ECE344_1 - ECE344 Semiconductor Devices and Materials M....

Info iconThis preview shows pages 1–4. Sign up to view the full content.

View Full Document Right Arrow Icon
ECE344 Semiconductor Devices and Materials M. Fischetti 201 D Marcus Hall Department of Electrical and Computer Engineering University of Massachusetts Amherst, MA 01003 Fall 2009
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
The crisis of Classical Mechanics At the end of the XIX century classical physics consisted of two main areas: 1. Classical Mechanics : Developed by Galileo and Newton, it predicted the motion of celestial objects with astonishing accuracy. It was also applicable (accounting for always present frictional eFects) to the motion of projectiles. 2. Classical Electromagnetism : Starting from the experiments of Coulomb, Amp` ere, ±araday, Lenz, etc., it culminated with the mathematical formulation by Maxwell and the prediction that electromagnetic waves are light (Hertz). Both areas were highly successful in most cases in explaining experimental observations. Yet, at the threshold of the XX century a few problems began to emerge, leading to the transition from ‘classical physics’ to ‘modern (or quantum) physics’. The following were the major ‘puzzles’ which classical physics could not explain: Black Body spectrum : The ultraviolet catastrophe. All objects emit ‘thermal radiation’, dependent on their temperature. Think of a wood log glowing red-to-white in the ²replace, or of molten iron, etc. Roughly speaking, the composition of the object does not matter, only its temperature: The hotter the body, the ‘bluer’ the body will appear. Many objects do not re³ect light (think of a chunk of coal) or re³ect it poorly. Empirically it is found that the light emitted by this class of bodies is almost completely independent of the details of their composition. Thus, the concept of ‘black body’ idealizes this class of objects. It is de²ned as a body which absorbs light ( i.e. , electromagnetic radiation), but does not re³ect any of it, instead re-emitting it after having ‘thermalized’ it. The ideal model would be a black box in which radiation enters through a tiny hole. Electromagnetic wave hitting the internal walls of the box reach equilibrium with the environment. Its spectrum ( i.e. , number of waves – or ‘modes’ – n ( ν ) in an interval ν - ν + of frequencies ν ) is measured as light exits from the tiny hole. ECE344 ±all 2009 1
Background image of page 2
Classical electromagnetic theory and equipartition ( i.e. , the energy is equally distributed among all modes, no matter what their frequency is) imply that the spectrum of a black-body should diverge at short wavelength. That is: if n ( ν ) is density of modes with frequency in ν , ν + , then n ( ν ) ν 2 (Jeans’ law) Experiments show, instead, n ( ν ) e - aν/ ( k B T ) Planck obtained the experimental spectrum assuming that: 1. the energy in each mode proportional to the ν of mode: E ν = with h a new (Planck) constant of nature 2. energy is exchanged between waves and the walls of the black-body only via ‘chunks’ (‘quanta’) of size Δ E ν = Planck’s assumption worked, but it lacked any justiFcation! 0
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 4
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 09/13/2010 for the course ECE ECE344 taught by Professor Polizinni during the Spring '10 term at University of Massachusetts Boston.

Page1 / 95

ECE344_1 - ECE344 Semiconductor Devices and Materials M....

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online