(see Fig. 2-4)
VBB forward biases the emitter diode, forcing the free electrons in the emitter to enter the base. The thin and lightly doped base gives almost all these electrons enough time to diffuse into the collector. These electrons flow through the collector, through RC, and into the positive terminal of the VCC voltage source. In most transistors, more than 95% of the emitter electrons flow to the collector, less than 5% flow out the external base lead.
Recall Kirchhoff's current law:
=> IE = IC + IB
Because IB is very small, for circuit analysis, we can do the following approximation:
IC is equal to IE
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