# Missing BJT parameter, $I_b$ max

After 40+ years you would think I would know this, somehow it never came up, but for the life of me I don't see it.

Typical transistors specs list a plethora of values but one that always seems to be missing is $I_{b_{MAX}}$. That is, the maximum current you can pass through the base emitter junction.

Obviously you can't exceed the power rating across the base-emitter diode forward drop, so 626mW/0.7V ~ 900mA, but I have a feeling there is a fusing number for the bonding wire. Perhaps it is the same as the max collector current.. 200mA.

What number is the right one, if any?

• I have seen it listed but I’m racking me brain to think on what. It was to do with a question on this site so maybe bc547, 2n3904 or maybe 2n2222..... – Andy aka Feb 1 '18 at 19:32
• It would be related to secondary breakdown due to current crowding, but that's normally spec in the SOA plot for IC-VC. It would be less than IC max – sstobbe Feb 1 '18 at 19:56
• – vofa Feb 1 '18 at 19:57
• Found one on an ST BD139 datasheet: 0.5A. – Oldfart Feb 1 '18 at 19:59
• Generally it is shown in power transistors' (BD139, BD243, 2N3055 etc) datasheets. But I cannot say the same thing for general purpose ones. – Rohat Kılıç Feb 1 '18 at 20:13