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Design >> RF Design >> Lower transistor Length -> Lower Vth ??
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Message started by wccheng on Jun 15^th, 2004, 8:55am

Title: Lower transistor Length -> Lower Vth ??
Post by wccheng on Jun 15^th, 2004, 8:55am

Dear all,

Is lower transistor length will give out lower threshold voltage? Actually, my supervisor said that it is not correct. Could anyone explain to me? Actually, I just know that smaller transistor length will give out smaller parasitic capacitor in order to give out higher Ft frequency.

Best Regards,

wccheng

Title: Re: Lower transistor Length -> Lower Vth ??
Post by sachin on Jun 29^th, 2004, 6:24am

Hi,
Actually that depends upon the technology you are using. I mean if you decrease channel length by changing the technology then vth will increased. For same technology output resistance will increased, no effect on vth. Is this answer you are expecting?

sachin

Title: Re: Lower transistor Length -> Lower Vth ??
Post by Shiva on Aug 1^st, 2004, 12:32pm

Hello,

for a given technology the VT of a transistor will DECREASE for decreasing L, so your teacher's wrong.

This can be easily understood by an intuitive reasoning:
a smaller L, means a smaller capacitor (W*L) so less charge is needed to charge this capacitor (which is the MOS channel).
This corresponds to a lowering of the VT of the transistor, which corresponds to the minimal gate voltage to build up enough charge in the channel.

Kind Regards

Title: Re: Lower transistor Length -> Lower Vth ??
Post by terryssw on Oct 3^rd, 2004, 7:26am

The voltage needed to charge the gate in order to inverse the channel is independent to the capacitance needed to charge, it depends actually on the capacitance per unit area. Actually Vth will have little changes (several tens mVs) with L, but depends on technology and biasing conditions, it can either increase or decrease. But the change surely is not related to length by the amount of capacitance.

Title: Re: Lower transistor Length -> Lower Vth ??
Post by chen on Oct 14^th, 2004, 5:25pm

I am not specialised in device physics so correct me if I am wrong. But I heard that as L drops, Vth also drops. However, a small Vth would mean large leakage current (current when VGS=0). Hence, tox is decreased in proportion also to maintain the Vth. However, the general trend is that Vth is still decreasing slowly.

Title: Re: Lower transistor Length -> Lower Vth ??
Post by Teddy on Dec 9^th, 2004, 11:38am

it's probably too late now but VTh modulation due to L change is caused by second order effects. Try to look for them. Also try to look for DIBL - drain induced barrier lowering.
For most of time you would have hard time to see it in sims. Vth is usualy part of model params and is not changing - depends on level of your model. Look at BSIM 4.
For design you should consider Vth as fixed value.