PHYSICS AND SEMICONDUCTOR DEVICE TECHNOLOGY
Learning outcomes of the course unit
The course is addressed to students which would like to understand the physical principles on which opto and micro electronic devices are based. A brief outlines is given also to low dimensional structures ( quantum devices) and to related nanotechnologies.
Starting from a survey of the principles of the semiconductor physics , the course consider in detail the optical properties of semiconductors from the interaction light-matter to the optical processes and the physics principles on photonic and optoelectronic devices are based.
Further , the principal equations used to describe the behaviour of electronic devices are introduced starting from the description of free carrier density ,both in uniform and non uniform semiconductor systems, under external perturbations. Milestones of the course are: (i) the physical properties of semiconductors related to the fundamental investigation techniques and to the growth and process technologies, (ii) the physical principles on which micro and optoelectronics devices are based, (iii) a comprehensive understanding of how it is possible to project particular materials and structures by tuning the physical properties for special applications.
The obligatory requirements for the students attending this course is a comprehensive knowledge of the principles of the quantum mechanics, solid state physics and physics-chemistry and technology of materials. In particular the possession of at least 12 credits is required from the following courses scheduled in the first year of the graduate degree course on Science and Technology of advanced materials :
• Quantum Physics (4 credits)
• Principles of the chemistry and physics of the materials technologies (4 credits)
• Solid State Physics (4 credits)
• Physics of the materials (4 credits)
• Laboratory of Physics of the materials ( 6 credits)
The course is scheduled in five parts and intentionally contains a number of lectures which overcomes the assigned number of credits; this scheme allows to better fit the knowledge and interest of the students as a function of the acquired credits and the content of the topics involved in their final project (thesis). Moreover, for the students which would like to complete the required number of credits for the specialist courses (18 credits) and obtain an advanced qualification in the field of Semiconductor Materials, the following courses, scheduled at the second year of the course degree, are highly recommended:
• Semiconductor physics (5 CFU)
• Laboratory of semiconductor physics (3 CFU)
• Growth technologies of electronics materials (4 CFU)
Course contents summary
I. Principles of the physics of semiconductor materials. (18 lectures)
1. Crystal properties and energy bands. Lattice structures, chemical bonds and energy bands. Effective mass approximation and electron dynamics. Energy band structure of the most important semiconductors. Energy band gap and its dependence on the external perturbations. Semiconductor compounds, overview on the principal epitaxial growth technologies: tailoring of the physical properties and lattice matching . A brief introduction to to low dimensional structures.
2. Equilibrium distribution of carriers . Elements of the statistical distribution. The Fermi-Dirac distribution and its classical approximation to the Maxwell.Boltzmann distribution. Density of states. Statistics of carriers at the thermodynamic equilibrium: intrinsic and extrinsic semiconductors. Mass-action laws. Electron and hole density variation with the temperature. Occupation of donor and acceptor levels: degenerate semiconductors. Outlines on deep levels and compensation effects.
3. Transport processes Outlines on Boltzmann equation. Charge transport: drift of carriers under an electric field. Electrical conductivity and mobility. Scattering processes: time relaxation approximation. Effects of the scattering processes on the mobility variation with the temperature and doping level. The Hall coefficient . The Hall effect and its application to the study of the electrical properties of semiconductors. Electrical conductivity and mobility measurements: an overview on the experimental and theoretical problems. Magnetoresistance High magnetic field effects: Landau levels, and optical processes in semiconductors (12 lectures)
1. Introduction:a. Dispersion laws . Reflection, transmission, interference processes. Optical absorption processes in semiconductors.
3, Optical processes Optical transitions and absorption coefficient. The microscopic model: outlines on the quantum theory of optical transitions. Transition probability and absorption coefficient for band to band transitions. Other optical transitions: excitons , impurities, free carriers. Outlines on other processes: intraband, high enerdy ecc.) . Deviations from the ideal behaviour: Burstein-Moss effect, non parabolicity of the energy bands, disordes and Urbach’s tail, Effects induced under external perturbations. Outlines on the modulation techniques and high energy spectroscopy (UPS, XPS). A brief overview on the experimental optical methods for the investigation of the electronic properties of semiconductors. Outlines on the optical processes in low dimensional systems.
III section: Excess carrier in uniform semiconductors. ( 10 lectures)
1, Generation- Recombination processes. Excess carrier with respect to the thermodynamic equilibrium. Injection level. Recombination and generation processes. Deviation from the equilibrium and relaxation time of the dielectric. Band to band recombinations. Auger processes. Recombination at intermediate levels and RSH theory. Carrier lifetime. Surface state recombination’s.
2, Continuity equation. Diffusion and drift. Diffusion length. Continuity equation. Ambipolar equation. Application of the continuity equation: examples. Lifetime and diffusion length measurement techniques: experimental examples.
1. Introduction Surface and bulk non uniformity of the doping level and of theband structure, Quasi Fermi level approximation.
2, Surface effects. Ideal surface and Tamm.-Schokley levels. Real surfaces: the Cowley-Sze model. The metal semiconductor contact. The Schottky effect and the metal semiconductor barrier. The Schottky barrier diode. The Mott barrier and ohmic contacts.
3. Bulk effects. Non uniform doping : the p-n junction. The p-n junction theory in the depletion approximation. Junction capacitance and electrical behaviour under forward and reverse bias. Current-voltage characteristic and deviation from the ideal behaviour. Heterojunctions: band discontinuity and interface states. Description of the charge current through the heterojunction: diffusion and generation-recombination models. Outlines to the low dimensional heterostructures : modulation doped and modulation composition multiple quantum wells (MQW) and superlattices (SL).
V section : Devices ( 16 lectures)
1. Electronic devices. The bipolar junction transistor (BJT): amplification and current gain. Outlines to the heterojunction bipolar transistors (HBT). The junction field effect transistor (JFET). The Metal-Oxide-Semiconductor structure: the MOS capacitor and the field effect MOS transistor (MOSFET). Outlines to Charge Coupled Devices (CCD) for memory application. A brief overview on the microelectronic technologies for the fabrication of the integrated circuits.
2. Optoelectronic devices. Light emitting diodes (LED's). Principles of operation of the semiconductor lasers ( double heterojunction, distributed feedback, quantum well lasers, ecc.). Photo detectors. Materials and structures tuned to particular spectral response. Photovoltaic devices. Principles of the the photovoltaic effect and applications to solar energy conversion devices.
1) C. Ghezzi “ Lecture notes from the corse “ Semiconductor Physics”
2) M.Wolf, N. Holonyak, G.E. Stillman ““Physical properties of semiconductors” Prentice Hall International Editions.
3) J. I. Pankove “Optical processes in semiconductors” Dover publ.inc.
4) M.S. Tyagi “Semiconductor materials and devices” John Wiley & sons.
5) S.Sze “Introduction to Semiconductor devices: Physics and technology” John Wil & sons.
6) R S. Muller, T.I. Kamins “Device electronics for integrated circuits“ John Wiley & sons.
7) P.Bhattacharya “Semiconductor optoelectronic devices” Prentice Hall Int. Editing.
By considering the advanced character of this course, the teaching work is generally addressed to a small group of students specialising in “semiconductor materials” , thus the teaching activity has a “tutorial” approach . The “ lectures” are planned in two steps: (i) presentation of the particular topics, (ii) discussion on particular remarks proposed by the students following an individual study (homework) on the arguments previously presented. In order to stress the strong connection between basic physics and experiments, this interactive teaching work is completed by discussions directly performed in laboratory in front of investigation instrumentation and technology facilities.
The course planning, based on an interactive approach, is useful to easily test if the students have acquired a satisfying level of knowledge of basic physical principles and semiconductor devices behaviour. Consequently, the assignment of the related credits is the result of the several discussions during the lectures , during which the student has the possibility to demonstrate his increasing understanding of the course contents.