BJTs as ultra low leakage protection diodes

We make one gadget whose operation involves forward biasing a big P-N junction diode by 48 volts.

John

Yeah, but you're insane.

Cheers

Phil Hobbs

waster

ftp://jjlarkin.lmi.net/99S260A.JPGftp://jjlarkin.lmi.net/99A260A3.JPGftp://jjlarkin.lmi.net/99A260A1.JPG

As I've mentioned, the damned RatShack banana terminal things turned out to be conductive to the tune of several nA, so I had to add the lexan insulator after the thing was all built. That was messy... chips and cutting fluid everywhere in my circuit.

And I recently received my ebay Keithley electrometer. It has a nice mirror-scale meter with lowest range 1e-14 amps full-scale.

I'm doing a preamp for a gadget that's a 10 pF source, an antenna of sorts, audio to 20 MHz, nanovolt signals, no DC allowed to accumulate on the sensor (screws up spins or something), horrible transmit overloads, runs in UHV. So I'm groveling around with fA's and nV's and surface-mount Tohm resistors.

John

Thanks, but it doesn't come natural; a guy's gotta work at it.

John

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I'm not so sure about zener input protection. Generally when you need to go above the rail, you just use a N mosfet with gate tied to source and the drain going to the pad. You depend on the breakdown of the fet to provide input protection. [If you are trying this in layout, antenna rules may apply.]

Every time I investigated chip damage after ESD testing, it was always due to over-voltage versus over current. I'm not a fan of high fields (as found in reverse bias) in semiconductors. They seem to find the weak spot in the junction and zap it. Forward biased junction might have hot spots, i.e. current hogging, but in general they are pretty rugged.

A manufacturer sells a device that passes a minimum breakdown. It is a one sided specification. That is, a minimum voltage is specified, but not a maximum.

Maybe the breakdown voltage will be much higher than where you want it to be.

I see all sorts of ideas in this forum on how to cheat and make a part do something that it is not intended to do. Fine, but you are on your own. I've seen specifications of parts change wildly over the years. Case in point was a SC filter which had a pretty good THD. The THD was not a guaranteed specification, but there was an internal spec to make sure things didn't go wild. Well somebody changed epi vendors, and the THD went about the internal limit. Since the parts didn't have a THD spec, they went out into the public, though the old epi vendor was used again.

If you want a bullet proof design, take every line item without limits and blot them out with a sharpie, then go design.

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Win

Cool, Thanks John I'm going to have to probe the inputs of some opamps. I've always assumed they had protection diodes on the front end. I should be able to find the series resistance if I'm careful.

George H.

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t -

Wow, Thanks Phil, I've scribbled the circuit down in my notebook and I'll analyze it if I ever need low leakage protection.

George H.

Transzorbs are pretty bulletproof--its job would be just to limit the overvoltage across the series protection resistor long enough for the polyfuse to switch. The resistor and the input diodes would protect the input devices, at least if it were done right.

Cheers

Phil Hobbs

You say that like its a bad thing.

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I am referring to on-chip protection and failure analysis I've done. I've used transorbs, but only on power rails. In any event, putting zeners on chip isn't done to my knowledge, at least in MOS/BiCMOS.

See the slide "ground gate NMOS"

Not at all. John is the most entertaining wild man I know--his delusions actually work.

Cheers

Phil Hobbs

I'm sure it isn't done anymore, but check out the CD4049UB for an example.

Cheers

Phil Hobbs

By design. Some of the most creative people have also been schizophrenics. In schizophrenia, the barriers between brain compartments become leaky, and ideas diffuse into different contexts. Very often, they make no sense when they get there. Moderated schizophrenia is great for systems and electronic design, churning up all sorts of ideas. People can even do it in groups.

The next stage is to winnow out the practical ideas from the insane ones. Last step is brutal, disciplined, cost-effective, no-risk implementation of the one that will sell best.

Not many people are good at spanning this range. But it can be taught and practiced.

"Crazy like a fox" is the popular version.

John

Schematic "representations" on data sheets are often just that, "representations". The "equivalent" IS done using a capacitively coupled NMOS.

...Jim Thompson

I'm not surprised that it isn't a normal zener--but those parts don't have normal protection diodes, which is where I was going with it. The input of a CD4049 isn't the most robust thing in the universe, largely because of having the weird protection network.

Cheers

Phil Hobbs

Most "modern" CMOS parts don't have classic diodes, particularly on output pins.

...Jim Thompson

What do they use exactly? Unless they're SOI, there's still the substrate diode.

Cheers

Phil Hobbs

On the outputs, yes there are the body diodes... _unless_ they are tri-state-when-unpowered, then things get a bit weird.

You'd be stunned at the complexity of today's CMOS protection circuits. I'll dig thru my drawings and see if there's something I can legally post.

...Jim Thompson