I've always wondered about this, theres got to be a limit to clock speed right? The processor obviously uses electricity to transfer data, and electricity travels slightly slower than light?? So, wouldnt there be a Ghz rating for the speed of light? Basically my question is what is the theoretical limit to the speed of a processor?
I see what you mean, but that is WAY theoretical. Before considering the speed of the electron, there are numerous other factors, such as the heat dissipation caused by their movement. Also, Hertz means 1/sec (or per sec), as in cycles per second. The number of cycles per seconds depend on what you call a cycle. There is a big difference between going from 0V to 5V then back (common logical cycle) and going from 0 to 1.3V (common CPU clock cycle).
As for the limitation, maybe the most important factor, is the switching time of the transistor. When you are working on very high frequencies (GHz), parasite capacitance will affect your system's performance (every electronic component stores a little bit of electric energy in form of electric field, acting like a capacitor). That stored energy needs to be discharged for the transistor to go back to its ideal LOW state, and that takes a little bit of time. If you set it too fast, it won't be able to go back at all, causing the system to fail.
Also, the electric rails on a CPU are tiny (i mean nanometricaly tiny), and cannot transfer a huge amount of charge at once. Too much current would cause them damage.
Intel and AMD engineers have to consider all this and a whole lot more when building a system, setting an ideal environment for the unit to work in (electrical parameters, heat condition, frequency), just so we can screw everything later.
I've always wondered about this, theres got to be a limit to clock speed right? The processor obviously uses electricity to transfer data, and electricity travels slightly slower than light?? So, wouldnt there be a Ghz rating for the speed of light? Basically my question is what is the theoretical limit to the speed of a processor?
Theoretically, your upper limit would be due to the propagation delay in sending electrons from one point to another, which would be the speed of electricity. Electricity travels close to the speed of light and light travels a foot in a femtosecond (10E-15 s) so the top speed is somewhere in the 10E22-10E23 Hz range. However, this limit will never be reached as this assumes no capacitance and no resistance in the wire. Semiconductors have a significant amount of resistance and capacitance in their circuitry. The capacitance in transistors (such as the hundreds of millions in your average CPU) is a direct function of the transistor size (actually, volume, but you get my point.) Murissokah explains why capacitance delays switching pretty well. Capacitance used to be the limiting factor for the shipping clock speed on CPUs up to about the 180 nm generation. Now it is thermal dissipation as we could get the transistors to switch faster than they are rated for at the present but we have to deal with a ton of heat in the process as he also explained.
I think the question you REALLY want answered is how fast do you think CPUs will actually get before they are replaced with something else. I highly doubt that silicon-based CPUs will exceed 10 GHz as transistor gate length cannot be shrunk too much more, lest the gates get thin enough that electrons can leak from the emitter to the collector without any voltage at the base. In fact, I doubt that CPUs will be clocked much above 5-6 GHz before we switch to another substrate or use a different technology. However, a 5-6 GHz CPU from 10-15 years from now will be significantly more powerful than a 5-6 GHz CPU of today (one that somebody uses liquid nitrogen to do a few-minute "suicide run" overclock stunt with.)
Theoretically, your upper limit would be due to the propagation delay in sending electrons from one point to another, which would be the speed of electricity. Electricity travels close to the speed of light and light travels a foot in a femtosecond (10E-15 s) so the top speed is somewhere in the 10E22-10E23 Hz range. However, this limit will never be reached as this assumes no capacitance and no resistance in the wire. Semiconductors have a significant amount of resistance and capacitance in their circuitry. The capacitance in transistors (such as the hundreds of millions in your average CPU) is a direct function of the transistor size (actually, volume, but you get my point.) Murissokah explains why capacitance delays switching pretty well. Capacitance used to be the limiting factor for the shipping clock speed on CPUs up to about the 180 nm generation. Now it is thermal dissipation as we could get the transistors to switch faster than they are rated for at the present but we have to deal with a ton of heat in the process as he also explained.
I think the question you REALLY want answered is how fast do you think CPUs will actually get before they are replaced with something else. I highly doubt that silicon-based CPUs will exceed 10 GHz as transistor gate length cannot be shrunk too much more, lest the gates get thin enough that electrons can leak from the emitter to the collector without any voltage at the base. In fact, I doubt that CPUs will be clocked much above 5-6 GHz before we switch to another substrate or use a different technology. However, a 5-6 GHz CPU from 10-15 years from now will be significantly more powerful than a 5-6 GHz CPU of today (one that somebody uses liquid nitrogen to do a few-minute "suicide run" overclock stunt with.)
Yes, the actual drift velocity (the net velocity due to an electrical current) of electrons in circuits that you would encounter is on the order of millimeters per second.
