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Every TO-92 BJT I've ever seen reads collector-base-emitter when read left-to-right facing the flat side. Then I found this lovely part, BC517. It reads emitter-base-collector.

Have I only been exposed to a strangely homogeneous subset of TO-92 transistors, or is this one really an exception? If it's really a weird transistor, what possible technical reason is there for defying what I understood to be a universal standard?

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  • \$\begingroup\$ You have E-B-C twice in your write up. \$\endgroup\$ May 24 '16 at 18:58
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    \$\begingroup\$ I think I've seen every possible configuration at least once. E-B-C and B-C-E are the ones which seem most common. \$\endgroup\$
    – brhans
    May 24 '16 at 19:01
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    \$\begingroup\$ See Russell McMahon's comments following this answer \$\endgroup\$
    – Tut
    May 24 '16 at 20:05
  • \$\begingroup\$ Countries that can make their own transistors go by their manufacturing process, including if it is for VHF or UHF frequencies. It is not a 'devil may care' attitude, but design needs that determine the pin layout. There is no world-wide standard. \$\endgroup\$
    – user105652
    May 24 '16 at 20:12
  • \$\begingroup\$ TO-92 pinouts are indicated by a suffix A,B,C ... up to at least L (not all of them bipolar transistors) \$\endgroup\$ May 24 '16 at 20:36
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There are historical reasons for the E-B-C configuration. Early junction transistors had Emitter and Collector pellets diffused onto either side of a Germanium Base, so the wires naturally came out of the case in that order.

Silicon planar transistors have the Collector connected to the substrate, with Base and Emitter connections on the top side. In a TO92 case it makes more sense to have the Collector in the middle with Base and Emitter leads either side of the substrate.

American and European manufacturers carried over the E-B-C order from diffused to planar, but the Japanese changed to B-C-E. That is why many 'BC' and '2N' series TO-92 transistors are E-B-C, but '2S' series are B-C-E.

enter image description here enter image description here

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You need look no further than transistor checkers on DMM's and in stand alone equipment.They have support for various pin-outs. The various pin-outs arise from how the transistors process flow works, if the base wafer was N or P type and the shape of the lead frame.

enter image description here

picture grabbed from engineersgarage.com

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  • \$\begingroup\$ This doesn't really explain why some transistors are C-B-E rather than E-B-C. \$\endgroup\$ May 24 '16 at 19:12
  • \$\begingroup\$ @IgnacioVazquez-Abrams if it was just that simple then you could insert the transistors backwards. The common configurations are EBC and BCE amongst others. Given the diversity of processes available it is sufficient to say that the differences arise from the process flow differences. \$\endgroup\$ May 24 '16 at 19:28

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