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Design >> Analog Design >> About the channel legth modulation
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Message started by Richardyi on Jan 14^th, 2008, 11:52pm

Title: About the channel legth modulation
Post by Richardyi on Jan 14^th, 2008, 11:52pm

Why the factor of channel legth modulation is different for nmos and pmos? Even for the channel length.
Can anyone help me?
Thanks.

Title: Re: About the channel legth modulation
Post by vivkr on Jan 15^th, 2008, 7:38am

Hi,

The answer is contained in your question. For submicron transistors, saturation typically occurs due to onset of velocity
saturation rather than due to pinchoff of the channel. Naturally, as the mobility of electrons and holes is very different,
so is the onset of saturation and the related behavior.

Hope this answers your question.

Vivek

Title: Re: About the channel legth modulation
Post by Richardyi on Jan 15^th, 2008, 7:22pm

Thanks for you answers.

But I still can not understand it.

For a Spice model, the channel length modulation parameter of NMOS is smaller than that of PMOS.

Title: Re: About the channel legth modulation
Post by thechopper on Jan 16^th, 2008, 6:10pm

Hi,

What Vivek stated is totally correct. Let me try to complete the whole picture by taking into account the following:
since the saturation velocity is the one driving the channel length modulation for sub-micron transistors and that the electrons mobility is larger than the pmos one, then the saturation velocity for the pmos will be smaller than the one for the nmos.

Therefore, under same bias conditions and geometry, it will be the pmos the one to get its channel modulated first. Hence, more susceptible to lenght modulation or equivalently larger modulation parameter.

Tosei

Title: Re: About the channel legth modulation
Post by vivkr on Jan 16^th, 2008, 11:19pm

Hi Tosei, Richard,

You need to be careful. The mobility of holes/electrons does not have a direct relation to the saturation velocity of the respective carrier in silicon although one might be inclined to think so.

I believe but I cannot find a suitable table anywhere that holes and electrons have almost similar values of saturation velocity in silicon.

As for why exactly you see more channel length modulation in your PMOS than in an NMOS, I could not offer an explanation at present. Perhaps someone else can.

Regards
Vivek