and over again if
choose to pursue a career in polymer/plastic materials.
2. Your task is three fold:
1. Select a plastic device and ask the instructor to approve your selection. If you do not have
the selection by Lecture #2, the instructor will offer you a set of selections to choose from.
2. Write a report on the article (10 page limit).
3. Make a 12-minute (a) interim and (b) final presentation to the class, respectively. The
presentation will be followed by a 3-minute question and answer period from the class.
3. Your report/presentation should contain a brief description of the plastic device, outlining the
most significant findings. The main part of the report should discuss the merits of the
conclusions of the value of the plastic device in term of cost, material, and technology to meet
the market needs. For example: Place these conclusions in the light of other people’s findings.
Is there a point that the plastic device failed to meet the requirement to expand market size?
What are the technological limits of the described devices? Speculate, on what are the next
steps additional developments to make this product/device better to expand the market size
by meeting additional customer needs. In essence, tell us how this plastic device contributes
to the knowledge of the market it explores.
Due Date:
1.
2.
3.
4.
Project Introduction/Dr. KW Lem's Expectations (Lecture #1)
Project Selection Finalize and Team Identify (Lecture #2)
Project Interim Report Presentation (Lecture #7)
Project Final Report Presentation (Lecture # Second to Last)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 6
Team Project – Interim Presentation
Class Rep: Kim Seung-Hee
1. Project OLED Displays
2. Team Leader: Lee Seung-Min
3. Members
•
•
•
Lee Seung-Min
Kim Seung-Hee
Lee Tae-Ho
Practice - 3:00 PM @ May 18, 2010
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 7
Hybrids Build-up Mechanics
Hybrids/Devices
Functional
Components
Structures
Hierarchy
Functional
Structures
Clusters
Functional
Materials
Molecules
Atoms
Functional
Chemicals
Features
Ground Work for Continuous Improvement
(Kaizen) in Materials Development
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 8
Shape and Size:
Polymer Structure/Properties
Introduction to High Performance Polymers
Structural Hierarchy in Semicrystalline Polymers
Structural/Properties in Block Copolymers
Structural/Properties in Amorphorous Polymers
Structural/Properties in Liquid Crystalline Polymers
Polymer and Hybrid Available
Form
Shape
Size
Thermoplastics (Engineering
Resins)
Thermosets
metal/ceramics/organic materials
Structural Foam
Elastomers
Polymers Alloys
Liquid Crystal Polymers
L/D Ratio
Micro vs. Nano
Structural
Hierarchy
Selection of Materials
Mechanical Properties
Viscoelastic Behavior
Degradations
Wear Resistance and Frictional
Properties
Electrical Properties
Optical Properties
Magnetic Properties
Thermal Properties
Barrier Properties
Other Functional Properties
What about the surface?
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 9
What is a Device?
Source: wordnetweb.princeton.edu/perl/webwn
1. An instrumentality invented for a particular purpose; "the
device is small enough to wear on your wrist"; "a device
intended to conserve water"
2. Something in an artistic work designed to achieve a
particular effect
3. Any clever maneuver; "he would stoop to any device to win
a point"; "it was a great sales gimmick"; "a cheap
promotions gimmick for greedy businessmen"
4. Any ornamental pattern or design (as in embroidery)
5. An emblematic design (especially in heraldry); "he was
recognized by the device on his shield"
Anything Useful is a Device!
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 10
Why Making a Device – First, there is a need!
(from a Market Pull)
INPUT
MATERIAL DEVICE
INPUT
OUTPUT
OUTPUT
Xi
f (Xi)
Process
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 11
How to make Device – Must have a need first!
INPUT
MATERIAL DEVICE
INPUT
OUTPUT
OUTPUT
Xi
f (Xi)
Process
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 12
Unmet Needs?
OUTPUT
INPUT
Xi
f (Xi)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 13
Making a Better Device is a Continuous Process
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 14
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 15
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 16
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 17
INPUT
OUTPUT
INPUT
MATERIAL DEVICE
Charge
Current
Magnetization
Strain
OUTPUT
Temperature
Light
Permittivity
Conductivity
Electromag.
effect
Converse
Piezo-effect
Electrocaloric
effect
Electro-opic
effect
Magneto-elect.
effect
Pemeability
Magnetostriction
Magnetocaloric
effect
Magnetooptic
effect
Stress
Piezoelectric
effect
Piezomagneto
effect
Elastic
constant
Heat
Pyroelectric
effect
Thermal
expansion
Light
Photovoltaic
effect
Photostriction
Sensor
Actuator
Electric Field
Magnetic Field
Photoelastic
effect
Specific heat
Refractive
index
Off-diagonal Coupling: Smart Materials
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 18
give up 1e
give up 2e
give up 3e
• Columns: Similar Valence Structure
H
accept 2e
accept 1e
inert gases
Source of Materials to Make Any Device
He
Li Be
O
F Ne
Na Mg
S
Cl Ar
K Ca Sc
Rb Sr
Se Br Kr
Y
Te
Cs Ba
I
Adapted from
Fig. 2.6,
Callister 7e.
Xe
Po At Rn
Fr Ra
Electropositive elements:
Readily give up electrons
to become + ions.
Kwok Wai Lem – (KU 4471, Spring 2010)
Electronegative elements:
Readily acquire electrons
to become - ions.
Lectures #12
Slide # 19
Device Build-up Mechanics
Devices
Structures
Hierarchy
Clusters
Molecules
Functional
Components
Functional
Structures
Functional
Materials
Functional
Chemicals
Features
Atom
s
Ground Work for Continuous Improvement
(Kaizen) in Materials Development
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 20
Typical TFT Structure
A
JFET
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 21
Key Difference with CMOS
Si MOSFET
A-Si:H TFT
Organic TFT
Oxide TFT
Process
Temperature
1000 °C
250 °C
Room Temp.
150 °C
Process
Technology
Photolithography
Photolithography
Roll-to-Roll /
Ink-Jet
RF
Sputtering
Min. Length
<= 65 nm
10 μm
50 μm
10 μm
Substrate
Si Wafer
Glass
/Plastic
Plastic/
Metal Foil
Glass
/Plastic
Device Type
N- & P-type
N-type
P-type
N-type
Mobility
1500 cm2/V-s
1 cm2/V-s
0.5 cm2/V-s
> 10 cm2/V-s
Cost/Area
High
Medium
Low
Low
Lifetime
Years
Months
Weeks
Years
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Source: Cheng, UCSB, 2009
Slide # 22
Definition of Terms
• Precision
– The degree of agreement (or variability)
between individual measurements or
test results from measuring the same
specimen(s)
• Accuracy (Bias)
– The difference between the average of
the measurement error distribution and
the reference value of the specimen
measured
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Source: Luftig, U Colorado at Boulder
Slide # 23
Precision
Precision vs. Accuracy
Accuracy
One Thing Only – Focus!
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Source: Luftig, U Colorado at Boulder
Slide # 24
Definition of Terms
• Repeatability
– The variation in repeated measurements of
the same items with a single measurement
system
– Within appraiser/system variation
• Reproducibility
– The variation in the average measurements
by different appraisers or systems
measuring the same items
– Between appraiser/system variation
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Source: Luftig, U Colorado at Boulder
Slide # 25
Measurement Error
Robust!
Distribution of repeated
measures on a single
specimen or part
Precision
- Repeatability
- Reproducibility
Accuracy
(Bias)
Reference Value
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Source: Luftig, U Colorado at Boulder
Slide # 26
Junction Devices
1. Metal/ Metal
2. Metal/ Semiconductors
3. Semiconductor/Semiconductors
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 27
Junction Devices
1. Metal/ Metal
2. Metal/ Semiconductors
3. Semiconductor/Semiconductors
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 28
STANDARD HYDROGEN ELECTRODE
• Two outcomes:
-- Electrodeposition
H2(gas)
Mn+ H+
ions
H+
e-
25°C
e-
ne -
2e -
metal, M
metal, M
ne -
e-
Platinum
e-
Mn+
ions
H+ 2e H+
Platinum
-- Corrosion
25°C
1M Mn+ sol’n 1M H + sol’n
1M Mn+ sol’n 1M H+ sol’n
-- Metal is the anode (-)
-- Metal is the cathode (+)
o
Vmetal
 0 (relative to Pt)
o
Vmetal
 0 (relative to Pt)
Standard Electrode Potential
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Adapted from Fig. 16.2,
Callister & Rethwisch 3e.
Slide # 29
Example
(a) Briefly explain the difference between oxidation and reduction
electrochemical reactions.
(b) Which reaction occurs at the anode and which at the cathode?
Solution
(a) Oxidation is the process by which an atom gives up an electron (or electrons)
to become a cation. Reduction is the process by which an atom acquires an extra
electron (or electrons) and becomes an anion.
(b) Oxidation occurs at the anode; reduction at the cathode.
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 30
STANDARD EMF SERIES
more anodic
more cathodic
• EMF series
metal
Au
Cu
Pb
Sn
Ni
Co
Cd
Fe
Cr
Zn
Al
Mg
Na
K
• Metal with smaller
o
o
Vmetal
Vmetal
corrodes.
+1.420 V
• Ex: Cd-Ni cell
+0.340
o  Cd corrodes
Vo < V Ni
Cd
- 0.126
- 0.136
+
- 0.250
o
- 0.277 DV =
- 0.403 0.153V
- 0.440
Cd
Ni
25°C
- 0.744
- 0.763
- 1.662
1.0 M
1.0 M
- 2.363
Cd 2+ solution Ni 2+ solution
- 2.714
Adapted from Fig. 16.2,
Data based on Table 17.1,
Callister & Rethwisch 3e.
- 2.924
Callister 7e.
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 31
EFFECT OF SOLUTION CONCENTRATION AND
TEMPERATURE
• Ex: Cd-Ni cell with
standard 1 M solutions
V V  0.153 V
o
Ni
o
Cd
-
Cd
+
25°C
Ni
1.0 M
1.0 M
Cd 2+ solution Ni 2+ solution
Kwok Wai Lem – (KU 4471, Spring 2010)
• Ex: Cd-Ni cell with
non-standard solutions
VNi  VCd
-
Cd
RT X
 V V 
ln
nF Y
o
Ni
o
Cd
+
T
Ni
XM
YM
Cd 2+ solution Ni 2+ solution
• Reduce VNi - VCd by
-- increasing X
-- decreasing Y
-- increasing T
Lectures #12
n = #eper unit
oxid/red
reaction
(= 2 here)
F=
Faraday's
constant
= 96,500
C/mol.
Slide # 32
GALVANIC SERIES
more anodic
(active)
more cathodic
(inert)
• Ranking of the reactivity of metals/alloys in seawater
Platinum
Gold
Graphite
Titanium
Silver
316 Stainless Steel (passive)
Nickel (passive)
Copper
Nickel (active)
Tin
Lead
316 Stainless Steel (active)
Iron/Steel
Aluminum Alloys
Cadmium
Zinc
Magnesium
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Based on Table 16.2, Callister &
Rethwisch 3e. (Source of Table
16.2 is M.G. Fontana, Corrosion
Engineering, 3rd ed., McGrawHill Book Company, 1986.)
Slide # 33
Fermi Level
• focus on the electrons near the filled/empty boundary.
• each material’s energy state distribution is unique; different EF.
Minimum
energy to
remove
electron
from
sample
E=0 (vacuum level)
EF (Fermi level)
EF (Fermi level)
Metal 1
Metal 2
• the closer an electron is to the vacuum level, the weaker it is bound to the solid
• or, the more energetic is the electron
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Source: Thomas, U of Guelph
Slide # 34
Two Conductors in Contact
–+
–+
–+
–+
–+
electron flow
leads to charge separation
Contact potential difference
Fermi level the same throughout sample
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Source: Thomas, U of Guelph
Slide # 35
(a) Electrons are more energetic in Mo, so they tunnel to the surface of Pt.
(b) Equilibrium is reached when the Fermi levels are lined up.
When two metals are brought together, there is a contact potential DV.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 36
There is no current when a closed circuit is formed by two different metals, even
though there is a contact potential at each contact.
The contact potentials oppose each other.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 37
Fermi Energy Significance
For a given metal the Fermi energy represents the
free energy per electron called the electrochemical
potential. The Fermi energy is a measure of the
potential of an electron to do electrical work (eV)
or nonmechanical work, through chemical or
physical processes.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 38
The Seebeck effect. A temperature gradient along a conductor gives rise
to a potential difference.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 39
Seebeck Effect
Seebeck effect (thermoelectric power)
is the built-in potential difference DV across a material
due to a temperature difference DT across it.
DV
S
DT
Sign of S
is the potential of the cold side with respect to the hot side;
negative if electrons have accumulated in the cold side.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 40
Seebeck coefficient for metals
S 
 k T
2
2
3eEFO
x
Mott and Jones thermoelectric power equation
x = a numerical constant that takes into account how various
charge transport parameters, such as the mean free path l,
depend on the electron energy.
x values are tabulated in Table 4.3
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 41
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 42
1. Consider two neighboring regions H (hot) and C (cold) with widths corresponding to
the mean Free paths l and l' in H and C.
2. Half the electrons in H would be moving in the +x direction and the other half in the –
x direction.
3. Half of the electrons in H therefore cross into C, and half in C cross into H.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 43
(a) If Al wires are used to measure the Seebeck voltage across the Al rod, then the
net emf is zero.
(b) The Al and Ni have different Seebeck coefficients. There is therefore a net emf in
the Al-Ni Circuit between the hot and cold ends that can be measured.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 44
Thermocouple
We can only measure differences between thermoelectric powers of
materials.
When two different metals A and B are connected to make a
thermocouple, then the net EMF is the voltage difference between the
two elements.
VAB   S A  S B dT   S AB dT
T
T
To
To
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 45
Thermocouple Equation
VAB  aDT  b(DT )
2
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 46
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 47
Thermocouples are widely used to measure the
temperature.
LEFT: A thermocouple pair embedded in a
stainless steel sheath-probe. The thermocouple
junction inside the probe is in thermal contact
with the probe tip, and, electrically insulated
from the probe metal.
|SOURCE: Courtesy of Omega
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 48
Output emf versus temperature (˚C) for various thermocouple between 0 and 1000 ˚C
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 49
Junction Devices
1. Metal/ Metal
2. Metal/ Semiconductors
3. Semiconductor/Semiconductors
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 50
Formation of a Schottky junction between a metal and an n-type semiconductor when
m > n.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 51
The principle of the Schottky junction
solar cell.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 52
Reverse biased Schottky photodiodes are frequently used as fast photodetectors.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 53
Cross section of a typical thermoelectric cooler.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 54
Typical structure of a commercial thermoelectric cooler.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 55
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 56
Junction Devices
1. Metal/ Metal
2. Metal/ Semiconductors
3. Semiconductor/Semiconductors
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 57
The Bipolar Junction Transistor: BJT
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 58
(a) A schematic illustration of
the pnp bipolar transistor
with three differently doped
regions.
(b) The pnp
bipolar
operated
under normal
and active
conditions.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 59
(c) The common
base (CB)
configuration with
input and output
circuits identified.
(d) The illustration of
various current
components under
normal and active
conditions.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 60
Emitter Junction: The Law of the Junction
Hole concentration just outside the depletion region in the base at the emitter end
 eVEB 
pn (0)  pno exp 

 kT 
where VEB is the forward bias applied across the emitter-base (EB) junction
Hole concentration just outside the depletion region in the base at the collector end
pn (WB )  0
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 61
The Emitter Current
Holes diffuse through the base, from the emitter end to the
collector end. This diffusion is driven by the hole
concentration gradient dpn/dx.
Assume that the hole concentration profile is linear; it
decreases from pn(0) to 0 over the neutral base width WB.
(Initially, neglect the recombination of holes with electrons
in the base.)
eADh pn (0)
 dpn 
I E  eADh 
 
WB
 dx  x0
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 62
BJT Common base (CB) dc characteristics
Emitter Current
eADh pno
 eVEB 
IE 
exp 

WB
 kT 
where VEB is the forward bias applied across the emitter-base (EB) junction and WB is
the neutral base width.
Definition of CB current gain
IC

IE
Typically  is less than unity, in the range 0.990 - 0.999
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 63
BJT Common base dc characteristics
Total emitter current
I E  I E ( hole)  I E ( electron)
Emitter injection efficiency

I E ( hole)
I E ( hole)  I E ( electron)
1

1
I E ( electron)
I E ( hole)
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 64
BJT Common base dc characteristics
Definition of base transport factor T
T 
IC
I E ( hole)
IC

I E
If the emitter were a perfect injector, IE = IE(hole), then the current gain  would be T
Base minority carrier transit time
2
B
W
t 
2 Dh
This diffusion time is the transit time of the minority carriers across the base
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 65
BJT Common base dc characteristics
Base transport factor
T 
IC
I E ( hole)
t
 1
h
CB current gain
 t 
  T   1  
 h 
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 66
BJT Common base dc characteristics
Base current
 t 
t
I B    I E ( hole)  I E ( electron)   I E  1   I E
h
 h 
or
I B  I E  IC
Base to collector current gain
IC
 h

 

IB 1  t
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 67
DC I-V characteristics of the pnp bipolar transistor (exaggerated to highlight
various effects)
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 68
The Early Effect
The Early effect. When the BC reverse bias increases, the depletion width WBC
increases to W'BC increases to W'BC which reduces the base width WB to W'B. As pn(0) is
constant (constant VEB), the minority carrier concentration gradient becomes steeper and
the collector current IC Increases.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 69
A pnp transistor operated in the active region in the common base amplifier configuration.
The applied (input) signal veb modulates the dc voltage across the BE junction and hence
modulates the injected hole concentration up and down about the dc value pn(0). The solid
line shows how pn(x) is modulated up and down by the signal veb superimposed on VEE.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 70
(a)
(b)
(a) An npn transistor operated in the active region in the common emitter (CE) configuration. The dc voltage across
the BE junction, VBE, controls the current IE and hence IB and IC. The input current is the current that flows between
VBE and the base which is IB. The output current is the current flowing between VCE and the collector which is IC.
(b) DC I-V characteristics of the npn bipolar transistor in the CE configuration (exaggerated to highlight various
effects).
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 71
Common Emitter dc characteristics
Active region collector current
I C  I B  I CEO
where
I CBO
I CEO
 I CBO
1   
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 72
Common Base Amplifier
Small signal input resistance
VEB kT
25
re 


I E eI E I E (mA )
CB voltage gain (small signal)
vcb RC
AV 

veb
re
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 73
An npn transistor operated in the active region in the common emitter amplifier configuration. The
applied signal vbe modulates the dc voltage across the BE junction and hence modulates the injected
minority concentration up and down about the dc value np(0). The solid line shows np(x) when only the
dc bias VBB is present. The dashed line shows how np(x) is modulated up by a positive small signal vbe
superimposed on VBB.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 74
Common Emitter dc characteristics
Emitter current and VBE
 eVBE 
I E  I EO exp 

 kT 
where IEO is a constant
Input resistance (small signal)
vbe VBE
VBE
 25
rbe 



ib
I B
I E
I C (mA )
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 75
Common Emitter dc characteristics
Transconductance, gm
ic
I E
I E (mA ) 1
gm 



vbe VBE
25
re
Voltage gain
AV  g m RC
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 76
Low frequency small signal simplified equivalent circuit of the bipolar transistor in the CE
configuration with a load resistor RC in the collector circuit.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 77
1.
2.
The basic structure of the junction field effect transistor (JFET) with
an n-channel. The two p+ regions are electrically connected and form
the gate.
A simplified sketch of the cross section of a more practical n-channel
JFET
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 78
(b) VDS has increased to a value that allows the two
depletion layers to just touch, when VDS = VP (= 5
V) when the p+n junction voltage at the drain end,
VGD = -VDS = -VP = -5 V.
A
B
C
(a) The gate and source are shorted
(VGS = 0) and VDS is small
(c) VDS is large (VDS > VP) so that a short length
of the channel is pinched off.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 79
Typical ID vs. VDS characteristics of a JFET for various fixed gate voltages VGS.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 80
The pinched-off channel and conduction for VDS > VP (=5 V)
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 81
(a) The JFET with a negative VGS voltage has a narrower n-channel at the start. (b) Compared to the VGS
= 0 case, the same VDS gives less ID as the channel is narrower. (c) The channel is pinched off at VDS =
3V sooner than the VGS = 0 case where it was VDS = 5 V.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 82
Junction field effect transistor (JFET)
Pinch-off condition
VDS ( sat )  VP  VGS
where VGS is a negative voltage (reducing VP). Beyond pinch-off when VDS > VDS(sat),
the point P where the channel is just pinched still remains at potential VDS(sat), given
by the above equation.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 83
1. When VGS = - 5 V the depletion layers close the whole channel
from the start, at VDS = 0.
2. As VDS is increased there is a very small drain current which is the
small reverse leakage current due to thermal generation of carriers
in the depletion layers.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 84
(a) Typical IDS versus VGS characteristics of a JFET. (b) The dc circuit where VGS in the
gate–source circuit (input) controls the drain current IDS in the drain–source (output)
circuit in which VDS is kept constant and large (VDS > VP).
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 85
Junction field effect transistor (JFET)
Beyond pinch-off
I DS
  V


GS


 I DSS 1 


V
  GS ( off ) 
2
where,
IDSS is the drain current when VGS = 0
VGS(off) = –Vp; the gate-source voltage that just pinches off the channel
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 86
(a) Common source (CS) ac
amplifier using a JFET.
Kwok Wai Lem – (KU 4471, Spring 2010)
(b) Explanation of how ID is modulated
by the signal vgs in series with the dc
bias voltage VGG.
Lectures #12
Slide # 87
Example - The JFET
Consider an n-channel JFET that has a symmetric p+n gate-channel structure as shown in Figures A-a
and B. Let L be the gate length, Z the gate width, and 2a the channel thickness. The pinch-off voltage is
a 2 eN d
given by VP 
 Vo .
2
The drain saturation current, IDSS, is the drain current when VGS = 0. This occurs when VDS = VDS(sat) = VP
(Figure C) so IDSS = VPGch, where Gch is the conductance of the channel between the source and the
pinched-off point (Figure 6Q30). Taking into account the shape of the channel at pinch-off, if Gch is
about 1/3 of the conductance of the free or unmodulated (rectangular) channel, show that
 1 (e e N d )(2a ) Z 
I DSS  VP 

L
3

A particular n-channel JFET with a symmetric p+n gate-channel structure has a pinch-off
voltage of 3.9 V and an IDSS of 5.5 mA. If the gate and channel dopant concentrations are Na = 1019
cm-3 and Nd = 1015 cm-3, respectively, find the channel thickness 2a and Z/L. If L = 10 m, what is
Z? What is the gate-source capacitance when the JFET has no voltage supplies connected to it?
Figure A
Figure B
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Figure C
Slide # 88
Solution (Part a)
Solution
The conductivity of the channel is
 = eNde
The channel width is Z and the depth is 2a. Therefore the area A is A = 2aZ. The conductance of
the channel is given as 1/3 of the conductance of the free channel, therefore
Substitute:
Gch 
1 A
3 L
Gch 
2 eN d  e aZ
3
L
The voltage across the channel is the pinch-off voltage VP. At pinch-off the drain current is IDSS, given
as:
IDSS = VPGch

 2 eN d  e aZ 
I DSS  VP 

3
L


(1)
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 89
Solution (Part a)
(a) The gate and source are shorted (VGS = 0) and VDS is small.
(b) VDS has increased to a value that allows the two depletion layers to just touch,
when VDS = VP (= 5 V) and the p+n junction voltage at the drain end, VGD = VDS
= VP = 5 V.
(c) VDS is large (VDS > VP), so a short length of the channel is pinched off.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 90
Solution (Part a)
Assume temperature T = 300 K and that the JFET is Si. The relative permittivity of Si is r = 11.9 and
the intrinsic concentration is ni = 1.0  1010 cm-3. The channel donor concentration Nd is given as 1015
cm-3 and the gate acceptor concentration Na = 1019 cm-3. The electron drift mobility with Nd = 1015 cm-3
is approximately e = 1350 cm2 V-1 s-1 (the dopant concentration is too low to affect e). Vo can be
calculated as follows:


 10 21 m 3 10 25 m 3
kT  N d N a 
  (0.0259 V) ln
ln
Vo 
2
 1.0 1016 m 3 2
e  ni 




 


Vo = 0.835 V
We can calculate a from the given pinch-off voltage, VP = 3.9 V:
a 2 eN d
 Vo
VP 
2




2 VP  Vo 
211.9 8.854 10 12 F/m 3.9 V  0.835 V 

a
1.602 10 19 C 10 21 m 3
eN d

a = 2.50  10-6 m or 2.50 m


From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 91
Solution (Part a)
Therefore the channel width (2a) is 5.00 m. IDSS is given as 5.5 mA. The Z/L ratio can then be found
from Eqn. (1) above:
 2 eN d  e aZ 
I DSS  VP 

3
L



3I DSS
Z

L 2VP eN d  e a

Z
3 5.5 10 3 A

L 23.9 V  1.602 10 19 C 10 21 m 3 0.135 m 2 V 1 s 1 2.50 10 6 m

Z
 39.2
L







The channel length L is given as 10 m, therefore:
Z = 39.2(10  10-6 m) = 3.92  10-4 m or 392 m
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 92
Solution (Part b)
The gate area is Agate and is equal to Z  L = (3.92  10-4 m)(10  10-6 m) = 3.92  10-9 m2. The
depletion capacitance per unit area is:
 eN d N a 

Cdep  Agate 
2
(
N

N
)
V
d
a
o 

1
2
and the gate capacitance Cgate is twice Cdep as the JFET is symmetric and the two gates are connected in
parallel.
 1.602 10
Cgate  23.93 10 9 m 2 


19
C 11.9 8.854 10 F/m 10 m
2 10 21 m 3  10 25 m 3 0.835 V 
12

21

3
10
25
m
3
 
1
2


Cgate = 7.9  10-13 F or 0.79 pF
This neglects stray capacitances (e.g. between gate and source leads, gate and drain leads etc.).
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 93
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 94
JFET Amplifier
Definition of the JFET transconductance (small signal)
dI DS I DS
id
gm 


dVGS VGS vgs
JFET transconductance (small signal)
dI DS
2 I DSS
gm 

dVGS
VGS ( off )
  V
 2I I 1/ 2

DSS DS
1   GS  
VGS ( off )
  VGS ( off ) 
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 95
JFET Amplifier
Small-signal voltage gain
vds  RDid
AV 

vgs
vgs
AV 
 RD (g m v gs )
v gs
 g m RD
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 96
Example - The JFET Amplifier
Consider an n-channel JFET that
has a pinch-off voltage (VP) of 5
V and IDSS = 10 mA. It is used
in a common source
configuration as in Figure A-a in
which the gate to source bias
voltage (VGS) is -1.5 V. Suppose
that VDD = 25 V.
a. If a small signal voltage gain
of 10 is needed, what should
be the drain resistance (RD)?
What is VDS?
b.If an ac signal of 3 V peak-topeak is applied to the gate in
series with the dc bias
voltage, what will be the ac
output voltage peak-to peak?
Figure A
What is the voltage gain for
positive and negative input
signals? What is your
conclusion?
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 97
Solution (Part a)
Solution
Given, VP = 5 V, IDSS = 10 mA, VGS = VGG =1.5 V.
a. If a small signal voltage gain of 10 is needed, what should be the drain resistance (RD)?
2
I DS

  VGS 
   1.5 V 
  (10 mA) 1  
 I DSS 1  
 = 4.90 mA

V

5
V

 
  P 
gm 
dI DS 2 I DSS

VGS
VP
2
  VGS  2(10 mA)    1.5 V 
-3
 
=
2.80

10
A/V
1



1  


5 V    5 V 
   VP  
The small signal voltage gain AV = gmRD, so that
RD = AV /gm = (10)/(2.80  10-3 A/V) = 3571 
VDS is given by (IDS = 4.9 mA):
VDS = VDD  IDSRD

VDS = 25 V  (4.9  10-3 A)(3571 ) = 7.50 V
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 98
Solution (Part b)
b. VDD is given as 25 V. Let RD = 3571 .
Negative input signal:
When vsignal =1.5 V at the input then,
VGSmin = VGG + vsignal = 1.5 V1.5 V =3 V
2
  VGS 
   3 V 
  (10 mA) 1  
 I DSS 1  
 = 1.60 mA

V

5
V

 
  P 
2

I DS min

VDSmax = VDD  IDSminRD = (25 V)(1.60 mA)(3.571 k) = 19.29 V
For negative going signals, the gain is,
AV - 

Change in output voltage VDS max  VDS 19.29 V  7.51 V


Change in input voltage
vsignal
 1 .5 V
AV- = 7.85
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 99
Solution (Part b)
Positive input signal:
When vsignal = +1.5 V at the input then,
VGSmax = VGG + vsignal =1.5 V + 1.5 V = 0 V
2
  V 
  0 V 
 I DSS 1   GS   (10 mA) 1  
 = 10.0 mA
   5 V 
   VP 
2

I DS max

VDSmin = VDDIDSmaxRD = 25 V(10.0 mA)(3.571 k) =10.7 V
This is nonsense as VDSmin can not be below zero. The peak to peak voltage is then 19.3 V0 V = 19.3
V. Therefore the JFET has saturated with VDS  0 and the voltage across RD being VDD. This occurs
when IDS = IDSsat.
The JFET amplifier saturates when
IDS = IDSsat  VDD/RD = 25 V / (3.571 k) = 7.00 mA
For positive going signals the output eventually becomes saturated and the gain is,
AV  

Change in output voltage VDS min  VDS 0  7.5


Change in input voltage
vsignal
 1.5
AV + = 5.00
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 100
Solution (Part b)
The JFET amplifier is operating non-linearly. The negative going output signal will be clipped for the
positive going input signal. The negative sign in both cases represents a phase shift of 180.
Can we find the signal vsignal for this saturation (clipping) condition in the positive going input
signal? Normally this would be done using a load line with the JFET characteristics (as in electronics
circuits courses). It may be thought that we can at least estimate VGSmin from
I DS
  V 
 I DSS 1   GS 
   VP 
2
with IDS = IDSmax to find the required VGSmin. However this equation is not valid when VDS < VDS(sat),
see Figure B, which will be the case when the drain current drops VDD across RD. As a very rough
estimate, using the above equation with IDSmax = 10 mA, gives VGSmin =0.82 V which can be
interpreted as a rough condition for approaching saturation (clipping). Thus when VGG + vsignal 
0.82 V, the output should be approaching saturation, or when vsignal  +0.68 V.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 101
Solution (Part b)
Figure B: Typical ID versus VDS characteristics of a JFET for various fixed gate voltages VGS.
VDS
19.3
7.5
time
0

1.5
time
–1.5
Figure C
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 102
The Field Effect
(a) In a metal-air-metal
capacitor, all the charges reside
on the surface
(b) Illustration of field
penetration into a p-type
semiconductor
(c) As the field increases
eventually when V > Vth an
inversion layer is created near
the surface in which there are
conduction electrons.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 103
The basic structure of the enhancement MOSFET and its circuit symbol.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 104
SEM cross section of a MOS Transistor
|SOURCE: Courtesy of Don Scansen, Semicondutcor Insights, Kanata, Ontario, Canada
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 105
The MOSFET ID vs. VDS characteristics
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 106
The MOSFET ID vs. VDS characteristics
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 107
(a) Typical ID vs VDS characteristics of
an enhancement MOSFET (Vth = 4
V) for various Fixed voltages VGS.
(b) Dependence of ID on
VGS at a given VDS (>
VDS(sat))
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 108
Example -Ultimate limits to device performance
a. Consider the speed of operation of an n-channel FET-type device. The time required for an electron
to transit from the source to the drain is t = L/vd, where L is the channel length and vd is the drift
velocity. This transit time can be shortened by shortening L and increasing vd. As the field increases,
the drift velocity eventually saturates at about vdsat = 105 m s-1 when the field in the channel is equal
to Ec  106 V m-1. A short t requires a field that is at least Ec.
1. What is the change in the PE of an electron when it traverses the channel length L from source to
drain if the voltage difference is VDS?
2. This energy must be greater than the energy due to thermal fluctuations, which is of the order of
kT. Otherwise, electrons would be brought in and out of the drain due to thermal fluctuations.
Given the minimum field and VDS, what is the minimum channel length and hence the minimum
transit time?
b. Heisenberg's uncertainty principle relates the energy and the time duration in which that energy is
possessed through a relationship of the form DEDt > . Given that during the transit of the electron
from the source to the drain its energy changes by eVDS, what is the shortest transit time, , satisfying
Heisenberg's uncertainty principle? How does it compare with your calculation in part (a)?
c. How does electron tunneling limit the thickness of the gate oxide and the channel length in a
MOSFET? What would be typical distances for tunneling to be effective?
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 109
Solution (Part a)
Solution
Assume temperature T = 300 K. The saturation velocity is given as vdsat = 105 m/s and the saturation
field is given as Ec = 106 V/m.
a. (1) The change in the PE is DPE. This is the charge times the voltage, i.e. DPE = eVDS.
(2) The lower limit to DPE due to thermal fluctuations is kT. Therefore, substituting into the
equation above:
eVDS = kT

VDS = kT/e = (1.381  10-23 J/K)(300 K)/(1.602  10-19 C) = 0.02586 V
This is the lower limit to VDS. The minimum channel length L can now be found from the minimum
electric field, given by Ec = VDS/L:

L = VDS/Ec = (0.02586 V)/(106 V/m) = 2.59  10-8 m
The minimum transit time t is then,
t = L/vdsat = (2.586  10-8 m)/(105 m/s) = 2.59  10-13 s
The above limit is the thermal fluctuation limit (thermal noise limit).
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 110
Solution (Part b & c)
b. Consider the Heisenberg uncertainty principle. Let  = Dt be the transit time. During this time the
energy changes by DE = eVDS. We are given DEDt > , therefore, substituting for the shortest transit
time:
eVDS = 


1.055 10 34 J s
 

= 2.55  10-14 s
19
eVDS 1.602 10 C 0.02586 V 


The uncertainty limit allows a shorter transit time down to 0.0255 ps. Thus thermal fluctuation limit will
operate at room temperature.
c. If the oxide becomes too thin then the electron tunneling will allow gate charge to tunnel into
the channel. This will lead to a gate current. The field effect will fail. Similarly, if the source and
drain are very close there will then be a tunneling current, a drain current, even when the
transistor is off, one can guess that the thickness should be less than 1 nm or 10 Å depending on
various material properties. The same order of magnitude also applies to the minimum sourcedrain separation.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 111
Enhancement NMOSFET
Enhancement NMOSFET constant
Z e
K
2Lt ox
where e is the electron drift mobility in the channel, L and Z are the length and
width of the gate controlling the channel, and  and tox are the permittivity (ro) and
thickness of the oxide insulation under the gate
Enhancement MOSFET
I DS  K VGS  Vth  1  VDS 
2
Where  is a constant that is typically 0.01 V-1.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 112
(a) The threshold voltage and the ideal MOS structure.
(b) In practice, there are several charges in the oxide and at the oxide-semiconductor interface that effect the threshold
voltage: Qmi = Mobile ionic charge (e.g. Na+), Qot = Trapped oxide charge, Qf = Fixed oxide charge, Qit = Charge
trapped at the interface.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 113
Schematic illustration of ion implantation for the control of Vth.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 114
(a) There is an overlap of the gate electrode with the source and drain regions and hence
Additional capacitance between the gate and drain.
(b) n+type ion implantation extends the drain and source to line-up with the gate.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 115
The poly-Si gate technology. (a) Poly-Si is deposited onto the oxide and the areas outside
the gate dimensions are etched away. (b) The poly-Si gate acts as a mask during ion
implantation of donors to form the n+ source and drain regions. (c) A simplified schematic
sketch of the final poly-Si MOS transistor.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 116
(a) The energy band diagram of a p-n+ (heavily n-type doped) junction without any bias.
Built-in potential V0 prevents electrons from diffusing from n+ to p side.
(b) The applied bias reduces V0 and thereby allows electrons to diffuse, be injected, into the
p-side. Recombination around the junction and within the diffusion length of the electrons in
the p-side leads to photon emission.
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 117
Special Topic In Polymer Materials (KU 4471)
Lecture #12
Part 2
Professor Kwok Wai Lem
Department of Materials Chemistry and Engineering
Konkuk University
May 18, 2010
Kwok Wai Lem – (KU 4471, Spring 2010)
Lectures #12
Slide # 118