TFTs and Memories Lecture 1 Thomas D. Anthopoulos EXSS Group Department of Physics and Centre for Plastic Electronics Imperial College London London April 2015

Thomas D. Anthopoulos 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. ActivitiesPercentage Homework20 Presentation10 Middle Term Exam20 Final Exam50 Grading Thomas D. Anthopoulos

What is a device?

Thomas D. Anthopoulos iPhone 5 launch, 2012iPhone 5C/S launch, 2013 iPhone 4S launch, 2011iPhone 4 launch, 2010

Thomas D. Anthopoulos iPhone 6 launch, 2014

Thomas D. Anthopoulos iPhone 6 launch, 2014 ← Hidden features…

Thomas D. Anthopoulos iPhone 6 launch, 2014 the future..? → ← Hidden features…

Thomas D. Anthopoulos New technologies have led to the development of flexible mobile phone prototypes…

Thomas D. Anthopoulos Inside an iPhone Touchscreen controller TEM of Samsung 45 nm transistor in cross section Apple A8 microprocessor Ten metals in the stack

Thomas D. Anthopoulos Outline This part of the course will focus on the nature of metal- semiconductor 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 7BJTs and emerging electronics Lectures 8 /9Electronics manufacturing (current & future technologies)

Thomas D. Anthopoulos 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

Thomas D. Anthopoulos 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: : “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: Advanced information: : 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: Advanced info: 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”

Thomas D. Anthopoulos A brief history of electronics Vacuum electronics 1897 K. Braun (cathode ray tube) 1904 A. Fleming (tube rectifier) L. De Forest (audion or vacuum triode) 1907 J. Bardeen W. Shockley W. Brattain (bipolar junction transistor) 1962 J. Kilby K. Lehovec R. Noyce (integrated circuit) N. Holonyak (Jr) (visible LED, semiconductor LASER) Microprocessor Tablets….Smart phones Personal computer (PC) J. Lilienfeld (solid-state amplifier, electrolytic capacitor) 1925 Solid-state electronics

Thomas D. Anthopoulos A brief history of electronics Vacuum electronics 1897 K. Braun (cathode ray tube) 1904 A. Fleming (tube rectifier) L. De Forest (audion or vacuum triode) 1907 J. Bardeen W. Shockley W. Brattain (bipolar junction transistor) 1962 J. Kilby K. Lehovec R. Noyce (integrated circuit) N. Holonyak (Jr) (visible LED, semiconductor LASER) J. Lilienfeld (solid-state amplifier, electrolytic capacitor) 1925 Solid-state electronics

Thomas D. Anthopoulos Splitting of atomic energy levels into bands Basics: the band theory of solids

Thomas D. Anthopoulos Formation of energy bands: Let's consider, a solid made up of a substance that involves only one type of atomic orbital. N = 1 N = 2 N = 3 N = 4 N = ∞ Energy (eV) Splitting of atomic energy levels into bands Basics: the band theory of solids

Thomas D. Anthopoulos Splitting of atomic energy levels into bands Basics: the band theory of solids Formation of energy bands: Let's now consider, a solid made up of a substance that involves two atomic orbital; s and p. n = 1 (s) n = 2 (p) Energy (eV) s-band (valence band) VB p-band (conduction band) CB Band gap (E G ) N = ∞ n = 1 (s orbital) n = 2 (p orbital) N = 1 s-p energy difference

Thomas D. Anthopoulos Energy bands in atom(s) and crystals 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 2p 2s 1s One atom 2p 2s 1s Two atoms Crystal 2p Conduction Band 1s 2s Valence Band Energy band gap (E G ) Range of allowed energies

Thomas D. Anthopoulos 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 (E G )

Thomas D. Anthopoulos 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 (E G ) Valence band (VB) Conduction band (CB) Energy band gap (E G >> 5 eV) Insulator VB CB Semiconductor E G < 3 eV VB CB Metal (overlappin g CB and VB ) Bands overlap

Thomas D. Anthopoulos Energy bands in solids (a few examples) Valence band (VB) Conduction band (CB) Energy band gap (E G >> 5 eV) Insulator VB CB Semiconductor E G < 3 eV SiO 2 : E G = 9 eV Diamond: E G = 5.47 eV GaAs: E G = 1.41 eV Si: E G = 1.12 eV Ge: E G = 0.66 eV Metals: Bands overlap Graphene: E G = 0 V VB CB Metal (overlapping CB and VB ) Bands overlap

Thomas D. Anthopoulos 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)

Thomas D. Anthopoulos 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 DiodesTransistorsIntegrated circuits

Thomas D. Anthopoulos 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 (10 9 ) transistors Transistors are also used for storing data Transistors dimensions <<100 nm

Thomas D. Anthopoulos Work function: Metals vs. semiconductors E FM = Fermi energy of the metal E FS = Fermi energy of the semiconductor located at the midgap for (undoped semic.) E V = Valence band energy E C = Conduction band energy E 0 = Vacuum energy Metal E FM  M E0E0  S = Semiconductor work function  M = Metal work function  = Electron affinity E G = Band gap E Fi = Fermi energy of intrinsic semiconductor Energy band structure of a metal

Thomas D. Anthopoulos Semiconductor EVEV ECEC E FS = E Fi  E0E0  S EGEG Work function: Metals vs. semiconductors E FM = Fermi energy of the metal E FS = Fermi energy of the semiconductor located at the midgap for (undoped semic.) E V = Valence band energy E C = Conduction band energy E 0 = Vacuum energy Metal E FM  M E0E0  S = Semiconductor work function  M = Metal work function  = Electron affinity E G = Band gap E Fi = Fermi energy of intrinsic semiconductor Energy band structure of a metal and an intrinsic semiconductor

Thomas D. Anthopoulos Traditional semiconductors Silicon (Si): The building block of modern electronics 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. Silicon crystallizes in a diamond cubic crystal structure. very large and nearly perfect single crystals can be grown

Thomas D. Anthopoulos Energy bands in solids – the case of Si Si Si nucleus outmost e - layer (4 electrons) Si Si nucleus

Thomas D. Anthopoulos Si Si nucleus outmost e - layer (4 electrons) Si bound electrons valence electrons Si nucleus Energy bands in solids – the case of Si

Thomas D. Anthopoulos Traditional semiconductors Silicon (Si): The building block of modern electronics Silicon atom forms crystal lattice with bonds to four neighbouring Si atoms. Pure silicon has no free carriers and conducts poorly.  Si atoms: covalently bonded (very strong bonds)  Si-crystal: strong, very brittle and prone to chipping  Excellent semiconductor when doped (carrier mobility (µ) ≈ 1000 cm 2 /Vs)  Melting temperature: >1400 °C

Thomas D. Anthopoulos Traditional semiconductors Silicon (Si): The building block of modern electronics Silicon atom forms crystal lattice with bonds to four neighbouring Si atoms. Pure silicon has no free carriers and conducts poorly.

Thomas D. Anthopoulos  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  Energy bands in solids – the case of Si

Thomas D. Anthopoulos  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 (E Fi ) is at the middle of the bandgap VB (Si) EVEV ECEC E Fi  E C +E G /2  E0E0  S EGEG Energy bands in solids – the case of Si

Thomas D. Anthopoulos  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) n-type doping of Si (As acts as donor - N D ) p-type doping of Si (B acts as acceptor- N A ) Energy bands in solids – the case of Si