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reltol1
Joined: 15 Dec 2004
Posts: 19
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 30 Dec 2004 9:21 about MOST |
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So far I have been thinking that
λ=1/KnL, where λ is channel modulation constant, Kn is constant and L is channel length. So, although L changes, Kn does not.
However, by using a BSIM3_3 model parameters in spice3, Kn is not constant for different length L (i.e. 0.25um, 0.5um). I am wondering why??
Does anyone have any idea about that?
Thanks a lot.[/code][/tex]
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sutapanaki
Joined: 02 Nov 2001
Posts: 488
Helped: 19
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| 30 Dec 2004 9:27 Re: about MOST |
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| This relationship is not accurate anymore for short channel devices. The trend still remains, that longer transistors have higher ro, but you can not model it like this. Reason is the physicsof the transistor with sybmicron channel length is not the same as it used to be for 1u, 2u .. technologies.
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reltol1
Joined: 15 Dec 2004
Posts: 19
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| 30 Dec 2004 9:34 Re: about MOST |
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Then, how can I get ro??
Should I derive ro for each length?
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sutapanaki
Joined: 02 Nov 2001
Posts: 488
Helped: 19
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| 30 Dec 2004 20:46 Re: about MOST |
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| That is one possibility, but you hardly need it. You may do a simulation and plot ro over Vds, or Id or L, just to get a feel of it. You'll see that with accurate models it changes with Vds even if you keep W and L constant. Ro is unprecise parameter and you don't need it for design. It is true that gain is gm.ro but why would you need to know the gain precisely? If you design an opamp you just need a large enough gain to take advantage of the feed-back - opamps always work with feedback. But you don't need to calculate the exact value of the gain. However, you need to know how to increase or decrease the gain. I hope you do.
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