Circuit design techniques for non-crystalline semiconductors

259 Pages·2012·13.188 MB·English

by  Sanjiv Sambandan

K12074 Cover 9/4/12 1:43 PM Page 1
C M Y CM MY CY CMY K
Materials Science/Electrical Engineering
CIRCUIT DESIGN S
a
m
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a
n
d
Techniques for Non-Crystalline a
n CIRCUIT
Semiconductors
Sanjiv Sambandan
Despite significant progress in materials and fabrication
technologies related to non-crystalline semiconductors,
fundamental drawbacks continue to limit real-world application
C
of these devices in electronic circuits. To help readers deal with
problems such as low mobility and intrinsic time variant
I D E S I G N
behavior, Circuit Design Techniques for Non-Crystalline
R
Semiconductors outlines a systematic design approach,
including circuit theory, enabling users to synthesize circuits C
without worrying about the details of device physics.
U
THIS BOOK:
I
• Offers examples of how self-assembly can be used as a
T
powerful tool in circuit synthesis
• Covers theory, materials, techniques, and applications
D Techniques for
• Provides starting threads for new research
E
This area of research is particularly unique since it employs a Non-Crystalline
range of disciplines including materials science, chemistry, S
mechanical engineering and electrical engineering. Recent
Semiconductors
progress in complementary polymer semiconductors and I
fabrication techniques such as ink-jet printing has opened G
doors to new themes and ideas. The book focuses on the
Sanjiv Sambandan
central problem of threshold voltage shift and concepts related N
to navigating this issue when using non-crystalline
semiconductors in electronic circuit design. Designed to give
the non-electrical engineer a clear, simplified overview of
fundamentals and tools to facilitate practical application, this
book highlights design roadblocks and provides models and
possible solutions for achieving successful circuit synthesis.
K12074
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CIRCUIT
D E S I G N
Techniques for
Non-Crystalline
Semiconductors
TThhiiss ppaaggee iinntteennttiioonnaallllyy lleefftt bbllaannkk
CIRCUIT
D E S I G N
Techniques for
Non-Crystalline
Semiconductors
Sanjiv Sambandan
Boca Raton London New York
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Version Date: 20120501
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Preface
Disorderedsemiconductorshavebeenofresearchinterestduetotheirpromise
of low temperature fabrication over large areas. The attempt to understand
the structure and electronic properties of these materials has generally been
driven by the need for a substitute for conventional mono-crystalline silicon
for applications in large-area electronics.
There has been significant progress made in the materials and fabrica-
tion technologies related to non-crystalline semiconductors. Recent research
on complementary polymer semiconductors and fabrication techniques such
as ink-jet printing has opened doors to new themes and ideas. However, all
these avenues for research meet the hard reality of low mobility and intrinsic
time variant behavior in field effect transistors based on these materials. Real
world application of these devices in electronic circuits are limited by these
fundamental drawbacks. One could argue that the main problem is not mo-
bility or lack of complementary devices, but the threshold voltage shift in the
devices that provide a bottleneck in lifetime and does not permit easy analog
circuit design.
What is missing is a circuit theory and systematic design approach that
would help a designer synthesize circuits without worrying about the nitty-
gritty of the physics of the device. In this book, I hope to highlight these
problems, provide models, and possible solutions to circuit synthesis with
these materials. The book revolves around the problem of threshold voltage
shift and the concepts related to the design of electronic circuits around this
problem.
This area of research is particularly unique since it brings people from
variousdisciplines—materialsscience,chemistry,mechanicalengineeringand
electrical engineering — over a common drawing board. This book has been
writtenwiththeaimofprovidingthe“non-electricalengineer”withthebasics
and tools that he or she could use to bring their research to the application
end and I sincerely hope that this book achieves this goal. While attempting
todosowithconstraintsontime,ashortcominginthedetailsandexposition
to very recent ideas may have appeared. I apologize for this and hope that
the book remains useful.
v
TThhiiss ppaaggee iinntteennttiioonnaallllyy lleefftt bbllaannkk
List of Figures
1.1 Series resistor-capacitor circuit. . . . . . . . . . . . . . . . . 9
1.2 Charge sharing between two capacitors. . . . . . . . . . . . . 11
1.3 Filtering Properties of RC circuits. . . . . . . . . . . . . . . 13
2.1 Band diagram of metals, semiconductors and insulators. . . 19
2.2 Nature of the density of states in a semiconductor assuming
equal effective masses for electrons and holes. . . . . . . . . 21
2.3 TheFermifunctionatdifferenttemperatures.Thevalueofthe
Fermi function is always 1/2 at the Fermi level. . . . . . . . 23
2.4 The free carrier density for an intrinsic, n-type doped and p-
type doped semiconductor. . . . . . . . . . . . . . . . . . . . 24
2.5 The formation of a rectifying metal-semiconductor junction. 32
2.6 Theformationofanohmicmetal-semiconductorjunction.The
semiconductor is n-type. . . . . . . . . . . . . . . . . . . . . 36
2.7 The formation of a p-n junction. . . . . . . . . . . . . . . . . 37
2.8 Metal-oxide-semiconductor (MOS) capacitor. The term “ox-
ide” is due to the typically used silicon oxide as a dielectric in
conventionalcrystallinesiliconMOStransistors.However,any
insulator can be present instead of the oxide and the device is
equivalently termed metal-insulator-semiconductor. . . . . . 40
2.9 Band bending at inversion in a MOS capacitor. . . . . . . . 42
2.10 Capacitance-Voltage (CV) characteristics of a MOS capacitor
with a p-type substrate. . . . . . . . . . . . . . . . . . . . . 45
2.11 MOS field effect transistor (MOSFET). . . . . . . . . . . . . 46
2.12 Transfer and output characteristics of the MOSFET. . . . . 47
3.1 Output characteristics of the MOSFET. . . . . . . . . . . . 52
3.2 Small signal gate bias fluctuations. . . . . . . . . . . . . . . 54
3.3 Small signal drain bias fluctuations. . . . . . . . . . . . . . . 56
3.4 Small signal output impedance of the MOSFET. . . . . . . . 57
3.5 Example for output impedance calculations in a MOSFET
circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.6 Example for gain or transfer function calculations in a MOS-
FET circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.7 Capacitances associated with a MOSFET. . . . . . . . . . . 61
3.8 Miller effect and Miller capacitances. . . . . . . . . . . . . . 62
vii
viii
3.9 Example for high frequency gain calculations in a MOSFET
circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.10 Noise analysis in a common source amplifier. . . . . . . . . . 70
4.1 States in the band gap of non-crystalline semiconductors. . . 78
4.2 Structure of the thin film transistor.. . . . . . . . . . . . . . 80
5.1 Fermilevelmovementinthestatesfordifferentgatebias.The
x-axis represents the density of states. . . . . . . . . . . . . 85
5.2 Typical transfer and output characteristics of a-Si:H TFTs.
The channel width is 200µm, and the channel length is 8µm. 87
6.1 Linear model for charge trapping. . . . . . . . . . . . . . . . 94
6.2 Model plotted against experimental data for a constant gate
bias. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
6.3 Model plotted against experimental data for a time varying
gate bias. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
7.1 Concept of a switch. (a) The open switch isolates point A
frompointBelectrically(b)Theclosedswitchconnectspoint
A and point B electrically (c) Voltage fluctuations at point A
are observed at point B when the switch is closed. . . . . . 104
7.2 A field effect transistor used as an electrical switch. The gate
terminal is used to control the switch, i.e., keep it open or
closed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
7.3 (a) Output characteristics of the n-channel TFT. (b)Output
characteristicsofthep-channelTFT.Forclosetoidealopera-
tion, a switch when closed must be biased in above threshold
linear region. . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
7.4 Simulation of the influence of TFT bias on switch perfor-
mance. (a) The circuit simulated. (b) Switch biased in linear
mode of operation. (c) Switch biased in saturation mode of
operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
7.5 (a)Switchcapacitorcircuitwherethedatavoltage,V ,istobe
d
writtenontothecapacitorviaanacessTFTswitch.(b)AIM-
SPICE simulations of the circuit illustrating the dynamics of
capacitor charging. . . . . . . . . . . . . . . . . . . . . . . . 109
7.6 (a) Equivalent circuit of the switch-capacitor using the TFT
current in linear operation. (b) Approximation of the TFT
switchwithanequivalentresistance.(c)Comparisonofmodels
of capacitor charging using the circuit of (a) and (b) with
comparisons to AIM-SPICE simulation of Figure 7.5 . . . . 111
7.7 Transfer characteristics of an a-Si:H TFT. . . . . . . . . . . 113
ix
7.8 (a) Mechanism of clock feedthrough where the time-varying
gate voltage influences the charge on the load capacitor via
coupling through the overlap capacitance. (b) Mechanism of
charge injection where the time varying gate voltage pumps
the channel charge in the TFT channel to the external capac-
itors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
7.9 (a) Circuit used for simulating the effects of charge injection
and clock feedthrough. (b) AIM-SPICE simulations of charge
injectionandclockfeedthrough.Arapidlytransitinggatevolt-
age induces error due to the mechanisms of charge injection
and clock feedthrough. . . . . . . . . . . . . . . . . . . . . . 117
8.1 (a) Conventional diode (b) Diode connected n-channel TFT
(c) Diode connected p-channel TFT. . . . . . . . . . . . . . 120
8.2 (a) TFT diode-capacitor series circuit. (b) Simulation of the
dynamicsofthevoltageonthecapacitortoapulseinputvolt-
age.Thediodeconfigurationenablesthecapacitortoholdthe
input peak after the pulse input has passed. There are losses
due to the diode leakage. . . . . . . . . . . . . . . . . . . . . 121
8.3 (a)Circuitsrequiringmanybiaslinesmusteitherbeprovided
with independent supplies, (b) or the bias provision can be
mademoreefficientlybytheuseofachainofdiodeconnected
TFTs of specific aspect ratios driven by one independent cur-
rentsource.(c)Simulationofthedynamicsofbiasgeneration.
Attimezero,thecurrentsourceissettotheappropriatevalue
and the dynamics is the response time of the diode chain to
charge the bias lines. . . . . . . . . . . . . . . . . . . . . . . 123
8.4 A two transistor LED driver circuit using TFTs. . . . . . . . 124
8.5 A modified LED driver circuit using a diode connected TFT
for threshold voltage shift compensation. . . . . . . . . . . . 125
8.6 (a) Application of the diode TFT as a compensation circuit
forthethresholdvoltageshiftinadrivingTFT(heretheTFT
with W=10um and L=20um). These circuits find application
in LED display architectures as will be discussed in the chap-
ters ahead. (b) Simulation of the dynamics of circuit operation.125
8.7 (a) Use of the diode-capacitor series circuit as a peak detect.
(b) Simulations of the dynamics of the peak detect operation
of the diode-capacitor series circuit. . . . . . . . . . . . . . . 126
9.1 Current mirror. . . . . . . . . . . . . . . . . . . . . . . . . . 131
9.2 Common source amplifiers with resistive, non-complementary
TFT and complementary TFT load.. . . . . . . . . . . . . . 132
9.3 Common drain amplifier with resistive, non-complementary
TFT and complementary TFT load.. . . . . . . . . . . . . . 133