I am reading Electronics and Devices by Boylestad and it says:
$$
\begin{align}
I_E &= I_C + I_B &(1)\\
I_C &= \alpha I_E + I_{CO} &(2) \\
I_E &= \alpha I_E + I_{CO} + I_B &(3)
\end{align}
$$
Is equation 3 valid for all regions of operation?
because if equation 3 is valid near cutoff region, if \$I_B\$ is decreased to zero,
either \$\alpha\$ has to increase or \$I_{CO}\$ should increase.
Also is there any relationship between \$I_B\$, \$\alpha\$ and \$I_{CO}\$?
How to connect the concept that, if there is no base current, no collector current flows in the transistor?
Best Answer
Equation 3 is totally derivable from eq2 and eq1 so the question really is should equation 2 be valid in all regions of operation.
In detail, equation 2 is saying collector current comprises the (large normally) fraction of emitter current that flows through the collector + leakage current through the collector-base reversed junction.
And, as far as I remember this is valid under in all regions of operation that I can think of.
This page might be useful.