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Dissecting a FET

July 12, 2015

This happened back when transistors were new and vacuum tubes still ruled the world of electronics.  I had built an oscilloscope from a kit.  It was full of vacuum tubes.  I wanted to look at the intermediate frequency signals of a receiver, also full of vacuum tubes.  The oscilloscope input had two long leads with aligator clips on the ends.  They wouldn’t do for sampling the IF signal.  Not only would they detune the amplifier stage, but they would act like antennas, radiating the signal all over the place.  I needed a probe that would put the lightest possible load on the IF stage, and would transmit the signal back to the oscilloscope over a shielded low-impedance cable.  A probe containing a triode tube in a cathode follower configuration was the usual way to solve this problem.

I had a better idea.  Field effect transistors had just come in.  They were a perfect replacement for vacuum tubes.  They were expensive, but I bought one.  The equivalent circuit was a source follower.  It only needed a 9 V battery for power.  I could build the whole thing in a very small case.  I put a large paper capacitor between the probe tip and the gate pin of the FET; it would pass a wide range of frequencies but block high DC voltages.  When I tested the probe with audio signals, it worked perfectly.

I was ready to try it on the IF amplifier stage of my receiver.  First I put the probe on the grid.  I saw a small signal on the oscilloscope.  Then I moved it to the plate.  I saw a much larger signal.  This was amazing.  I was impressed.  It was working very nicely.  Then I tried the grid again.  Nothing!  Gloom descended.  It never worked again.

What happened?  I pondered this question for some time, until the aweful truth came to me all of the sudden.  The grid of the IF amplifier was grounded through the IF transformer.  The plate was at 150 V DC.  Nothing happened when I first put the probe on the grid, but when I moved it to the plate, that capacitor charged up slowly through a resistor.  When I moved it back to the grid, the capacitor discharged very quickly through the FET, with so much current that it destroyed the FET.  Quick as a flash, my expensive FET was turned into a tiny piece of junk, never to amplify again.

I had never seen an FET before.  It was expensive.  I decided to at least examine it before I threw it away.  It was a white ceramic disk with three leads coming through it, with a black cap made of a drop of epoxy.  When I tapped it gently with a hammer, the black cap popped off.  Underneath, I saw that one lead was bent over and flattened against the ceramic disk, but the other two stuck straight up.  I needed magnification to see more.

I brought the FET into work, where we had a binocular microscope.  With that, the tiny details of the FET loomed into view like buildings and streets of a city block.  The die was really tiny.  It was sitting on top of the flattened lead.  That was the gate lead.  Hair-like wires ran from the other two leads to pads on the die.  These were the source and drain leads.  Down the centre of the die ran a serpentine gap, with source and drain fingers projecting from either side.  I could even see a damaged area in the gap, no doubt the result of the high current I had put through the FET.  I was amazed at what I saw, even though I was still disgusted with what I had done.

Other people at work were not impressed.  They looked in the microscope, but really didn’t care about what they were seeing.  Maybe they didn’t understand.  Maybe they just weren’t interested in electronics or those new transistors.  I suppose I was the only one who was interested.  I had also learned an expensive lesson.  It was a long time before I bought another FET.




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