reversed transistor

It'll turn on with about 0.7V.

Graham

"Eeyore" wrote in message news: snipped-for-privacy@REMOVETHIS.hotmail.com...

I tried with only 5V through a 1K limiting resitor and...nope, it is still off.

Joer

Eeyore a écrit :

???

What he's looking for is VEBo.

Joer Breton a écrit :

No. But note as Vceo is lower than Vcbo. Why?

Your best bet is looking at Vebo. If you've answer the previous question, then you know what to do with Vebo.

Like with any junction breakdown.

Not weird. Unusual.

I rechecked with a higher input voltage and you're right, it is similar to Vebo, the voltage tops at 6.5V accross the transistor. It makes a good zener though, the voltage is quite stable from an input of 7V to 15V. Wonder if it can be used that way or it will deteriorate over time?

Joer

Because the emitter material is doped to a much higher concentration than the collector material is, the breakdown voltage of the emitter-base junction is much lower than the collector-base junction. For the same reason, the reverse beta (current gain) is also lower.

I think that the emitter-base break down voltage is usually between 4 and 10 volts.

Typically around 6 volts. The c-b junction will be forward biased, and the b-e junction will zener. Many conventional npn's will act like 6.2 volt reference zeners (namely a 5.6 volt zener in series with a forward-biased diode) with a very low zener voltage tc. For some reason, pnp e-b junctions tend to zener at higher voltages, sometimes in the 12 volt range.

The datasheets usually specify a maximum allowed back-biased b-e voltage, Vbr e-b or something like that.

Longterm b-e zener current degrades beta, but I don't have any numbers.

John

Or the reverse-biased Vceo, which probably doesn't have a name of its own. Veco?

John

I've observed degradation of the breakdown voltage as well, in phenolic packing at 140°C (automotive applications).

...Jim Thompson

Yes, and yes. Emitter base zeners are used in lots of circuits, but, eventually, this breakdown degrades the beta of the transistor. Problems only occur if the design alternately needs both zener operation and gain.

In message , dated Mon, 4 Sep

That's the emitter-base voltage with collector open. Not the same as VECo, emitter-collector voltage with base open.

I suspect that it depends on how much surface leakage there is. A sniff of base current will change things quite a bit.

Hello John,

For really hot RF transistors that can be as low as 1V. Ohming out a circuit with a regular meter can destroy such a transistor. Or worse, partically damage it, making it difficult to figure out what's going on.

I've seen it from 3V in RF transistors to 6~8V for most types, out to 30V or so for symmetrical types (2SC2828 is one I have) and germanium transistors (which seem to have similar Vcb and Veb limits, haven't tested a whole lot).

Tim

John Larkin a écrit :

I'd expect Vebo/beta-reverse when it latches back.

...Jim Thompson

John Woodgate a écrit :

Yep, but probably not much different since the transistor reverse beta is quite low.

The "rating" would be Vbeo, approximately. In other words, the E-B junction would eventually zener and the B-C junction would be forward biased. The zener voltage *used* to be specified. These days, mfgs are lazy and give 5V as a max reverse VBE, knowing full well that ther are no silicon transistors that zener before 6V (a number of them have a breakdown in the 8-12V region).

Actually, reverse current in the base of a transistor will cause crystalline damage similar to radiation; the damage is based on the dose rate and total dosage (amount of current and time). The product determines the amount of damage. For reverse current flow, the dmage can be annealed out (250F or so, forget actual temp and time).