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TFTs and Memories
Lecture 1
Thomas D. Anthopoulos
EXSS Group
Department of Physics and Centre for Plastic Electronics
Imperial College London
London
April 2015
Course Description
Course Description (5 ECTS)
This module will offer an introduction to organic thin-film transistors (TFTs) and
memory devices. The band theory of solids will guide the way to the energy band
diagram of metal-semiconductor (MS) contacts as a fundamental constituent of
electronic devices. By means of the metal-oxide-semiconductor (MOS) capacitor
concepts such as accumulation and depletion of charge carriers will be discussed. This
leads over to the structure and operating principle of field-effect transistors, their
device architectures, considerations on switching speeds and scaling. As part of their
applications, the role of TFTs in (unipolar/complementary) logic circuits, displays, and
memories will be introduced. The module will then look at general properties and
requirements of memories such as writing/reading speeds, retention time, endurance,
and scalability/integration. Different memory concepts (e.g. capacitive, resistive,
floating-gate) are introduced.
Grading
Activities
Homework
Presentation
Middle Term Exam
Final Exam
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Percentage
20
10
20
50
What is a device?
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iPhone 4 launch, 2010
iPhone 4S launch, 2011
iPhone 5 launch, 2012
iPhone 5C/S launch, 2013
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iPhone 6 launch, 2014
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← Hidden features…
iPhone 6 launch, 2014
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← Hidden features…
the future..? →
iPhone 6 launch, 2014
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New technologies have led to the development of flexible mobile
phone prototypes…
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Inside an iPhone
Apple A8 microprocessor
Touchscreen controller
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http://www.chipworks.com/
Ten metals in the stack
TEM of Samsung 45 nm transistor
in cross section
Outline
This part of the course will focus on the nature of metalsemiconductor contacts and various solid-state electronic devices
and their applications covering:
Lectures 1/2 Introduction & metal-semiconductor (MS)
contacts
Lecture 3 The metal-oxide-semiconductor (MOS) capacitor
Lecture 4 Introduction to field-effect transistors
Lecture 5 TFTs / MOSFETs and frequency response
Lecture 6 Applications of MOSFETs & TFTs
Lecture 7 BJTs and emerging electronics
Lectures 8 /9 Electronics manufacturing (current & future
technologies)
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From metals and insulators...
Benjamin Franklin advanced the ideas of positive and
negative charge, the electrical nature of lightning and the
use of good electrical conductors as lightning conductors.
You can read his letters to the Royal Society online!
E.P. Krider,
Physics Today, 2006
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Semiconductors
What is a semiconductor? Why are they so widely used?
Their use has been acknowledged in several Nobel prizes:
1956: Shockley, Bardeen and Brattain “for their researches on semiconductors and their
discovery of the transistor effect”. For the award ceremony speech, see:
http://nobelprize.org/nobel_prizes/physics/laureates/1956/press.html
2000: “for basic work on information and communication technology”: Alferov, Kroemer “for
developing semiconductor heterostructures used in high-speed- and opto-electronics” and Kilby
“"for his part in the invention of the integrated circuit"
Popular information: http://nobelprize.org/nobel_prizes/physics/laureates/2000/public.html
Advanced information: http://nobelprize.org/nobel_prizes/physics/laureates/2000/phyadv.pdf
2009: Boyle and Smith “for the invention of the imaging semiconductor circuit – the CCD
sensor” (half of prize; the other half being awarded to Kao for optical fibre communication)
Popular information:
http://nobelprize.org/nobel_prizes/physics/laureates/2009/info_publ_phy_09_en.pdf
Advanced info:http://nobelprize.org/nobel_prizes/physics/laureates/2009/sciback_phy_09.pdf
2014: Isamu Akasaki, Hiroshi Amano and Shuji Nakamura “for the invention of efficient blue
light-emitting diodes which has enabled bright and energy-saving white light sources”
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A brief history of electronics
K. Braun
(cathode
ray tube)
1897
J. Lilienfeld
(solid-state
amplifier,
electrolytic
capacitor)
N. Holonyak (Jr)
(visible LED,
semiconductor
LASER)
1925
1904 1907
A. Fleming L. De Forest
(tube
(audion or
rectifier)
vacuum
triode)
Microprocessor
Personal computer (PC)
1962
1947
1958
J. Bardeen
W. Shockley
W. Brattain
(bipolar junction
transistor)
J. Kilby
K. Lehovec
R. Noyce
(integrated circuit)
Solid-state electronics
Vacuum electronics
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Smart phones
Tablets….
A brief history of electronics
K. Braun
(cathode
ray tube)
1897
J. Lilienfeld
(solid-state
amplifier,
electrolytic
capacitor)
N. Holonyak (Jr)
(visible LED,
semiconductor
LASER)
1925
1904 1907
A. Fleming L. De Forest
(tube
(audion or
rectifier)
vacuum
triode)
1962
1947
1958
J. Bardeen
W. Shockley
W. Brattain
(bipolar junction
transistor)
J. Kilby
K. Lehovec
R. Noyce
(integrated circuit)
Solid-state electronics
Vacuum electronics
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Basics: the band theory of solids
Splitting of atomic energy levels into bands
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Basics: the band theory of solids
Splitting of atomic energy levels into bands
Energy (eV)
N=1
N=2
N=3
N=4
Formation of energy bands:
Let's consider, a solid made up
of a substance that involves
only one type of atomic
orbital.
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N=∞
Basics: the band theory of solids
Splitting of atomic energy levels into bands
Energy (eV)
n = 2 (p)
n = 1 (s)
p-band
n = 2 (p orbital)
(conduction band)
CB
s-p energy
difference
Formation of energy bands:
Let's now consider, a solid
made up of a substance that
involves two atomic orbital; s
and p.
s-band
n = 1 (s orbital)
N=1
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Band gap (EG)
(valence band)
VB
N=∞
Energy bands in atom(s) and
crystals
2p
2p
2p
2s
2s
1s
1s
One atom
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Conduction
Band
2s
Energy
band gap (EG)
Valence
Band
1s
Two atoms
Crystal
Range of allowed energies
The classic explanation for conduction difference between materials uses
the energy band model of solids that derives from quantum mechanics.
The figure below shows the allowed energy levels of a hydrogen atom
electron
Energy bands in solids
The electrical conductivity is a measure of the number of charge carriers
available for electric-field acceleration. Hence the nature of the band
picture of each solid should be indicative of conductivity. It turns out that
the fundamental difference is the size of the energy band gap (EG)
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Energy bands in solids
The electrical conductivity is a measure of the number of charge carriers
available for electric-field acceleration. Hence the nature of the band
picture of each solid should be indicative of conductivity. It turns out that
the fundamental difference is the size of the energy band gap (EG)
Conduction band (CB)
Energy band gap
(EG >> 5 eV)
CB
CB
(overlappin
g CB and VB
)
EG < 3 eV
Bands overlap
Valence band (VB)
VB
Insulator
Semiconductor
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VB
Metal
Energy bands in solids
(a few examples)
SiO2: EG = 9 eV
Diamond: EG = 5.47 eV
GaAs: EG = 1.41 eV
Si: EG = 1.12 eV
Ge: EG = 0.66 eV
Metals: Bands overlap
Graphene: EG = 0 V
Conduction band (CB)
(overlapping
CB and VB )
Energy band gap
(EG >> 5 eV)
CB
CB
EG < 3 eV
Bands overlap
Valence band (VB)
VB
Insulator
Semiconductor
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VB
Metal
Metal-Semiconductor (M-S) Contacts
Additional reading
[1] S.M. Sze, Physics of Semiconductor Devices, 2nd Edition, Wiley (1981)
[2] E.H. Rhoderick and R.H. Williams, Metal-Semiconductor Contacts, Oxford
University Press (1988)
[3] M.J. Cooke, Semiconductor Devices, Prentice Hall (1990)
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Metal-semiconductor (MS)
contacts
 Metal-semiconductor contacts are an obvious component of any
modern solid-state semiconductor device
 Few examples of solid-state electronic devices and integrated
circuits containing metal-semiconductor contacts are shown below
Diodes
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Transistors
Integrated circuits
MS contacts are indeed very
important
The Microprocessor
• Microprocessors can
be found in most
advanced
electronics (digital
watches, phones,
PCs…)
• State-of-the-art
microprocessors
contain several
billions (109)
transistors
• Transistors are also
used for storing data
• Transistors
dimensions <<100
nm
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Work function: Metals vs.
semiconductors
Energy band structure of a metal
E0
M
EFM
Metal
EFM = Fermi energy of the metal
EFS = Fermi energy of the semiconductor located
at the midgap for (undoped semic.)
EV = Valence band energy
EC = Conduction band energy
E0 = Vacuum energy
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S = Semiconductor work function
M = Metal work function
 = Electron affinity
EG = Band gap
EFi = Fermi energy of intrinsic
semiconductor
Work function: Metals vs.
semiconductors
Energy band structure of a metal and an intrinsic semiconductor
E0
E0

EC
M
S
EFS = EFi
EFM
EG
Metal
EFM = Fermi energy of the metal
EFS = Fermi energy of the semiconductor located
at the midgap for (undoped semic.)
EV = Valence band energy
EC = Conduction band energy
E0 = Vacuum energy
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Semiconductor
EV
S = Semiconductor work function
M = Metal work function
 = Electron affinity
EG = Band gap
EFi = Fermi energy of intrinsic
semiconductor
Traditional semiconductors
Silicon (Si): The building block of modern electronics
1
2
4
3
Silicon atom has four valence electrons - just
like carbon (C). Si is the second most
abundant element on earth – nearly a quarter
of the planet crust by weight.
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Silicon crystallizes in a diamond cubic
crystal structure. very large and nearly
perfect single crystals can be grown.
Energy bands in solids –
the case of Si
Si nucleus
Si
Si
Si
Si
Si
Si
Si
Si nucleus
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outmost e- layer
(4 electrons)
Energy bands in solids –
the case of Si
Si nucleus
Si
bound electrons
valence electrons
Si
Si
Si
Si
Si
Si
Si nucleus
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outmost e- layer
(4 electrons)
Traditional semiconductors
Silicon (Si): The building block of modern electronics
Si
Si
Si
Si
Si
Si
Si
Si
Si
Silicon atom forms crystal
lattice with bonds to four
neighbouring Si atoms. Pure
silicon has no free carriers and
conducts poorly.
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 Si atoms: covalently bonded (very strong bonds)
 Si-crystal: strong, very brittle and prone to chipping
 Excellent semiconductor when doped (carrier

mobility (µ) ≈ 1000 cm2/Vs)
Melting temperature: >1400 °C
Traditional semiconductors
Silicon (Si): The building block of modern electronics
Si
Si
Si
Si
Si
Si
Si
Si
Si
Silicon atom forms crystal
lattice with bonds to four
neighbouring Si atoms. Pure
silicon has no free carriers and
conducts poorly.
Thomas D. Anthopoulos
Energy bands in solids –
the case of Si
 Devices such as transistors and integrated circuits are built on a
silicon substrate (i.e. single crystal wafers)
 Silicon is a Group IV material
 Forms crystal lattice with bonds to four neighbors
 Pure silicon has no free carriers and conducts poorly
Representation of a
single crystal of Si 
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Si
Si
Si
Si
Si
Si
Si
Si
Si
Energy bands in solids –
the case of Si
 Devices such as transistors and integrated circuits are built on a
silicon substrate (i.e. single crystal wafers)
 Silicon is a Group IV material
 Forms crystal lattice with bonds to four neighbors
 Pure silicon has no free carriers and conducts poorly
 The Fermi level of undoped Si (EFi) is at the middle of the bandgap
E0

Si
Si
Si
EC
S
EFi  EC+EG/2
Si
Si
Si
EG
Si
Si
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Si
VB (Si)
EV
Energy bands in solids –
the case of Si
 Pure silicon has no free carriers and conducts poorly
 Adding dopants increases the conductivity
 Group V: extra electron (n-type) – e.g. doping Si with donor
semimetals such as Arsenic (As)
 Group III: missing electron, called hole (p-type) – e.g. doping Si
with acceptor semimetals such as Boron (B)
Si
Si
-
+
Si
Si
Si
+
-
Si
As
Si
Si
B
Si
Si
Si
Si
Si
n-type doping of Si
(As acts as donor - ND)
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Si
Si
Si
p-type doping of Si
(B acts as acceptor- NA)