Zenering a big transistor

Ummm- how are you breaking down the BE junction of a diode connected transistor? The junction is forward biased in diode connection.

Beta will gradually diminish. It will be accelerated if zenered at high temperature. ...Jim Thompson

The usual meaning of "diode-connected" is CB shorted. In one direction you get normal conduction, and in the other you can zener BE because there's no reverse transistor action.

If you were to use the CB junction as your sensor, you'd be OK reverse-biasing it up to BV_CBO.

Alternatively if the reverse beta is big enough to be useful, short BE and use it upside down.

A third fun possibility is to connect B to a voltage divider between C and E so that you get transistor action in both directions as well as a bit of voltage gain.

Cheers

Phil Hobbs

I'm reverse biasing the whole thing, C+B shorted and held negative wrt to base (pnp).

GH

Thanks Jim, I saw what looks to be a small change (in V_f) in 15 minutes. I'll let it 'cook' for a few hours.

George h.

Why are you ever zenering the poor thing?

You'll have to measure temperature accurately, too, to see if zenering is affecting the junction tempco. Or compare the drop of two transistors, one that you zener and one that you don't.

But TIP31C is an NPN. NPNs tend to zener BE at lower voltages than PNPs. Usually.

Right I could sue a different 'diode'. One thing I don't really understand is that the diode connected transistor seems to give a more consistent* temperature sensor than just the C-B or E-B junction. It's not something I looked at very closely, and maybe just a fluke with the TIP31.

George H.

Oh snap, sorry tip32C. (I'm always mixing things up.) I'm only zenering it to see if my over voltage protection works. Maybe I'll need some other diode for protection... an LED?

George H.

Why not just protect your TIP31C from zenering? A few diodes would do it.

Cheers, James Arthur

e.g., | +----. | | TIP31C |

It appears that zenering any junction causes dislocations that act like ionic contamination and like radiation damage. Like radiation damage, the accumulation is total dose related, and can be annealed out.

A larger current is just like a higher radiation level; a one-to-one correspondence.

Why not a resistor?

Reference zeners are stable to parts per million for years. There's something special about the b-e junction of a transistor.

Really? How come beta degrades when a b-e junction is used as a zener? ...Jim Thompson

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Probably this is due to base spreading resistance. The TIP32C part number indicates (the 'C' part) high breakdown voltage (collector-base); the 'B' and 'A'' parts were lower rated. So, there's significant manufacturing range in these parts' characteristics, and that particular characteristic goes with the base doping level, i.e. base material resistivity.

When using C-B or E-B diodes, the (thin, lighter-than-emitter-doped) base adds a bit of electrical resistance. The E-(B+C) connection, in forward bias, has only a fraction of the base current that an E-B connection does, so the base's resistance adds a couple of orders of magnitude lower voltage drop.

I'd guess that the larger size and higher doping of the emitter region means that the potential barrier generated by reverse biasing the junction is too wide for thermally-generated minority carriers to quantum tunnel through, they can only cross via avalanche breakdown

Beats me. But reference zeners are super stable, usually in the 5-10 mA range, for years.

There are also "reference transistors" (or "reference amplifiers") which are, I think, an NPN transistor with a separate zener in series with its emitter. Fluke used them in their differential voltmeters, which are damned accurate for decades.