Why Vth changes with transistor size and bias condition

how to calculate the vth of the transistor

Vth seems not to be a function of transistor size or bias condition if Vsb=0? Why in simulation Vth changes whenever I change the size or the bias current?

short channel effects on vth

dear vistapoint,
Vth for long channel device is independent og gate length and drain voltage.
The effect of body bias you are aware of.
But for short channel devices, the Vth shows greater dependence on channel length and drain voltage. The effect of body bias on Vth for short channel becomes weaker as body bias has less control on depletion region.
hope it helps

vth vs vsb graph

vt is a parameter of channel lenght in both pmos and nmos. this effect is called channel modulation effect considered with parameter lambda in the equation of cmos current equation.
the best way to bypass this altering effect is to connect source to bulk of the mosfet.

pmos vsb vth

arno_a_boy said:

vt is a parameter of channel lenght in both pmos and nmos. this effect is called channel modulation effect considered with parameter lambda in the equation of cmos current equation.
the best way to bypass this altering effect is to connect source to bulk of the mosfet.

Click to expand...

I think what you said should be body effect not channel modulation effect, right?

lambda modulation effect cmos

Yes Vth is not function of transistor design parameters for long channek devices
but for short channel it will have some dependcy
I think that the manuals of a certain design technology should provide information about this
So, I think you should review you design kit manual

thanks

vth transistor

Yes I think he is wrong with that. channel length modulation is the effect of Vds on Ids. Body effect is the effect of body potential on Vth.

Just say it out so I won't be confused.

Alles Gute said:

arno_a_boy said:

vt is a parameter of channel lenght in both pmos and nmos. this effect is called channel modulation effect considered with parameter lambda in the equation of cmos current equation.
the best way to bypass this altering effect is to connect source to bulk of the mosfet.

Click to expand...

I think what you said should be body effect not channel modulation effect, right?

Click to expand...

vth calculation

Vt=Vt0+γ(√(Vsb+2ΦF)-√(2ΦF))

γ=Body effect constant
ΦF=fermi potential
Vsb=Vsource-Vbulk
Vt0 threshold potential with Vsb=0

channel length modulation vth

if you open a model file from MOSIS test results, you will see Vth depends on transisotor size. It's just not included in our simple hand calculation model.

vth body effect

the lambda always changes with transistor length, how to simulate this curve in HSPICE for a specific process?

vth short channel

u can plot I/V curve and take two points in that curve ,one at the edge of saturatio and other in deep saturation and calculate lambda using
Id1/Id2=(1+λvds1)/(1+λvds2)

repeat the same for two lengths .

Old way to plot it on graph .

ids equation in mos long channel vsb=0

Dear Mady79,
I did not get what you are saying. Can you be more explanative?

Thanks
ravi

Added after 22 minutes:

Vth for long channel devices is dependent on body bias. But as the channel length becomes small, Vth depends on Non-uniform doping and short-channel effects. As the channel becomes smaller the effect of body bias on vth becomes smaller because body bias has less control over depletion region.
Due to short channel, the length of the device effects the Vth. And due to narrow channel, the width of the device comes into picture. When the channel width is narrow the fringing fields also considerbly affect the depletion width comparable with that of vertical field.

I think this helps.

ravi

short channel modulation lambda

I thought the value of Vth is fixed during fabrication process. Isnt that the value of Vth range from 0.5V to 1 Volt..

size of transistor lambda

this is called canal lenght modulation
check in razavi book

how has a transistor changed in size

Hi
Vt of a normal MOSFET (long and wide enough channel MOSFET) "majorly" depends on folowing patameters
1). Doping Conc.
2). Vsb (Substrate Bias / Body Bias)
3). tox (Oxide Thickness)
4). dielectric material used.
5). Gate metal used.
you can find more parameters if you look at second or third level eq.s of Vt.
But for short/narrow MOSFETs, the potential/Gate bias needed to achive inversion varies due to the fringing field and hance needs to be counted.
With Vsb the effective potential diff between gate and the substrate changes and hence Vt changes.
But Channel Length modulation has nothing to do with the change in Vt.
In Saturation region of operation, the channel lenght of the MOS starts decreasing after pinch off and this results in an increase in Id due to reduced channel resistence. This effect is called Channel Lenght mmodulation. it apears only in Saturation region of operation and hence has nothing to with the increase/decrease of Vt.
I hope i am clear.
let me know if any further explaination is needed.

vth test condition

you can search short channel effect and narrow width effect. briefly speaking, it is due to the shape of depletion area.

does threshold depend on transistor size

Hi, all,

Please refer to the attached file.