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Design >> Analog Design >> A basic question?
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Message started by average on Aug 8^th, 2006, 2:23am

Title: A basic question?
Post by average on Aug 8^th, 2006, 2:23am

Does the mosfet have the channel_length modulation effect when it operates in the triode region?

Green

Title: Re: A basic question?
Post by jeffyan on Aug 8^th, 2006, 2:47am

average wrote on Aug 8^th, 2006, 2:23am:

Does the mosfet have the channel_length modulation effect when it operates in the triode region?

Green

hi,
as i know, the channel_length modulation effect is defined in mosfet's saturation region.
jeff.yan

Title: Re: A basic question?
Post by loose-electron on Aug 8^th, 2006, 10:23am

Channel length modulation is applicible only in saturation region. You need to have a pinch off scenario in place, so that the pinch off location changes.

When you are in triode region the entire channel is inverted (drain to source fully) so length modulation is not applicable.

hope that helps

Jerry

Title: Re: A basic question?
Post by ACWWong on Aug 8^th, 2006, 10:56am

loose-electron wrote on Aug 8^th, 2006, 10:23am:

When you are in triode region, the entire channel is indeed inverted, but you can still have pinch-off due to velocity satuation (short channel effect). Although this is not "channel length modulation" in the traditional sense, the velocity saturation point is dependant on Vds, so the effects are similar to channel length modulation (ie, you see the knee in the Ids vs vds graph within the triode region).
This effect only occurs (as the name depicts) on short channel devices/high field strength situations.

Title: Re: A basic question?
Post by loose-electron on Aug 8^th, 2006, 11:18am

Well yes, agreed -
But there is a lot of bad information in textbooks on velocity saturation

Pinchoff Region – Electron carriers in the pinched-off region are velocity saturated,

Many texts use the illustration: "velocity saturation is similar to a gas under pressure traveling through a very small tube“

I still don't know who the originator of that one is. It is a nice illustration, but not valid.

Carrier velocity saturation is caused by horizontal electric fields to the channel. end.

For an in depth discussion see:
"Carrier Mobility and Current Saturation in the MOS transistor"
IEEE Transactions on Electron Devices, March 1965.

I wish I had an e-copy of the above, but it predates most computers.

- Jerry

Title: Re: A basic question?
Post by average on Aug 8^th, 2006, 5:57pm

I think your replies are very helpful for me, thanks all.

Title: Re: A basic question?
Post by qiushidaren on Aug 21^st, 2006, 1:46am

jeffyan wrote on Aug 8^th, 2006, 2:47am:

average wrote on Aug 8^th, 2006, 2:23am:

Does the mosfet have the channel_length modulation effect when it operates in the triode region?

Green

hi,
as i know, the channel_length modulation effect is defined in mosfet's saturation region.
jeff.yan

I agree with you, jeff.