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Design >> Analog Design >> substrate current effect?
https://designers-guide.org/forum/YaBB.pl?num=1142903450

Message started by jaylin79 on Mar 20^th, 2006, 5:10pm

Title: substrate current effect?
Post by jaylin79 on Mar 20^th, 2006, 5:10pm

Hi Guys,

I am designing a bipolar circuit, its input is +/- 20V, and there is a problem that when the collector of NPN is lower than -0.7V, there will be substrate current flowing from P-sub to N collector, will this current give a bad effect to the whole circuit if we don''t consider the power dissipation effect.

Best regards
Jay

Title: Re: substrate current effect?
Post by RobG on Mar 20^th, 2006, 7:44pm

I dunno, do you consider latching up a bad effect? ;). Seriously, if you forward bias the collector base of an NPN you don't just turn on a diode... you (generally) turn on the parasitic pnp that goes along with it (PNP = p+ base/N- Collector/Substrate). This pnp often isn't modeled. Yes, the effect can be quite bad, and if the substrate currents are large enough they might even turn on a npn and you'll latch everything up. You really don't want to forward bias the BC of a bipolar.

Title: Re: substrate current effect?
Post by jaylin79 on Mar 20^th, 2006, 9:03pm

RobG, thanks a lot!

Title: Re: substrate current effect?
Post by judean1 on Mar 21^st, 2006, 2:48am

Making the collector base junction forward bias will effectively take away the very essence of the bipolar circuit as an active device.
Regards,
V