Transistor currents

(see Fig. 2-4)

V_BB 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 R_C, and into the positive terminal of the V_CC 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.

Fig. 2-4: NPN Transistor

Recall Kirchhoff's current law:

=> I_E = I_C + I_B

Fig. 2-5: Transistor currents

Because I_B is very small, for circuit analysis, we can do the following approximation:

I_C is equal to I_E