Mos switch can be over several tens of um?????

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ljy4468

Joined: 20 Jul 2005
Posts: 199
Helped: 10
Location: South Korea

19 Jan 2006 3:44 Mos switch can be over several tens of um?????

Hi
everyone!
I've question about mos switch size.
Can be switch size over 70um, 100um(gate) without using multiplier or finger?
Of course There are no problem in simulation If I use big size switch
But I think there are problem in layout
I heard that multiplier or finger structure is recommended for gate over 20um.
Which is true?

thanks in advance.

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eda_freak

Joined: 20 Jun 2005
Posts: 67
Helped: 9

19 Jan 2006 7:08 Re: Mos switch can be over several tens of um?????

Hi!!
i think that by switch u mean the width of the MOS device...yes actually u can simulate for larger widths of device but you should break it down into multiples i.e. fingers...this ideally should also be reflected in your spice netlist by multiples(M value) for the device simulation..... larger widths have larger gate and diffusion capacitances associated with them... so its always smart way to break em down into smallerer managable fingers....now this also helps in making good layouts especially when u are making digital cell libraries...which have size specifications(technology definations).

venkateshr

Joined: 01 Feb 2005
Posts: 63
Helped: 2

19 Jan 2006 8:31 Re: Mos switch can be over several tens of um?????

It is highly advisable to use fingers if u want to emulate a high width mosfet.the reasons are

1. Capacitances(diffusion) will be heavily reduced due to sharing of drains

2.gate resisntace will be heavil reduced and this aids the freq. response .

3. layout is easy (when space is concerned)

paulux

Joined: 16 May 2005
Posts: 132
Helped: 8

20 Jan 2006 10:45 Re: Mos switch can be over several tens of um?????

Is there any optimum way to split this multiple finger? Any criteria, consideration from real example? Thanks for comments

jutek

Joined: 21 Oct 2005
Posts: 274
Helped: 5

21 Jan 2006 14:47 Re: Mos switch can be over several tens of um?????

hello

when i divide large (10m) PMOS into 1000 fingers, nothing changes. Capacities are still the same.

should i split it into 10 fingers and connect 100 transistor parallelly to see the result?

it also doesn't change antyhing. i use tsmc 0.18um

regards

rockycheng

Joined: 04 Feb 2005
Posts: 91
Helped: 4

22 Jan 2006 0:15 Mos switch can be over several tens of um?????

jutek,

You are right. Cgs and Cgd are not affected, when you devide the long gate into many fingers, since the total transistor width remains the same. However, when the finger number is large enough, the drain and source capacitances shall become almost half.