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The commercial application of TFTs has been driven by the use of hydrogenated amorphous silicon, a-Si:H, which is the workhorse of the industry. However, a greater range of TFT materials, such as polycrystalline silicon, amorphous oxide semiconductors and organic semiconductors, are now commanding interest, and this book has been written as an accessible introduction to the field.
Whilst there are a number excellent books aimed at the specialist working with these individual materials, this book was conceived to meet the needs of a less specialist audience.
The book was written for a target audience including undergraduate and postgraduate students taking display-related courses, postgraduate and new researchers to the field, engineers supplying equipment to the flat panel display industry, researchers in one specialist TFT field looking for an overview of another and technical managers within the FPD industry.
Whilst some familiarity with the background physics of semiconductor device operation has been assumed, the book is intended to be self-contained, with introductory chapters covering the device physics concepts needed to follow the later chapters on the TFTs themselves. These introductory chapters deal with band bending effects at semiconductor surfaces, electron-hole pair generation and recombination and the operation of classical MOSFET devices. A further chapter covers the operation of active matrix displays.
This book has grown principally out of a series of lectures on TFT technologies given to an Italian research consortium, and which incorporated material from specialist lectures on active matrix displays and TFTs presented in Masters courses at Southampton and Dundee Universities.
It is a pleasure to acknowledge the contribution made by the referees with their insightful and helpful comments, and these have undoubtedly improved the book. In addition, colleagues and other contacts within the research community have generously responded to my requests for background information and access to published material. These have included Dr. The major application area is flat panel, active matrix liquid crystal displays, AMLCDs, and, with the range of screen diagonals currently available, this is the most ubiquitous and successful display technology to date.
Screen sizes from 1 inch to more than inches dominate applications in most areas of life, from small, portable mobile phone displays, through medium-sized tablets, netbooks and lap-tops, to large-screen monitors and HD televisions.
In the s, there was early work into a variety TFT materials and applications, and an engaging and personal account of that work has been given by Brody [1,2], who was a pioneer of both TFTs and active matrix displays. However, the present interest in thin film transistors can be dated back to the research of Spear and Le Comber [3,4] in the mids.
They showed that glow discharge hydrogenated amorphous silicon, a-Si:H, had a low enough trap state density that it could be doped, and used in thin film transistor structures. This stimulated worldwide research and development of the a-Si:H TFT, and led to its application in active matrix liquid crystal displays.
From the first, small scale AMLCD production facilities in the late s, processing A4-sized glass plates, to the current 10th generation plant, processing 2.
Whilst AMLCDs continue to dominate the current product range in terms of volume, other display media are now appearing. These are electro-phoretic displays, EPDs, for e-readers, and organic light emitting diode, OLED, displays for smart phones, and which are predicted to be of longer-term interest for TV. The contents of these sections progress from basic semiconductor physics to speculative new materials and device structures.
In the introductory Part I, Chaps. Part II, containing Chaps. These newer materials are attracting much interest as they have considerably higher carrier mobility than a-Si:H, but, due to their amorphous nature, are expected to have comparable excellent uniformity. In the preferred implementation, these TFTs contain organic materials for both the semiconductor and the gate dielectric layers. They are currently low carrier mobility devices, but can be produced by low temperature solution-processing, including printing, and are seen as a cost-effective route to flexible electronics on polymer substrates.
Finally, Chap. This has markedly different characteristics from conventional TFTs, and may be an interesting vehicle for achieving improved performance from low-mobility disordered materials, including organic semiconductors. Hence, as an introduction to the TFT field, Chaps. In particular, Chap. This includes the bending of the semiconductor energy bands, and the change in free carrier concentration at the semiconductor surface, in response to an orthogonally applied electric field.
The relationships are derived analytically for a single crystal substrate, and these expressions are then used as reference points in later chapters to clarify the changes needed to describe TFT behaviour. The other topic covered is electron-hole pair recombination and generation, and these are processes governing non-thermal equilibrium effects in TFTs, such as photoconductivity and leakage currents.
Whilst most practical TFT analysis makes use of 2-D numerical modelling packages, an analytical approach is used here in order to illustrate the underlying physics. However, the relevant 2 1 Introduction IntroductionIn this chapter, device physics topics are introduced, which are relevant to TFTs, and for following the discussion in later chapters. The emphasis is on background understanding of basic device physics principles, and an analytical approach is followed, using single crystal semiconductor equations.
These concepts and equations are modified in later chapters for the more complex situation of the nonsingle crystal TFT materials. The first topic presented deals with semiconductor surface physics, focussing on band bending and surface charge in the metal-insulator-semiconductor, MIS, system. This underlies the relationship between the voltage on the metal gate and the induced charge in the semiconductor surface, and introduces the concepts of the flat band voltage and the threshold voltage for surface inversion.
In Chap. The second topic is electron-hole pair generation and recombination. These are basic processes underlying both leakage current effects, and steady state carrier densities in devices under injection conditions, such as optical illumination. Much current research into device behaviour makes use of commercial device simulators to solve these latter equations.
These simulation packages give a deeper insight into device behaviour by relating its current-voltage characteristics to internal field and carrier distributions. However, in published work, the fundamental equations are rarely listed. In the final Sect. As is apparent, the range of device physics topics covered here is limited, and, for a broader coverage of this field, there are many excellent books available, such as Sze and Ng .
Examples of device simulation packages can be found in Refs. Some of the key simplified equations, from Sects.
These are useful for cross-reference purposes in later chapters, and also for direct, analytical calculations. In the treatment below, the following conventions will be used: the Fermi potential, V F , will be measured from the bulk intrinsic level, E i , and will be taken as positive beneath E i and negative above it. Similarly, the band bending, V s , will be measured from the bulk intrinsic level, and the polarity convention will be the same as used for V F.
Semiconductor Surface Physics Ideal MIS Capacitor and Surface Band BendingWhen a positive charge, Q G , is placed on the metal gate, it will induce an equal and opposite negative charge in the semiconductor, Q s , and this negative charge will consist of an increase in the electron density and a decrease in the free hole density, thereby, leaving behind immobile, ionised acceptor centres, N a.
In order to accommodate these changes in free carrier density, the bands within the semiconductor will have to bend downwards near its surface, as shown in Fig. It will also be seen that the positive charge on the gate results from a positive bias being applied to the gate relative to the semiconductor.
The situation shown in this diagram is for a small positive potential on the gate, such that the surface electron density, n s , is small compared with N a , and the surface is said to be depleted of free holes. For a larger positive gate bias, the situation shown in Fig. In this case, there is a corresponding increase in band bending, V s , and the free electron concentration at the surface is larger than N a : the surface is now said to be inverted.
Further positive band bending beyond this point will lead to n s [ p s. Finally, as shown in Fig. This is associated with the bands bending upwards by an amount -V s. In this case, the surface is said to be accumulated. In these two regimes, the main contributors to Q s are the free carriers, so that the hole and electron densities increase exponentially with V s in accumulation and inversion, respectively. In the third regime of majority carrier depletion, Q s increases with HV s , and extrapolating the inversion arm of the curve back into this regime shows that the ionised acceptor charge dominates Q s.
Hence, in the three regimes, either free carriers or fixed space charge constitutes the major part of Q s. As will be seen in Chap. For the calculation in Fig. This simplified expression, with the physically obvious terms, provides a good approximation to the full Eq.
Between flat bands and inversion, when the ionised acceptor space charge dominates, Eq. Equation 2. Alternatively, if Eq. These will be confined close to the semiconductor surface, by virtue of the electrons being in a parabolic potential well of depth V s. The terms in Eq. The variation of x dmax with N a can be calculated from Eq. From a TFT point of view, assuming a certain equivalence between N a and the TFT trap state density, a curve of this type can be used to determine whether a thin film is fully depleted or not at inversion.
As a result of this, the threshold of inversion will occur at a lower value of band bending than for a thicker film having the same trap state density. This is discussed further in Chap. It can also be used to calculate the threshold voltage, V T , of the structure, i. The more heavily doped substrate requires a larger gate bias to achieve a given degree of band bending in depletion, and, equally, has a larger value of V T , as shown by Eq.
It is equally widely used for similar calculations in TFTs. Hence, the threshold voltage is a key parameter in determining on-state device characteristics, and the dependence of threshold voltage on substrate doping level is shown in Fig.
These will change the zero-gate-bias band bending, and affect the relationship between V G and V s and Q s. Work Function DifferencesIn general, the Fermi level position in a free metal will be different from the Fermi level in a free semiconductor. These differences are usually expressed in terms of a work function difference, where the work function is the energy required to remove an electron from the Fermi level to the vacuum level.
When the two materials are connected, and in thermal equilibrium, the Fermi levels need to be coincident, and there will be a flow of electrons from one material to the other, resulting in an interfacial dipole, which establishes this equilibrium. In the semiconductor, this will result in surface band bending, the extent of which will be much greater than in the metal due to the short screening length of the latter's high electron density.
In an MIS structure, it is conventional to reference the Fermi levels to the dielectric's conduction band edge  rather than to the vacuum level , so that the quoted barriers, U Mi and v i , are the energies needed to remove an electron to the dielectric conduction band as shown in Fig. Figure 2.
However, the synthesis of these Si-based inks is generally complex and expensive. Here, we prove that a polysilane ink, obtained as a byproduct of silicon gases and derivatives, can be used successfully for the synthesis of poly-Si by laser annealing, at room temperature, and for n- and p-channel TFTs. The proved capacity to fabricate polycrystalline silicon poly-Si thin-film transistors TFTs employing liquid-phase silicon precursors opened a flourishing route toward large-area and flexible electronics with high performance. The liquid silicon L-Si precursors of poly-Si studied so far are cyclosilanes, such as cyclopentasilane Si5H10 and cyclohexasilane Si6H12 , which need, in general, dedicated expensive and complex manufacturing. Regarding the environmental sustainability, the perspective to fabricate microelectronics devices employing a byproduct material would, moreover, shade a positive light on the future wide diffusion of disposable, single-use devices and sensors. The poly-Si thin film has been prepared at room temperature RT by excimer laser annealing of the PS layer; thus, it is possible to fabricate TFTs on lower thermal budget substrates, e. The transistors in this work have been fabricated see fabrication schematic in Figure S1 on top of a crystalline Si wafer, capped with nm of Si3N4, deposited by plasma-enhanced chemical vapor deposition PE-CVD.
Introduction to Thin Film Transistors reviews the operation, application and technology of the main classes of thin film transistor (TFT) of current interest for DRM-free; Included format: EPUB, PDF; ebooks can be used on all reading devices.
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Introduction to Thin Film Transistors reviews the operation, application, and technology of the main classes of thin film transistor TFT of current interest for large area electronics. The TFT materials covered include hydrogenated amorphous silicon a-Si:H , poly-crystalline silicon poly-Si , transparent amorphous oxide semiconductors AOS , and organic semiconductors. Poly-Si TFTs facilitate the integration of electronic circuits into portable active matrix liquid crystal displays, and are increasingly used in active matrix organic light emitting diode AMOLED displays for smart phones. The organic TFTs are regarded as a cost effective route into flexible electronics. As well as treating the highly divergent preparation and properties of these materials, the physics of the devices fabricated from them is also covered, with emphasis on performance features such as carrier mobility limitations, leakage currents and instability mechanisms.
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It seems that you're in Germany. We have a dedicated site for Germany. Introduction to Thin Film Transistors reviews the operation, application and technology of the main classes of thin film transistor TFT of current interest for large area electronics. The TFT materials covered include hydrogenated amorphous silicon a-Si:H , poly-crystalline silicon poly-Si , transparent amorphous oxide semiconductors AOS , and organic semiconductors. Poly-Si TFTs facilitate the integration of electronic circuits into portable active matrix liquid crystal displays, and are increasingly used in active matrix organic light emitting diode AMOLED displays for smart phones. The organic TFTs are regarded as a cost effective route into flexible electronics.
Request PDF | Introduction to thin film transistors: Physics and technology of TFTs | Introduction to Thin Film Transistors reviews the operation.Miles G. 28.05.2021 at 20:04
The Journal of Applied Research and Technology JART is a bimonthly open access journal that publishes papers on innovative applications, development of new technologies and efficient solutions in engineering, computing and scientific research.Prewitt B. 30.05.2021 at 00:02
A thin-film transistor TFT is a special type of metal—oxide—semiconductor field-effect transistor MOSFET  made by depositing thin films of an active semiconductor layer as well as the dielectric layer and metallic contacts over a supporting but non-conducting substrate.