I've searched many references, but now I'm stuck. What I understand so far is that a transistor is made up from two diodes, and as far as I know, the function of diode is only allow a current flow from one direction (ideal diode).

I've looked at this diagram of transistors:

enter image description here

For PNP transistor: If I trigger the Base with some small current, Ib, how can the current from the Collector flow to the Emitter, if there's a diode there (in the Emitter)?

Same question with NPN transistor.

I'm stuck here....

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    \$\begingroup\$ The two-diode model of a transistor simply doesn't work. Throw that book away. \$\endgroup\$ – Chu Feb 11 '17 at 13:32
  • \$\begingroup\$ So, what's the diagram of the transistor? i've search google and have that \$\endgroup\$ – Anton Io Feb 11 '17 at 13:38
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    \$\begingroup\$ Yeah, you should stop thinking of them as two diodes as that will probably just confuse you. They say this because transistors are physically constructed with a P-N-P or N-P-N configuration, but they are all together in one piece of silicon. You can't model that with two discrete diodes. Firstly you would have P-N-N-P (or N-P-P-N), but also there is a lead between each. An actual BJT has more complex physics going on in the base layer and between the junctions. It's hard to give a short explanation of 'basic' BJT operation. \$\endgroup\$ – AngeloQ Feb 11 '17 at 13:40
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    \$\begingroup\$ There are three main modes (cut-off, active, saturation) and there are various properties for each. I think the Wiki article does a decent job explaining it, and there are lot's of other good resources online - just stop thinking of it as two diodes from a functional perspective. en.wikipedia.org/wiki/Bipolar_junction_transistor \$\endgroup\$ – AngeloQ Feb 11 '17 at 13:41
  • \$\begingroup\$ The diagrams of the transistors are as you showed in your question (the top ones, not the diode representations). \$\endgroup\$ – AngeloQ Feb 11 '17 at 13:42

The 'two diode' model of a transistor is OK if you want to identify which is the base lead with a multimeter, but it leaves out too much behaviour to be useful beyond that.

Although the transistor has two junctions, it is the way the junctions interact through their thin common layer that makes a transistor amplify current like it does. In the two diode model, the interaction between the junctions is completely ignored.

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