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zhangjerome

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Why does substrate current so large??
Mar 18^th, 2007, 8:33pm

I am now designing a CML D latch biased with cascoded current source. With transient simulation, the drain current and source current of the common-gate transistor in the current source differs from each other by about 400uA. I checked the node current and found out that the current goes to B node.

The substrate of the transistor is grounded, and the transistor is in active region. I totally have no idea how
this coulde happen.

Is this a model problem or simulation problem? Or there is something wrong with my circuit??

Thanks, bow~~

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Geoffrey_Coram

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Re: Why does substrate current so large??
Reply #1 - Mar 19^th, 2007, 4:17am

Can you run a dc sweep across the active operating region, corresponding to the operating point of the transient? You should use this to determine if the current is dc (caused by impact ionization -- or dc diode current, if you have a pmos where you thought you had an nmos because of a severe modeling problem) or if the current is capacitive (s/b or d/b diode capacitances charging/discharging during transient might cause a "glitch" in current -- how long does the large body current persist?).

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zhangjerome

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Re: Why does substrate current so large??
Reply #2 - Apr 9^th, 2007, 6:55am

Geoffrey_Coram wrote on Mar 19^th, 2007, 4:17am:

Can you run a dc sweep across the active operating region, corresponding to the operating point of the transient? �You should use this to determine if the current is dc (caused by impact ionization -- or dc diode current, if you have a pmos where you thought you had an nmos because of a severe modeling problem) or if the current is capacitive (s/b or d/b diode capacitances charging/discharging during transient might cause a "glitch" in current -- how long does the large body current persist?).

Thank you. I now know that it's bacause of the large voltage swing at the drain node and the current is capacitve.

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