I bought some BF245C's from a German eBay seller ( bra-elektronik ).

I wanted to replenish my stock, but I am increasingly sceptical of buying semiconductors on eBay, so I only bought 10 pcs.

I still had a couple of old BF245c's, so I used my simple component tester to compare their specifications with the new one.

The result is, that all the 5 old BF245C's show consistent results (on average):

Id = 3,4 - 4,1 mA
Vf = 2,17 - 2,86 V

The new BF245C from Germany shows much lower values:

Id = 0,65 - 0,7 mA
Vf = 450 - 550 mV



The tester show them all to be N-FET's with GDS connections, so it's safe to say that whatever the difference, the new device is still a FET.

But then my question is, what conclusion should I draw from these much lower values for Id and Vf?

Are these fakes, poor imitations or possibly even an improved version?

What kind of FET would come closest to these specifications?

 

Apparently a depletion-mode symmetrical device - unusual indeed. None of your devices appear to meet the datasheet from NXP however.

 

What parameter are you calling Vf?
Vgs(cuf-off) can vary from -0.25V to -8V so there is a pretty wide spread of parameters.
Your new one appears to be from Fairchild, and your older ones are a mixture of Motorola and Philips, so must be quite old.

 

I wanted to replenish my stock, but I am increasingly sceptical of buying semiconductors on eBay, so I only bought 10 pcs.

A wise move. I treat most semiconductors on eBay as counterfeit. At least you got something that was a JFET.

I still had a couple of old BF245c's, so I used my simple component tester to compare their specifications with the new one.

The result is, that all the 5 old BF245C's show consistent results (on average):

Id = 3,4 - 4,1 mA
Vf = 2,17 - 2,86 V

The new BF245C from Germany shows much lower values:

Id = 0,65 - 0,7 mA
Vf = 450 - 550 mV

None of the devices meet the requirements in the datasheet.

Does Id mean Idss? What does Vf represent? The datasheet specifies Idss at Vgs=15V under pulsed conditions.

 

Honestly, I do not expect my tester to be too accurate, but see it more as a way of getting an indication and be able to compare components.


The picture shows the result of one of the discussed BF245C's. Sorry, I wrote Vf, it should have been Vg.

The datasheet made one thing clear, the GDS connections do not correspond with those from either NXP, OnSemi and Fairchild.

And yes, most of these 5 types are old, from 1990/2000. I might have bought some from Segor Electronics in Germany about 10 years ago.

 

I do not expect my tester to be too accurate

At least the qualitative results showed you that the eBay parts aren't equivalent to your authentic parts. 10 years ago is probably on the cusp of when counterfeiting on the likes of eBay really got bad.

 

The datasheet made one thing clear, the GDS connections do not correspond with those from either NXP, OnSemi and Fairchild.

Drain and source are interchangeable.

 

ON Semiconductor also makes one and appears to have similar specifications
https://www.nxp.com/docs/en/data-sheet/BF245A-B-C.pdf

An aside:
There are some things you just need a transistor for and don't care much exactly what kind of transistor you are actually using. Transistors from eBay, Aliexpress, etc. fall into this category. When I receive parts from sources like those I mark the storage package so I know it is suspect if I go to use it.

For cases in which I care a lot about the devices specifications I buy from the manufacturer or an established authorized distributor. It is often worth a few cents to know for sure what I am getting.