I am just getting back into electronics after a long while. For the life of me I can't figure where the \$\alpha_F\$ variable in the Ebers–Moll model comes from when you are calculating \$I_C\$ or \$I_B\$ using the formulas given under the Ebers–Moll section on the BJT Wiki Page. My first instinct is that it is \$h_{fe}\$ , if I am to take the meaning of 'common base forward short-circuit current gain' literally, since \$h_{fe}\$ was the closest rating I could find in any datasheet.

So what I am asking is: Where do you get \$\alpha_F\$ when you are using the Ebers–Moll model of a BJT?


2 Answers 2


αF is not the Hfe of the transistor. Per the model, αF of the emitter current reaches the collector.

This means that αF of the diode current passing through the base-emitter junction contributes to the current flowing through the base-collector junction.

Typically, αF has a value of between 0.98 and 0.99. the forward beta (~hfe) is αF/(1-αF).

  • \$\begingroup\$ I did not think it was hfe! So what would I put into the equation then? \$\endgroup\$
    – Zack Frost
    Feb 15, 2017 at 20:48
  • \$\begingroup\$ Use hfe on the datasheet and the relation in the answer to estimate αF: aF = hfe/(1+hfe). \$\endgroup\$
    – John D
    Feb 15, 2017 at 20:52
  • \$\begingroup\$ Ok! That makes sense now. I missed that part at the end for whatever reason. \$\endgroup\$
    – Zack Frost
    Feb 15, 2017 at 20:56

It is defined as function of collector and emitter current. More specifically a ratio of the exponential functions which describe the current at the emitter and collector. You should find that the forward ratio is near unity.


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