Regarding the collector current Ic equation in the below figure:

enter image description here

The above figure tells:

Ic = Is * (exp(Vbe/Ut)-1)

But I thought it should be:

Ic = α * Is * (exp(Vbe/Ut)-1) + Icbo

1-) Is α and Icbo neglected in the figure?

2-) Is the reverse saturation current Is the reverse saturation current between the base-emitter junction? Is Is that the same thing called Ieo in some texts?

  • \$\begingroup\$ I'm not familiar with \$I_{EO}\$, itself. Is it just another name for \$I_{EBO}\$? In any case, I recommend studying Level 1 BJT Model. Pretty much all the details are there for you. (I am assuming you mean 'area' when you write \$a\$. You will see that term in the above document.) I find plotting these things on a log chart helps a great deal in understanding the factors, though. For example, \$I_S\$ is just what you get at the y-axis when you apply a ruler to the flat part in region II curve of the BJT operating mode. \$\endgroup\$ – jonk Feb 10 '18 at 18:18
  • \$\begingroup\$ No that is alpha \$\endgroup\$ – floppy380 Feb 10 '18 at 18:27
  • \$\begingroup\$ Okay. Then I recommend reading the document I gave you. All answers are in there and detailed well. Start with the section on DC operation. \$\endgroup\$ – jonk Feb 10 '18 at 18:45
  • \$\begingroup\$ In that document there is not a single term called "reverse saturation current" The terminology is horrible. \$\endgroup\$ – floppy380 Feb 10 '18 at 18:47
  • 2
    \$\begingroup\$ Sorry, then. I started to consider what I wanted to write about this, because what you wrote isn't consistent with the way I think of things. This means we'd be at cross-purposes until we'd established mutual foundations we understood and agreed to share. And then we could move forward. Then I just gave up. Too much work establishing terms. I apologize for my lack of energy. \$\endgroup\$ – jonk Feb 10 '18 at 19:04

As per Ebers–Moll Model of an NPN transistor:

$$I_E = I_{ES} (e^{V_{BE}/V_T} -1) -\alpha_RI_{CS}(e^{V_{BC}/V_T}-1)$$ $$I_C = \alpha_FI_{ES} (e^{V_{BE}/V_T} -1) -I_{CS}(e^{V_{BC}/V_T}-1)$$ where:

\$ I_{ES}\$ = reverse saturation current of Base-Emitter Junction.

\$ \alpha_R\$ = reverse current gain

\$ \alpha_F\$ = foward current gain

\$ I_{CS}\$ = reverse saturation current of Base-Collector Junction.

Approximated Ebers-Moll Models look like: $$I_E \approx I_{ES} (e^{V_{BE}/V_T} -1) $$ $$I_C \approx \alpha_FI_{ES} (e^{V_{BE}/V_T} -1) $$ which satisfies the normal convention: \$ I_C = \alpha_F I_E \$

Therefore, whatever source you took the figure from, By \$ I_S \$ they mean, \$ \alpha_F I_{ES} \$.

If you are looking for where is \$ I_{CBO} \$, it is there in the original equation of \$ I_C \$ , the term --> \$-I_{CS}(e^{V_{BC}/V_T}-1) \$ is nothing but equals to \$ + I_{CBO}\$.

From the original Ebers-Moll Model equations, you can see that even \$ I_C = \alpha_F I_E + I_{CBO} \$ is an approximation, as it neglects the second term in the expression for \$I_E\$.


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