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This question already has an answer here:

I have a doubt related to back to back connection of PN diode....

Does back to back connection of PN diode works exactly as transistor?

It looks like a small doubt... But I want detailed information about it. So, can anybody explain about it in detailed manner?

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marked as duplicate by nidhin, Ricardo, Scott Seidman, Dave Tweed Feb 20 '15 at 23:58

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  • \$\begingroup\$ The English of that article isn't very well written. It doesn't look like a good learning resource. \$\endgroup\$ – pjc50 Feb 10 '15 at 14:20
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    \$\begingroup\$ Depending on how deep you are studying BJTs just think of the carriers distribution: the profile in the base can be something meaningful only if the base lenght is comparable to the diffusion length, that's in the 10um ballpark. When you connect two diodes back to back the base is some mm long at least, so you have a transistor but with very, very, very poor performances. \$\endgroup\$ – Vladimir Cravero Feb 10 '15 at 14:26
  • \$\begingroup\$ I don't think it's fair you're being downvoted; you read an article, you were curious about a concept, and asked it clearly. English isn't your first language so how are you supposed to know that it is not a good resource? \$\endgroup\$ – Nick Williams Feb 10 '15 at 15:29
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No. A BJT shares one of the regions with both junctions, plus the regions in a BJT are asymmetrical in both size and doping strength. Both of these factors contribute to the fact that BJTs act as current amplification devices rather than just pairs of rectification devices.

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