FET Transistors

[R.G. Keen]

Most jfets are symmetrical regions of semiconductive N or P type 
material, the "source" and "drain" being nominal designations of
the ends of the region. The gate is an abutting region of the 
opposite type of material.

USUALLY ... the gate will conduct like a diode when forward
biased to either source or drain. No surprise - it IS a forward
biased diode.

There are non-symmetrical jfets where source and drain are not
interchangeable, but you should be able to read the differences
by the difference in forward resistance. 

So. Find a lead that conducts to both others. That's the gate. 
If there is one that has a higher resistance from the gate, that 
is likely to be the drain.


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     You know that's a good question. My typical answer (an unprofessional 
     one, I know) is "move it around till it works".:)
     
     I would offer an answer to this one if I could, but I'm weak on the 
     theory behind the various junctions and such. It would be nice if 
     somebody with such knowledge could post a long description of 
     transistor physics. Like - okay what's the real difference between an 
     emitter and a collector in a transistor? In an NPN they're both "N" 
     with a "P" between them, but you can't swap them in a circuit. Why 
     not? Similarly, what's the difference between a Source and a Drain in 
     a FET? Such a post would be great archive/FAQ material.
     
______________________________ Reply Separator _________________________________

The discussion that went on the other week about FET's and pinouts, leaves 
me wondering if there is a way of determining the pinout of a transistor 
such as a 2n4391 or MPF102 without having a datasheet.  I had found a way in 
Electronotes of determining the pinout of a small signal NPN or PNP 
transistor and am wondering if anything similar can be done for FET's with a 
simple circuit.