Actually, the flow of electrons in a semiconductor doesn't really carry the signal. it caries the current, to be sure, but the signal is carried by the electic field of the electron-hole pairs in the semiconductor. The changing fields propagate at whatever the speed of light is inside the semiconductor, and as Mr. Summerfeild says, limited by waveguide effects. The actual speed of flow of the electrons is called the "drift velocity" and is quite slow, measured in centimeters per second depending on the semiconductor and the temperature.
And I agree with you, John. It's been entirely too many decades since I did my Master's in Solid State Physics. I'm surprised I remember all this stuff. "A friend will help you move. A really good friend will help you move the body." Gordon W. Wolfe, Ph.D, (425) 865 - 5940 VM Technical Services, the Boeing Company -----Original Message----- From: Joseph Temple [mailto:[EMAIL PROTECTED]] Sent: Thursday, February 20, 2003 9:48 AM To: [EMAIL PROTECTED] Subject: Re: URGENT! really low performance. A related question... John Summerfield wrote "Why do people keep referring to the speed of light? In what I learned of electronics, signals are carried by electrons travelling round in conductors (and semiconductors). AFAIK electrons are quite a bit slower than photons." Well, according to a guy named James Clerk Maxwell light is electromagnetic radiation. Electrical current (the flow of charge) in a conductor is induced by an electromagntic wave. At low frequency and short distance the idea of voltage and current works fine and the wave is ignored. As things get faster or longer (ie POwer Lines) the conductors need to be treated as "wave guides" more than as conductors. As far as the math goes, I believe that you have to start using transmission line characteristics as soon as the delay on the line matters. The conductors in a modern chip are not treated as simple "wires" with no impedance or delay but are modeled with inductance, capacitance, and resistance, in much the same way that a transmission line is modeled. An upper bound for electromagnetic wave speed is "C", unless you get into some really hairy quantum physics paradoxes. (Read Shroedingers Kittens, I forget the author's name) On the other hand practical physical limitations slows waves down. How close to C you get depends on the medium and practical things like the need to dampen reflections on the line. That is the fastest line is useless if the signal on it "rings" enough to prevent further use of the line. Wow I thought I had forgotten all that stuff 30 years ago when I burned my fields and waves book... Joe Temple [EMAIL PROTECTED] 845-435-6301 295/6301 cell 914-706-5211 home 845-338-8794
