Thread by @TubeTimeUS

here's a fun little mystery! see this 2N2222A transistor in a TO-18 metal case? there's 12V through a 1K resistor from the emitter to the base (reverse biased junction) and OUT goes to the meter (10 meg internal impedance). SPICE says OUT=0V, but i measure -0.353V! why?

it also gives me a negative voltage if i use a differential voltmeter measurement, which has close to infinite input impedance.

H/T to @PhilipFreidin for this puzzle.

scope shot of the collector voltage. it's not oscillating or anything. just a constant negative voltage.

let's tear the lid off the transistor and see what's going on in there.

closeup of the die. when connected in this circuit, it lights up! compare the two photos.

ok, now let's disconnect the 12V supply and instead shine a laser pointer at the die. now we're getting -0.551 volts!

this circuit was presented by Bob Pease in one of his columns. i'm not sure where it originally came from.

all right that's enough guessing, let's look at Bob's solution.

basically in avalanche mode the transistor emitter-base junction emits light, which is then picked up by the collector-base junction which acts like a tiny solar cell. 🌞

so the winner is @tedyapo ! his description came the closest to the correct answer.

also i really love how everyone broke out their oscilloscopes and voltmeters and breadboards and microscopes! it's such a simple riddle -- it DEMANDS experimentation! 😍

transistors and other junction devices that use avalanche breakdown modes are pretty fascinating. a good read is rfcafe.com/references/ele… which discusses the infamous 4-layer diode.

infamous because William Shockley, the co-inventor* of the transistor, was absolutely convinced the 4-layer diode was the future for his company, Shockley Semiconductor. his employees thought he was wrong, left, and founded ALL THE OTHER CHIP COMPANIES.

*it's complicated

the Shockley Semiconductor building was in Mountain View, CA, on San Antonio Road. it's since been torn down, but someone installed electronic component sculptures! the grey transistor is a 2N696. there's also two 4-layer diodes, including one cut-away!