Reliability of NPN (3904) with increased base current. 1

[sogaard]

Can anyone provide any information on how base-emitter current affects a NPN transistors long term reliability.

With a 3904 type transistor will the long term reliability suffer if the transistor is driven into saturation with a 2-4mA Ibe current.

 

[MacGyverS2000]

Under what conditions, and how do you intend to force a particular Ibe current? Most TO-92 packages are only good for a couple hundred mA, and betas are typically north of 250-300. That gives an Ibe of <1mA. Try to shove much more in there and you'll be going way beyond package specs.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[IRstuff]

Assuming that you're just overdriving the base, which is bad for circuit performance reasons, and that there's no increase in collector current, the incremental increase in junction temperature is quite miniscule, on the order of 0.03ºC, which is very small, so a tiny change in reliability from a thermal acceleration perspective.

However, doubling the base current could increase the electromigration failure of the base contact.

TTFN

FAQ731-376

 

[sogaard]

IRstuff,

Exactly the kind of information I am looking for. The base current is comming from a contact where I want to make sure that sufficient current is flowing to keep the contacts clean, the collector is only driving a weak pullup.

Do you have any links for more information about the electromigration failures?

 

[itsmoked]

macgyvers2000; A 2N3904 is only guaranteed to have a beta of 100. If you design one as a switch and don't plan on a beta of only 50 you're asking for trouble.


sogaard; You are considering one of the most basic and robust parts ever conceived, why are you considering its longevity? Probably anything else in your circuit is a better candidate for failure.

5mA is the base current specified in the data sheet. They place no limit on I_B. They have package power limits.

If you remain below 5mA I would expect no early failure. Most failure will be a gradual decline of the transistor's gain. Having a higher then needed (initially) I_B may actually keep the product running longer.

As with all semiconductors heat is the issue. Hotter => shorter life.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[sogaard]

itsmoked,

you are right, maybe I should just stop whining...

The only reason I started to focus on this was that I remembered someone telling that having a high bias current was bad for reliability. I was never able to find anything backing this statement up and just wanted to make sure.

 

[MacGyverS2000]

Keith,

Looking at a Fairchild sheet right now... Hfe is a min of 100 at 10mA of Ic, max of 300. Even at 1mA it's still a min of 70.

The 2N2222 that I use every day, on the other hand, is a little more dodgy spec-wise... you have to get up to 150mA Ic before you get a min of 100 Hfe. In practice, however, I'm seeing closer to 200 @ 25mA, and that has been fairly stable across countless reels.

I guess my "north of" figures should have been 150-200, not 250-300.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[IRstuff]

"High" base current would be where Ic/Ib is on the order of 10 or less, with the spec Icsat. When I was in school, back in the stone age, our textbooks never used betas over about 20 for design basis.

I would agree with the others that the level of base current you're discussin is quite low-risk, compared to other parts of your circuitry.

TTFN

FAQ731-376

 

[VEBill]

"...The base current is coming from a contact where I want to make sure that sufficient current is flowing to keep the contacts clean..."

Can't you just add a resistor to ground (or similar) to increase the contact current without increasing the base current?

 

[itsmoked]

Oh, contacts eh?

If you are running 5V and not gold contacts you can run all the current you want, eventually the contacts will fail to run your transistor. (Shesh, talk about a failure mode!) (And you were worried about the transistor failing?!)

If you are running 12V or more you should not have a problem.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[DHambley]

2-4 mA will not cause any signifigant failure in the base. If you need current to keep wetting the contact when it switches then VE1BILL's idea to add resistance may be the thing to do.