I was under the impression that any reverse current, even small, through the BE junction could cause permanent gain loss, with the gain loss increasing with the total charge moved across the junction.
Hmm. I might have to just test this. Gentle reverse breakdown in my intended application will be quite extended, not just one event. Perhaps I'll characterize a bunch of transistors, stick a couple mA through them in reverse for a long time and re-characterize them at intervals...
Hi
On EB reverse junction breakdown I did a few quick measurments some years ago to find effects of different reverse currents and found the beta reduction even with only small reverse currents of 1 mA or so could easily reduce the beta of Very low power BJT's by upto ~80%. The maximum beta reduction I could achieve seemed to be related(exponentially) to EB junction size. i.e the larger the transistor is the smaller was the max beta reduction I could achieve.
Going from lowish power devices like a bc557 I easily got gain reduction of 50-60%
whilst for larger power to3 device I could not reduce gain by more than ~10% no matter how much reverse current I put through eb.
The gain loss need not actually be permanent you can recover the junction by passing a large current
of 1-2 Amps for a few tenths of a second forward through the EB junction ( is very easy to overheat junction when attempting this ).
I think the beta reduction you sometimes see in bjt's that are old and have been sitting unused for years maybe the same physical proccess and if you have ever fixed much old equipment thats being sitting idle for many years you might have already came across faults related to loss of beta due to age/idleness in small signal bjt's. If you do and you can't find a suitable new replacement bjt you could always try a 'rejuvination' like I did below.
Heres a small selection of the rough measurments I did I found in my old notebook.:-
I found a box of 2sc2320 (npn 50v 200mA) being sitting in a parts bin since about 1990 the beta on most of these was ~ 190 . I passed a 1mA reverse current (breakdown voltage was 8.6v) through the EB junction for 1 minute
Hfe was reduced to 116 . I tried to see how low it would go so then did 10mA reverse current for 1 minute which reduced gain to ~ 100 I could not reduce gain further with higher currents. I repeated this on a few more of these and got a sim response. I then tried to 'restore' the gain by passing 1.25 A for ~ 1 second which then revovered the gain to ~ 170 . I tried to see how high I could get it so a second attempt using 2 Amps for ~ 500mS the 2sc2320 gave a gain of ~ 240 (so 30 % higher than the original 190 ).This was the max forward eb current I could go before the eb junction started to burn out.
I repeated the above on various bjt's and they where typical figures for similar size
devices so I wont post them all here .On smaller bjt's the larger loss/then boost gain %'s where seen . larger bjt's and saw little beta loss and/or regain where seen .
I also tried power Darlington's heres a small bit from my notes:-
Darlington bd646 (pnp 8A) initial reverse breakdown was ~9V and it had intitial
Ice = 2.13A @ Ibe of .5mA (an Hfe ~ 4050) ,Ice=.95A @ Ibe .25mA (Hfe ~3950) both well above spec given in data sheet.
Found difficult to reduce the Hfe and put upto < 400mA reverse and hardly reduced beta by more than 5% .
Then 500mA for 10 seconds reverse Ibe was applied beta was reduced to 3600 @ .5mA Ibe (1.8A Ice)
and 2000 @ .25mA Ibe (Ice .54A) .These are both still above the minimun spec for this device in the data sheets.
Then tried 750mA reverse for 10 second (it got hot) this hardly reduced the higher Ice current Beta figure but @ lower Ice current (Ibe=.25mA Ice =.15A). the gain fell to 600 .
Signifying only some beta reduction of the smaller first stage bjt of the darlington .
Regards