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For MOSFETs, lower channel resistance doesn't actually mean higher current (well it does up to a point). Resistance is what it is, and limits current through a MOSFET due to heating from losses. So if you could improve cooling you could just keep pushing more and more current regardless of what the actual resistance was. But after a certain amount of current you hit the limit of channel saturation. Channel saturation is the physical limit to the amount of current you can stuff into the channel regardless of temperature.To my understanding, you are correct, short channel means lower channel's resistance, therefore higher current.
Thanks for the book!
Regarding 'the shorter the channel the lower the IDS' issue, i read this in the book:
"In short channel devices, the horizontal field in the channel is high enough to reach the velocity saturation regime, which degrades the drain current as well as the transconductance"
Do you understand why reaching velocity saturation degrades the drain current?
I *think* that in a shorter channel the electric field can remain stronger over the length of the channel for the same voltage than a longer channel (less distance to degrade over), so the same Vds can generate a stronger overall electric field in the a shorter channel than it could in a longer channel. The charge carriers can only move so fast and this speed limit manifests itself as the saturation current limit. In a shorter channel, less effort (a lower Vds) is needed to generate an electric field with the strength capable of pushing the electrons to this limit than in a longer channel. So a lower Vds is required to reach the saturation current in a short channel than in a longer one.Regarding reaching saturation region, i meant to ask why in shorter channel devices, the MOS reaches saturation faster, with respect to VDS?
So maybe it is right that in short channels the drain current is lower.Velocity saturation is saturation that occurs because the charge carriers can only move so fast. As the electric field increases, there is a point when the speed the charge carriers are moving at can no longer be increased. This manifests itself as a limit in the current that the channel can conduct.
I *think* that in a shorter channel the electric field can remain stronger over the length of the channel for the same voltage than a longer channel (less distance to degrade over), so the same Vds can generate a stronger overall electric field in the a shorter channel than it could in a longer channel. The charge carriers can only move so fast and this speed limit manifests itself as the saturation current limit. In a shorter channel, less effort (a lower Vds) is needed to generate an electric field with the strength capable of pushing the electrons to this limit than in a longer channel. So a lower Vds is required to reach the saturation current in a short channel than in a longer one.
I think we've agreed that the saturation current can be reached with a lower Vds for a shorter channel. BUt just because the saturation current can be reached with a lower Vds does not necessarily mean that the value of the saturation current is less than a longer channel. (A short channel certainly has increased current capability due to reduced heating from resistive losses, but I don't see why the saturating current would be less than a longer channel).So maybe it is right that in short channels the drain current is lower.
Maybe in short channel, the drain current reaches its saturation faster, and therefore it doesnt keep growing with VDS.
Saturation current is reached only when velocity saturation is reached?
I mean, if the channel is pinched off at the darin side, but velicity saturation is not reached, then we the current will keep growing with the rise of VDS?