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vlsi_design Junior Member Offline Posts: 23	Simulation for finding cgs and cgd Mar 08^th, 2015, 6:23pm Hi, Is there any way to find cgs and cgd of transistor using simulations? I know that AC can be done to find imaginary part but that will give cgs+cgd=cgate. I also know about DCop point and captab. I am looking for either AC or S param sim that allows to seperate cgs and cgd
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Geoffrey_Coram Senior Fellow Offline Posts: 1999 Massachusetts, USA	Re: Simulation for finding cgs and cgd Reply #1 - Mar 13^th, 2015, 11:06am If you put an ac voltage on drain and measure the gate current, this will be related to cgd. If you put the ac voltage on the source instead, this will give you cgs. If you put the ac voltage on the gate (still measuring the gate current), this is cgs+cgd (+cgb).
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vlsi_design

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Re: Simulation for finding cgs and cgd
Reply #2 - Mar 21^st, 2015, 7:40pm

Hi Geoffrey_Coram,
Thank you very much for the reply! I tried to simulate it but I find that Cgate < Cgs + Cgd. Actually it should be Cgate> Cgs + Cgd due to Cgb. I have attached the circuit and curves

Also why these caps decrease with frequency? Is it due to inversion layer trap phenomenon as explained in device books? But that occurs at 100KHz onward and only when inversion layer is completely isolated.

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Cg_Cgs_Cgd.JPG

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vlsi_design Junior Member Offline Posts: 23	Re: Simulation for finding cgs and cgd Reply #3 - Mar 21^st, 2015, 7:41pm Here is the simple circuit
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Geoffrey_Coram

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Re: Simulation for finding cgs and cgd
Reply #4 - Mar 26^th, 2015, 7:44am

vlsi_design wrote on Mar 21^st, 2015, 7:40pm:

Hi Geoffrey_Coram,
Thank you very much for the reply! I tried to simulate it but I find that Cgate < Cgs + Cgd. Actually it should be Cgate> Cgs + Cgd due to Cgb. I have attached the circuit and curves

Your plot shows Cg (M6, in green) above the other two (Cgs from M9 and Cgd from M12).

Quote:

Also why these caps decrease with frequency? Is it due to inversion layer trap phenomenon as explained in device books? But that occurs at 100KHz onward and only when inversion layer is completely isolated.

It's not the inversion layer trap phenomenon. I think it's just a voltage-divider effect with all the different capacitors and conductances (gm, gds -- but maybe also RD,RS, and RG).

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