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shot-channel MOS vs. long-channel MOS.

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alphadog

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I have two questions pleaes on this subject:
1. Why in short-channel MOS, the devices enters saturation region faster than in long-channel MOS?
2. Why in short-channel MOS, IDsat is lower than in long-channel MOS?

Thank you.
 

Sceadwian

Banned
Are you planning on designing cmos technology anytime soon?
 

dknguyen

Well-Known Member
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Do you understand how the inversion channel in a MOSFET works?

ANd what exactly so you mean by "enter saturation faster?" Earlier with respect to time? Or something else?

Are you able to get anything useful from page 115 and 116 of this book? THe answer is in there.
The electrical engineering handbook - Google Book Search

FOrmula 3.23 on page 115 of the book seems to imply the opposite though- longer channel width means the saturation current is less which seems more consistent with what I remember which is (keep in mind my memory could be wrong since it's been 4 years since I learned this and I've never had to use it since):
Longer channel = can block higher voltages across source-drain, but higher resistance, lower saturation current.
Shorther channel = can only block lower voltages across source-drain, but lower resistance and higher saturation current.
 
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alphadog

Banned
To my understanding, you are correct, short channel means lower channel's resistance, therefore higher current.
Thanks for the book!

Regarding reaching saturation region, i meant to ask why in shorter channel devices, the MOS reaches saturation faster, with respect to VDS?
 

alphadog

Banned
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?
 

dknguyen

Well-Known Member
Most Helpful Member
To my understanding, you are correct, short channel means lower channel's resistance, therefore higher current.
Thanks for the book!
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.

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?
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.

That's one reason why wider channels can conduct more current. They don't increase current by increasing the maximum speed of the charge carriers, they increase the maximum number of charge carriers that can move at the same time (similar to increasing the throughput of a computer technology that can't run any faster by using multiple cores in parallel). Wider channels also have less resistance which reduces losses increasing the current capability due to thermal limits as well.

Regarding reaching saturation region, i meant to ask why in shorter channel devices, the MOS reaches saturation faster, with respect to VDS?
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.
 
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alphadog

Banned
Thank you very much!!!


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.
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.



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.


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?
 
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dknguyen

Well-Known Member
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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.
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).

I'm not saying you're wrong, I'm just saying the Vds for current saturation and the amps of the saturating current are not necessarily related proportionally (at least from what I've been reviewing so far. They could be.). It's just from all the FETs I've looked at that come from the same series, the lower voltage FETs seem to have a higher saturation current (and higher current capability due to lower resistance) than an otherwise equivelant higher voltage FETs.

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?
I don't know this without reviewing some more. There might be more than one cause for current saturation, just like in diodes where there is more than one way for the diode to breakdown. It could very well be that people just say saturation to save the effort of saying velocity saturation all the time.

EDIT - FURTHER UPDATES:
Here is more reading that may be helpful:
The electrical engineering handbook - Google Book Search
The first page (right hand columns) confirm that the Vds required to reach saturation is less in short channels than long ones (woo hoo! I learned enough in school to relearn this stuff and surmise things about it!).

But the equation they give short afterward (equation 3.4) seems to imply that the value of the saturation current is dependent on channel WIDTH not length. So I'm starting to think that MOSFETs that had different channel lengths, but were otherwise identical would have the same saturation current, regardless of channel length (although Vds required for saturation would be different since that does depend on channel length). Earlier, I was thinking that this would be the case if velocity saturation IS saturation (the only effect causing saturation) but didn't say anything because I had no evidence that velocity saturation was the only effect causing saturation.
 
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