Emitter Base Breakdown Voltage of NPN Used as Zener

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.

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?

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.

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.

Regards

All tests are on Si devices.

Have some Ge ones handy? Try!

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

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.