Now that i am home i have finally time to think about it and read up about it , i think i understand all the factors now.
When the process gets smaller, the resistance of the "wires" increases but since high speed signals also propagate through it, skin effects also becomes a role. As does capacitive effects.
Mainly relative permittivity (dielectric constant) and to some extend permability are the issues. At least that is what i understand of it.
Not so sure about permability, but the rest of it, yes.
Long time ago we had at work some experiments with a time domain reflection meter and the principle behind it. That was all about the velocity factor. And that was dependent on the material surrounding the conductor. Also was all about matching impedance to prevent reflections.
Even at chip layouts, the impedance must be matched yes to prevent reflections ?
Not sure what you mean by the bolded, but yes, velocity factor or signal propagation is dependent on the effective relative permittivity of the material surrounding the microstrip line as well as the ratio of the width of the microstrip line to the height (or thickness) of the dielectric material.
For reflections to occur, the wire must be way longer so that the propagation time of the signal is much longer than the rise time of the signal and the impedance is not matched.
'This is true, the wire needs to be sufficiently long such that a VSWR (voltage standing wave ratio) can form. You are also correct that the rise (and fall) time of the signal is what is important as a pulse signal is represented by a very broad range of frequencies in the frequency domain. I found these slides online that explain it so I don't have to run it through Matlab myself,
http://eeweb.poly.edu/~yao/EE3414/signal_freq.pdf . From those slides, you can see a single pulse signal in the frequency domain is actually a sinc() function:
To accurately transmit a periodic pulsed signal, you need the sum of many sin functions reaching high levels of harmonics which means your actual signal needed to transmit in order to achieve a square wave in time contains frequencies much, much higher than your switching frequency.
An advantage of smaller process technology would be that the propagation time of the signal is shorter but at the same time the relative permittivity increases. And thus the propagation speed goes down as well. There goes the advantage out of the window again.
For the bolded, do you mean propagation distance of the signal is shorter? Also, I don't work with finfet processes so I am unfamiliar with the characteristics of their dielectric material, do you know for sure the relative permittivity is increased from prior nodes?
Because the relative permittivity increases, the coulombforce on the electrons increase. That would make for less easy passing on of the " charge" signal from electron to electron.
And there is something that is confusing me.
I always understood that electron do not really travel that fast through a material because of mainly scattering and other atomic forces.
Thus when a signal is applied with very short rise time, the passing of the charge (the signal wave front) goes with the velocity factor or signal propagation speed but the electrons move at a relative slow pace through the material. But the electrons are very good at passing on information.
Not really sure where coulombforce comes into play in context of the rest of the post. I think you need to remember that FETs work off of transconductance. They take a voltage signal at the input and put out a current at the output. The potential on the gate forms a channel within the FET which begins conduction between the source and drain. The dielectric material between the gate and the channel as well as the dielectric material of the channel itself will be different than the material surrounding the metal interconnects. The channels are specially designed for this process to occur while finfets help greatly in controlling leakage while feature sizes decrease and cutoff frequency increases.
With that said, in order for one stage to drive the next, you have to drive the input capacitance of the next gate with a sufficient current from the first gate. The rate at which the electrons from the first gate can drive the voltage on the input capacitance of the next gate is what forms the signal.
I recently read about ballistic conduction where an electron can travel through a material without scattering. Without encountering resistance but also not being a super conductor.
It is just that at small sizes, the electron encounters less interaction from surrounding atoms. At least i think that is the case of everything influences everything when interacting.
Ballistic conduction happens as you said, when scattering is eliminated. The electrons will still interact with the walls of the conductor, but shouldn't be interfering with each other within the conductor.
*Hopefully I made sense as well and didn't make any silly mistakes. I'm running on very little sleep lately due to a newborn in the house.
