Quote:
Originally Posted by G0HZU_JMR
Process 51 is another popular JFET process, and this process is used to make JFETs that are suited for use as switches or choppers. You can see there are some familiar part numbers in the image below.
The J111, J112 and J113 span the parameter range of process 51. These parts are still available today.
The die shape is very different for process 51 and I guess this is done like this to make the JFET more suited to switching applications.
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You'll find the 2N4391, 2N4392, 2N4391 group in that listing. They are a graded set of high Idss devices. 2N4391 ha the highest Idss grade (and lowest RDSon) then the higher JEDEC numbers are reducing Idss (Increasing RDSon)
HP used the 2N4391 as i855-0020, its preferred analogue switch FET.
Marconi used, I think it was 2N4392 or it may have been 2N4391 as a high bias current HF amplifier in some of their high performance HF receivers. I first came across it in the H2900 'Sosin's folly' star of the famously withdrawn receiver comparison article sosin wrote.
The FET channel is long and is folded in a serpentine under the gate fingers. the fine-ness of the fingers shows how short the channel is, and the total length of all the gate fingers shows you how wide the channel is. Process 51 are wide, short channel FETs, folded to fit on an economical area of silicon.
There used to be a sort of industry rule of thumb of so many dollars per square millimetre, plus packaging. Can't remember the number, but it held for quite some time and even worked for specialist devices like digital camera sensors!
David