Operation on NPN and PNP Transistor

OPERATION ON NPN TRANSISTOR AND PNP TRANSISTOR

NPN Transistor Operation

As shown in Fig.1.35, the forward bias applied to the emitter base junction of an NPN transistor causes a lot of electrons from the emitter region to crossover to the base region.

As the base is lightly doped with P-type material the number of holes in the base region is very small and hence the number of electrons that combine with holes in the p-type base region is also very small. Hence few electrons combine with holes to constitute a base current I_B. The remaining electrons (more than 95%) crossover into the collector region to constitute collector current I_C. Thus the base and collector current summed up gives the emitter current i.e., I_E = - (I_C + I_B).

In the external circuit of the NPN bipolar junction transistor, the magnitudes of the emitter current IE, the base current I, and the collector current Ic are related by I_E = (I_C + I_B).

PNP Transistor Operation

As shown in Fig.1.36, the forward bias applied to the emitter base junction of PNP transistor causes a lot of holes from the emitter region to crossover to the base region as the base is lightly doped with N type material. The number of electrons in the base region is very small and hence the number of holes combined with electrons in the N-type base region is also very small. Hence a few holes combined with electrons to constitute a base current Ig. The remaining holes (more than 95%) cross over into the collector region to constitute a collector current I_C. I_E = - (I_C + I_B).

In the external circuit of the PNP bipolar junction transistor the magnitude of the emitter current IE, the base current I_B and the collector current I_C are related by

I_E = I_C + I_B ................(1)

This equation gives the fundamental relationship between the currents in a bipolar transistor circuit. Also this fundamental equation shows that there are current amplification factors α and β in common base transistor configuration and common emitter transistor configuration respectively for the static (dc) currents and for small changes in the currents.

Large-Signal Current Gain (α)

The large signal current gain of a common base transistor is defined as the ratio of the negative of the collector-current increment to the emitter-current change from cutoff (I_E = 0) to I_E i.e.,

Where I_CBO (or I_CO) is the reverse saturation current flowing through the reverse biased collector-base junction, i.e., the collector to base leakage current with emitter open. As the magnitude of I_CBO is negligible when compared to I_E, the above expression can be written as

Since I_C and I_E are flowing in opposite directions, a is always positive. Typical value of α ranges from 0.90 to 0.995. Also a is not a constant but varies with I_E, V_CB, and temperature.