NPN brain teaser puzzle

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echo47

Joined: 07 Apr 2002
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30 Mar 2005 6:56 NPN brain teaser puzzle

Can you solve this little puzzle?
If you already know the (surprising) answer, please don't reply too quickly and spoil the fun for everyone else.

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_i386sx25_

Joined: 02 Jun 2002
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30 Mar 2005 18:08 Re: NPN brain teaser puzzle

10V ?

wwfieee

Joined: 09 Sep 2004
Posts: 55

30 Mar 2005 20:55 Re: NPN brain teaser puzzle

0V? or floating.

the transistor acting as diode, and collector is open.

echo47

Joined: 07 Apr 2002
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Helped: 567

30 Mar 2005 21:06 NPN brain teaser puzzle

No, try it on your bench. Many engineers find the results astonishing.

You can use almost any NPN transistor.
Be sure to use a DVM or other high-impedance voltmeter.

flatulent

Joined: 19 Jul 2002
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30 Mar 2005 21:24 Re: NPN brain teaser puzzle

The BE junction will be broken down with a zener effect to make about 5 V. There will be leakage current to the collector. You will probably measure 5 V or so.

truebs

Joined: 21 Jan 2005
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30 Mar 2005 21:36 Re: NPN brain teaser puzzle

10.7 V , As the base emitter junction is reverse biased it would at up to Vbe .

But if leakage is considered (say for eg Ico = 1 ma ) then volatage recieved would be 9.7 V !

aryajur

Joined: 23 Oct 2004
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31 Mar 2005 0:11 Re: NPN brain teaser puzzle

It goes to a negative value !?

movingbait

Joined: 23 Apr 2004
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31 Mar 2005 0:52 Re: NPN brain teaser puzzle

ok i did a sim using circuitmaker

So how is the result explained, i assume the sim is right(a damn long shot) and varied the resistance of the voltmeter appropriatly?

movingbait Ü

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chinito

Joined: 05 Jul 2004
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01 Apr 2005 2:05 Re: NPN brain teaser puzzle

VBE is negative. NPN in cut-off --> zero volts reading

echo47

Joined: 07 Apr 2002
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01 Apr 2005 5:11 NPN brain teaser puzzle

Good job Aryajur, you actually tried it! Simulators and textbooks won't give the correct answer.

Now who can explain it?

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01 Apr 2005 5:11 Ads

aryajur

Joined: 23 Oct 2004
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01 Apr 2005 9:26 Re: NPN brain teaser puzzle

The explanation I can think of is:
The excessive reverse bias across the base emitter junction, causes the depletion region to expand in the base region. Some of the charge that comes to expand the depletion of the base emitter region comes from the depletion of the base collector region. And so the base collector region depletion width decreases, causing a sort of forward bias, and thus we get a negative voltage in the voltmeter.
Please comment.

Jack// ani

Joined: 02 Dec 2004
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01 Apr 2005 10:55 NPN brain teaser puzzle

Now its your turn echo47!!!!!! Please reveal it.....

echo47

Joined: 07 Apr 2002
Posts: 4205
Helped: 567

01 Apr 2005 12:35 NPN brain teaser puzzle

Yes this circuit does output a negative voltage. I measure about -0.35V. Although the available current is small, only a fraction of a microamp, the transistor really does convert from positive to negative without using any capacitor, inductor, or charge pump.

Semiconductor equations usually don't give negative values. The circuit is not oscillating. So what's happening? A new form of energy? An April fool's joke?

No, it's an old form of energy.

I'll give the explanation soon.

You'll see!

Literally.

pixel

Joined: 16 Sep 2004
Posts: 574
Helped: 63

01 Apr 2005 14:20 Re: NPN brain teaser puzzle

It seems like C and E are inversed.

It should be off (at the edge of inverse active region), but then voltatge should be -0.6V?

echo47

Joined: 07 Apr 2002
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Helped: 567

02 Apr 2005 11:04 NPN brain teaser puzzle

Here's the unexpected answer:

The reversed-biased base-emitter junction breaks down and emits yellow light. The light illuminates the base-collector junction, where the photoelectric effect generates the negative voltage.

Just for fun, you can cut the top off a metal-case transistor and examine the die under a microscope. A large transistor such as 2N3055 looks quite pretty, like an aerial view of a small town at night.

Note: Driving a transistor's B-E junction into reverse breakdown my injure the device, so don't put it back into your stock bin.

Karthikeya

Joined: 09 Jan 2005
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03 Apr 2005 11:36 Re: NPN brain teaser puzzle

it was indeed an unexpected answer........
can u plz explain this photoelectric effect in CB jn in little more detail.....

regards

wwfieee

Joined: 09 Sep 2004
Posts: 55

03 Apr 2005 21:14 Re: NPN brain teaser puzzle

how in the world did you find this out? This is like a DC-DC converter.

echo47

Joined: 07 Apr 2002
Posts: 4205
Helped: 567

03 Apr 2005 22:08 NPN brain teaser puzzle

A few years ago, Bob Pease, a famous analog engineer at National Semiconductor, briefly mentioned this effect in his "Pease Porridge" monthly magazine column. I wish I could give you more technical info, but he did not go into detail, and I can't find any literature on this little-known effect.

Bob is an infamous practical jokester. He sent this puzzle to the engineering team at a rival company. The joke apparently worked, because those guys started tearing their hair out trying to figure out why their cherished semiconductor theories were suddenly flying out the window.

Heeeeeerrrr's Bob!
http://www.national.com/nationaltv/

flatulent

Joined: 19 Jul 2002
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03 Apr 2005 23:32 Re: NPN brain teaser puzzle

Is this a thermal effect? Can someone who has measured it adjust the transistor temperature with spray and see?

echo47

Joined: 07 Apr 2002
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04 Apr 2005 0:52 NPN brain teaser puzzle

I freeze-sprayed my 2N3904. The output voltage increased slightly, from -0.32V to -0.40V.

Right now I can't find my decapitated 2N3055. I recall that increasing the input current caused more and more tiny pinpoints of yellow light to appear, like turning on streetlights. Fascinating!

wwfieee

Joined: 09 Sep 2004
Posts: 55

04 Apr 2005 2:56 Re: NPN brain teaser puzzle

yes, i've seen some of bob pease's show in the national website. strange yet funny guy. i've also read his book. very informative. I haven't had the chance to read his side column in the journal though.

does it work with pnps too? I've a couple lying around that hopefully i can tear them apart to see this. We need to be in dark room for it?

nand_gates

Joined: 19 Jul 2004
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04 Apr 2005 4:37 Re: NPN brain teaser puzzle

Gr8, Please post more such exciting things about transistors!

echo47

Joined: 07 Apr 2002
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04 Apr 2005 5:15 NPN brain teaser puzzle

Yes I see similar behavior with PNP. Today I tested a 2N3906 and 2N2907A.

I cut open a Motorola 2N2907A. With 10mA of input current, I see a faint speck of greenish yellow light with my naked eye in a dark room. Through a 100x microscope I see eight rows of tiny lights. As I increase the current, more lights appear along those rows. Most curious!

The 2N3055 was the most impressive. I recall its light was yellow or amber.

I just now found a somewhat relevant paper, "Degradation of a Light Emitting Silicon Junction of a Bipolar Transistor". Does anyone have references [6] or [7]?
http://www.j-elec-dev.org/JED-v1n1/A2-v1n1.pdf

I found another paper that begins with this sentence, "It is well known that when a p-n-junction is biased into avalanche, light is emitted. This light is absorbed in the surrounding substrate and generates electron-hole pairs."

Karthikeya

Joined: 09 Jan 2005
Posts: 114
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04 Apr 2005 11:24 Re: NPN brain teaser puzzle

hi echo47
it is really an amazing puzzle . here is the .pdf files u asked for.....

thank u

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wwfieee

Joined: 09 Sep 2004
Posts: 55

07 Apr 2005 6:43 Re: NPN brain teaser puzzle

i cut off the top of a 2n2222 by ST and biased it as shown but without a resistor. it begins to glow at around 9.5Vs. Truly remarkable. Yellow and its real nice. I showed another guy and had him guess what the component is.

He never guessed it. Haha.

flatulent

Joined: 19 Jul 2002
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Location: Middle Earth

07 Apr 2005 17:43 Re: NPN brain teaser puzzle

I am not convinced that this is caused by the photo-electric effect. The with of the base would prevent the photons from getting to the base-collector junction. Can someone put their meter in the current mode and measure the current with different values of resistance in series with the meter. Then calculate the voltage across the BC junction and plot a voltage-current curve. If it is the photoelectric effect you should get a horizontal line of constant voltage for currents increasing up to a point and then the voltage should rapidly decline.

An alternative theory is that the current flowing around the input loop injects many electrons into the base. These have kinetic energy. Most of them are attracted by the electric field across the B-E junction and flow in that direction. A small portion use their kinetic energy to go to the B-C junction.

Mazi3

Joined: 29 May 2002
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07 Apr 2005 23:26 Re: NPN brain teaser puzzle

I had to try it myself. One old 2N3055 from ST was sacrified to see it Smile

current was 160mA and picture was taken at night, long exposure time.
The transistor is still good, but I didn't make any measure of his parameters, only checking PN junction. Breakdown voltage is 11.5V (thats more than 7V as stated in datasheet-but this is only maximum voltage, which can EB junction withstand). Current was tried up to 0.6A-be aware, there is a lot of heat generated..

here is picture of transistor glowing Smile

one for background... Smile

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movingbait

Joined: 23 Apr 2004
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08 Apr 2005 1:30 Re: NPN brain teaser puzzle

Forgive the question, but how do you remove the top from a TO3 package and/or a TO92 metal can.

movingbait Ü

wwfieee

Joined: 09 Sep 2004
Posts: 55

08 Apr 2005 4:34 Re: NPN brain teaser puzzle

you just use a clipper and clip the top end off. becareful not to clip too close to the base. 2 tiny wires are connected from the chip to the legs.

i'll measure the currents on the 2n2222 next week.