The effact of W and L for vth of MOS transitor

[shqzhao]

find vth in nmos

hi, can you give me some advices?
for MOS transitor why vth decrease when L decrease? and vth increase when W decrease?
Thanks

 

[spminn]

vt vth nmos

Well in submicron process, when L decreases, the short-channel effects are more prominent. Consequently, effects like DIBL reduce the vth of the transistor. I have seen that vth changes with W in submicron technologies but I don't know the exact reason behind it. When the width is too small (comparable to L), then there is something called narrow-width effect, which reduces the vth of the device.

 

[shqzhao]

narrow mos

Thanks ! Ican't find material that how W effect the Vth too.

 

[ricklin]

l and w in pmos

For short channel Vth vs. W and L correlated with process type. I think vth increase while w decrease may be a shallow trench isolation. The parasitic capacitor at the width direction ends get more weight when the width get narrow.

 

[manissri]

vth increases with decrease in width

vt decrease with decrease of L due to short channel effect and the vt increase due to width decrease due to narrow width effects

 

[octocreation]

find vth of nmos

Hi,

In simple terms...., A channel is a conducting path for carriers from source to drain....and as we define threshold voltage as the minimum gate voltage required to form a conduction path....and as channel decreases the gate voltage required o turn on the device decreases...thus vth also decreases...

regds,
anup

 

[stoica.dan]

mos decrease width vt increase + shallow trench

Ok, but some formulas where could we find?

 

[octocreation]

vth vs width

Hi,

If you read any of the books on CMOS basics you will find explaination along with the mathematical part. CMOS VLSI Design By Weste is very good book.

 

[pixel]

vth formulas nmos

there is also inverse vth moudulation effect if you further decrease L

for equations read book from Tsividis...

 

[semiartist]

vth vs l

I don't think so!
Actually, Vth is almost nondepent on L or W when L or W is large (not short or narrow MOS). However When L or W decrease, the Vth would increase. You will see the reason and the formula on Neman's book, sorry, I can't remember the exact name of the book, it's on semiconductor device.

Actually, we can deduce the result from foundry's process file. It tells us if you want to use low vth process, we could not use minimun L any more, but use several times of the minimun L.

BR,
Alex

 

[drabos]

formula of vth for mos

The formerly mentioned DIBL is the drain induced barrier lowering. Quite good explanation here: https://en.wikipedia.org/wiki/DIBL