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 Thomas D. Anthopoulos Percentage 20 10 20 50 What is a device? Thomas D. Anthopoulos iPhone 4 launch, 2010 iPhone 4S launch, 2011 iPhone 5 launch, 2012 iPhone 5C/S launch, 2013 Thomas D. Anthopoulos iPhone 6 launch, 2014 Thomas D. Anthopoulos ← Hidden features… iPhone 6 launch, 2014 Thomas D. Anthopoulos ← Hidden features… the future..? → iPhone 6 launch, 2014 Thomas D. Anthopoulos New technologies have led to the development of flexible mobile phone prototypes… Thomas D. Anthopoulos Inside an iPhone Apple A8 microprocessor Touchscreen controller Thomas D. Anthopoulos 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) 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: 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” Thomas D. Anthopoulos 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 Thomas D. Anthopoulos 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 Thomas D. Anthopoulos Basics: the band theory of solids Splitting of atomic energy levels into bands Thomas D. Anthopoulos 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. Thomas D. Anthopoulos 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 Thomas D. Anthopoulos Band gap (EG) (valence band) VB N=∞ Energy bands in atom(s) and crystals 2p 2p 2p 2s 2s 1s 1s One atom Thomas D. Anthopoulos 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) 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 (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 Thomas D. Anthopoulos 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 Thomas D. Anthopoulos 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) 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 Diodes Thomas D. Anthopoulos 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 Thomas D. Anthopoulos 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 Thomas D. Anthopoulos 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 Thomas D. Anthopoulos 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. Thomas D. Anthopoulos 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 Thomas D. Anthopoulos 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 Thomas D. Anthopoulos 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. Thomas D. Anthopoulos 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 Thomas D. Anthopoulos 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 Thomas D. Anthopoulos 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) Thomas D. Anthopoulos Si Si Si p-type doping of Si (B acts as acceptor- NA)