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Emitter Base Breakdown Voltage of NPN Used as Zener

09/27/2011 1:55 PM

Has anyone seen the reverse breakdown voltage of a bipolar transistor used as a sort of Zener diode?

Most data sheets list a VEBO (that's Voltage from emitter to base with collector open) of 5 or 6 Volts in their absolute maximum section.

But, I have come across a design which specifically uses this junction as a "sort" of Zener diode. Does that seem like a good idea?

One of my colleagues has an ancient Tektronics curve tracer so we decided to have a look at the characteristics of a couple of transistors. There is no USB port on this machine, so I had to use the camera in my phone.

First this is the result of testing a 5.1 Volt Zener for comparison (and, because we wanted to check the "calibration of the instrument). The vertical scale is 1 mA/div. and is offset by one division to get the picture clearly on the screen. The horizontal axis is 1 V/div. So this shows no current up to about 4½ Volts then a "knee" and the rest of the curve which is asymptotic to about 5.1 V: just what you'd expect.

The other two "readouts" are only applicable when testing transistors (as transistors).

This is the result of testing a BC107 emitter base junction:-

The scales are the same. The junction avalanches at about 8.2 to 8.3 Volts; the knee is astonishingly sharp, but, the trace then rises as a straight line not quite vertical.

This is a BC846B:-

The knee is again at about 8.2 to 8.3 Volts, and, the slope is steeper.

We kept turning up the current on the 846. This is it going right up to 100 mA; we didn't have the offset on the vertical axis, and, we had to change both scales.

Vertical 10 mA/div.; Horizontal 2 V/div.

The knee moved further up the horizontal axis as the device warmed up and down as it cooled down.

At first I couldn't imagine what kind of "mechanism" could cause this behaviour, but on reflection it looks like a perfect Zener with a resistor in series.

What does everyone anyone think? Is this a well known characteristic that no one ever told me about?

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#1

Re: Emitter Base Breakdown Voltage of NPN Used as Zener

09/27/2011 2:28 PM

All PN junctions show this behavior. When you place a P-type (excess holes) material against an N-type (excess electrons) material, the holes attract the electrons across the junction. This causes a "null" section right at the junction. The amount of voltage to cross the junction is usually referred to as the barrier potential. Once the barrier potential is reached you have current flow.

This is also why diodes emit red light. To cross the barrier, the electrons are actually "pumped up" (charged) into the next orbital shell. As they cross the barrier, they drop back down into their own shell and release their charge as a photon. Just moving from one shell distance turns out to be red light.

Tunnel diodes (heavily doped n and p type materials) have a 'negative resistance' region. As the barrier is breached, the width of the barrier junction actually decreases for a small amount of current of change. This negative resistance region is used to create small, multi GHz oscillators in hand-held electronics.

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#2

Re: Emitter Base Breakdown Voltage of NPN Used as Zener

09/27/2011 5:31 PM

Interesting to know that the knee is sharper than normal zener.

Could you do a scan on the breakdown of the Base Collector junction , to see if it has same effect?

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#3

Re: Emitter Base Breakdown Voltage of NPN Used as Zener

09/28/2011 2:03 AM

Every P-N iunction of a diode or a transistor breaks down at some reverse voltage, and displays Zener type behavior. The breakdown voltage depends on the construction and doping of the layers. Common, diffused transistors have high doping concentration in the emitter / base area, hence they break down fairly consistently at around 5Volt in silicon. Collector is normally much less doped and thicker, yielding (much) higher breakdown voltages. Germanium has overall lover voltages, SiGe used for some microwave transistors is used with such small dimensions, that even the collector breakdown may come in under 3Volts.

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#4

Re: Emitter Base Breakdown Voltage of NPN Used as Zener

09/28/2011 6:05 AM

Thanks to all for your comments. I guess I was so surprised at how sharp the knee was that I didn't really ask the most pertinent questions.

To what extent can you rely on the behaviour?

And what sorts of currents would you expect to be likely to damage the device?

I suppose that since the spec says that the maximum peak forward current for the base emitter junction is 200 mA, and, the voltage would probably be around 1V for that current: then the peak reverse current should be about 20mA to keep the peak power to about the same level.

Sorry bravo88: my colleague has taken the curve tracer home, and it's not what you would call the most transportable instrument.

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#5

Re: Emitter Base Breakdown Voltage of NPN Used as Zener

09/28/2011 7:36 AM

Hi,

this is a typical behavior. I used to work with assembly and test of such transistors. The zener you get is quite reliable, but remember the failure mode for a wire bonded device is open circuit. A "normal" zener diode has metallurgical bonds on both sides of the chip, so the failure mode is short. A higher voltage is more likely to damage something else in your ckt than zero...

8.2 V is exactly what we would expect for a BC107. Devices with voltages above 6 volts will have a sharper knee, those below will be rounded. Also the voltage temperature coefficients have different signs... the transition from Zener effect to avalanche effect is at around 6 V. You can estimate your maximum current assuming total power dissipation at around 300 mW.

Bring back that Tektronix and try to see the curve you get from collector to emitter (inverted polarity). The CB junction starts carrier injection into your zener junction and you get negative resistance (similar to a neon lamp or a diac). You can make a simple relaxation oscillator using this effect. In fact the device will probably start oscillating while on the curve tracer, your display will show some small "twisters" (HF oscs).

Try measuring beta at a standard point (say 2 mA, 5 V). Then inject some bigger current in the inverted mode as above (till "just before the poor thing will fail"). Then cool it down and measure hfe again, there may be a reduction. You can trim it up again by injecting some 2 amps on the BE junction (conduction direction; reduce collector R) for some milliseconds... The more you get this effect, the worse is the passivation on the chip. It's easier to get with old metal case transistors.

brgds

Snel

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#6

Re: Emitter Base Breakdown Voltage of NPN Used as Zener

09/28/2011 10:50 AM

Regards

All tests are on Si devices.

Have some Ge ones handy? Try!

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#7

Re: Emitter Base Breakdown Voltage of NPN Used as Zener

09/28/2011 12:06 PM

Hi All:

Speaking as a Prof., EET (retired now), my Solid State Devices students had to ID concealed 2-terminal devices to pass that course...anything was fair game. They had to determine material (Ge, Si), polarity, zener-or-rectifier (if diode), R, C, L, shorts, I(rev) leakeage, opens, etc.; using a simple curve tracer I created back in 1976. Still in use.

Most Si BJTs exhibited very sharp breakdowns (90 degrees), very low Z(zt), from 5 V to 6.5 V; . Used as zeners, these could handle only limited I(rev) currents, <3 mA, but did work as V(z) references in series (pass) regulators. Also they could serve as voltage level detectors in high Z circuits.

Karl Hunter

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#8

Re: Emitter Base Breakdown Voltage of NPN Used as Zener

09/28/2011 2:32 PM

I just got daily E-mails from EDN (Electronic Design News), and there was a link to an old Bob Pease article from 1983. Coincidentally, the article was about semiconductor "bounding and clamping" and talks about how different junctions behave.

http://www.edn.com/blog/Designing_Ideas/41356-Bob_Pease_on_bounding_and_clamping_techniques.php

Thought it was worth sharing.......

Tom D.

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#9
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Re: Emitter Base Breakdown Voltage of NPN Used as Zener

09/28/2011 2:51 PM

Great link.

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