Electrical Engineering ⇒ Topic : Transistor Action

TRANSISTOR ACTION

Figure (a) shows the N-P-N transistor whose emitter junction is forward biased by the dc source V_EE and hence the depletion region at this junction is reduced. The collector base junction is reverse biased. Thus the depletion region at this junction is increased

               Figure (a) Forward biased EB junction and reverse biased CB junction.

The electrons in the N-type emitter flow towards the base due to the forward biased EB junction constituting the emitter current IE. These electrons flow through the P-type base and they combine with the holes in P-region. Since the base is very thin and lightly doped, very few of them combine with the holes and hence the base current (I_B) is very small. The remaining large number of electrons cross the depletion region and move through the collector region to the positive terminal of the external dc source constituting the collector current (I_C). Therefore, the flow of electrons constitutes the dominant current in an N-P-N transistor. The collector current is larger than the base current because very few of the electrons combine with the holes.The emitter current is equal to the sum of collector current and base current.

Therefore,

                                              I_E = I_C + I_{B            .................}.(1)

The base current is a small fraction, 2% or less, of the emitter current. Hence, the emitter current is very nearly equal to the collector current.

The directions of conventional currents in an N-P-N and P-N-P transistors are shown in Figures 2 (a) and 2 (b) respectively

              Figure (2) Directions of conventional currents in the N-P-N and P-N-P transistors.