Hacker News Reader

Comment by jbaczuk | Hacker News Reader

> "It's like driving on a gravel road versus driving on a freeway," de Heer said. "It's more efficient, it doesn't heat up as much, and it allows for higher speeds so that the electrons can move faster."

Does it allow for higher speeds? I thought resistance increases the number of collisions the electrons make with the molecules in the material, heating up the material, does it actually change the "speed of electricity"? If anything, I would think it might actually increase, similar to water in a smaller pipe (see current formula below). Either way, I don't know if this (possible) change in speed is in any way significant when it comes to computing.

I = NeAVd

Where Ne is the number of free/conduction electrons per unit volume, A is the cross sectional area of the wire, and Vd is is the drift velocity.

View on HN

Latency is an issue even inside a transistor itself, something known as propagation delay. When you have a bunch of logic gates chained together the propagation delay increases and the chance for a race condition occurring also increases. The propagation delay of a logic gate is one of the limiting factors of the speed of a circuit.

Transistor gates act as capacitors in a way. This capacitance increases the delay between switching on/off. [1]

To lower this delay, we can shorten the transistor gate. We've hit a wall with how small we can shorten this length, and it is exceedingly difficult to make smaller gates without running into quantum effects.

Higher electron mobility means that the delay inside the gate is reduced, allowing faster circuits.

[1]http://ece-research.unm.edu/jimp/vlsi/slides/chap4_1.html Switching speed of MOS systems strongly dependent [on]: Parasitic capacitances associated with the MOS transistor. Interconnect capacitance of "wires". Resistance of transistors and wires.

I imagine they're not talking about changing the drift velocity, but the rate of information propagation through the EM wave: https://en.wikipedia.org/wiki/Speed_of_electricity versus https://en.wikipedia.org/wiki/Drift_velocity

I don't know if increasing the speed of electricity has real implications for small circuits; I guess it could make a nearly factor of 2 difference for long copper wires, but we use fiber for long connections anyway, and speed of light in glass is higher than speed of electricity in copper.

Distance is a limiting factor for memory bandwidth and other long circuits, but I'm not sure a speed up of 2x is feasible?

Reply on news.ycombinator.com