Quantitate also varies with I E, V CB and temperature. Further is always less than unity but very close to it. Hence as defined by equation 10 comes out to be positive number. Where and are small changes in I C and I E. Quantity is also always positive and less than unity. This is referred to as the dc current gain of CB transistor and is denoted by. If I COaddition to current I pC, there is another current at J C namely reverse saturation current I CO (or I CBO). Since, the width of the base region is very small, almost all the holes injected into the base reach the collector junction and get collected by the p-type collector. The holes on reaching J C across it easily (since they travel down the potential barrier) and enter the p-type collector region. The hole current I pc on reaching the collector is, therefore, slightly less than I pE. The holes while diffusion through n-type base region, meet majority carrier electrons ad some of these holes recombine with electrons giving rise to a small base current. Thus,Īll these currents I E, I pE and I nE are positive in a pnp transistor. Total emitter current I E crossing J E is the sum of I pE and I nE. Where, D n is the diffusion constant for electrons. Similarly, I nE is proportional to the electron density n p at J E and is given by, Where D p is the diffusion constant for holes and A is the cross-sectional area. This current is proportional to the slope of the hole density p n at K E. These minority holes diffuse through the base constituting the hole diffusion current I pE. In the active region, the transistor acts as an amplifier. In the active state, the collector current is β times the base current, i.e.Forward bias at J E results in injection of holes across J E into the base region. In the active region, emitter to the base junction is forward biased and the base to collector junction is reverse biased. In the saturation region, the B-E and B-C junction are forward biased and IC=IE ACTIVE REGION: TRANSISTOR AS AN AMPLIFIER: In cut off region the output of the transistor VCE, IC, IB and IE=0 SATURATION REGION: In the saturation region, both the junctions are in forwarding bias,and the transistor acts as a closed switch. In cut off region, both emitter to base and base to collector junction is in the reverse bias and no current flows through the transistor. Based on biasing, the transistor can be operated in cut off, active and saturation region of the transfer characteristics of the transistor.In this post, we will discuss operation of BJT in Active, Saturation and Cutoff Region TRANSISTOR AS A SWITCH : CUT OFF REGION: The transistor can be used as a switch or as an amplifier by forward/reverse biasing the emitter to base and base to collector junctions.
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