Channel length modulation

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mike_bihan

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Hi, gurus:

I have a question on channel length modulation equation.

Rout = 1 / (lamda*I)
Lamda will decide mos output resistance.

According to some books, 1/Lamda = Constact*Leff

Then, if channel length is long, Leff~~L. Then 1/Lamda should be propotional to L.

However, it is not true.

Can anyone recommend any papers on that?
 

Please look at the curve, it is not propotional to Length.

1/lamda does not double when Length double.
 

Hi mike_bihan,
recently i had the question of how to calculate the channel length modulation for a given technology. well, since i am using 0.5um tech, the bsim3v3.2 gives PCLM as the model parameter. but it involves a little complex equation. so i exeuted my test transistor alone in spice and sw the I-V curve for expected VGS and ramp VDS from vss to vdd. this can be obtained in hardcopy and get approximate lambda or (this is what i did) go and check the analog circuit design forum and look for channel length modulation: spice problem. may be this would help you.
srivatsa
 

if L is large, the para. cap.is also large.
 

I can use parallel device to reduce cap.
 

possibaly,

channal lenth modulation occur due to lateral elecric field in device gettign stronger and important as channel leth reduces and so lambda is correction factor for the assumption about L considered in physics of device.

so i suppose the relation derived for lambda and ids remain valid in shot channel only and not when L=Leff.

correct me if i am wrong.
/ue
 

Lambda which is defined for the saturation current is defined taking an approximation.

Leff = L - dL

1/Leff ~ (1+dL/L) / L

and dL/L is defined as Lambda*Vds an approximate model

So the approximation is true only of dL/L << 1

Also Rout = 1/(Lambda *I )

is also an approximate expression

Rout = (1+Lambda*Vds) / (Lambda * I)

This is true when the Lambda approximation holds, so I don't think you should compare these results to accurate plots from a simulator since Lambda is just a parameter useful to model Channel Lendth modulation for hand calculations, it does not even appear as a separate parameter in the BSIM3 model.
 

THe issue is that when I used calculated parameters in simulation, the mismatch in lamda kills the gain of the amplifier.

Lamda vary so much between different Vds/Vgs/I, and I lost control over my circuits' output impedance.

Any good idea from design considerations?
 

This is interesting. I think the hand calculations should give u a result that is reasonable approximation to the simulated output, i think about 10%. Could you give a circuit example for this I think that would be helpful.
 

You can see attached waveform.

The transistor is in saturation region always. (how deep is a problem)

When current through the transistor is almost constant, the Rds change much.
 

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