11/13/2023 0 Comments Transistor biased in active region![]() Physical meaning of equation 11 is that the current in J c (acting as a pn diode) gets supplemented by a fraction of I E coming from the emitter.Įxample 1: In a PNP transistor in CB configuration, only 0.5% of the holes injected into the base through J E recombine with electrons in the base. If V C is negative and large in magnitude compared to V T, Equation 11 reduce to equation 6. Then from equation 6 we get the following generalized expression for I C. where V c is the voltage drop across J C from p-side to n-side and V T is the volt equivalent of temperature. In that case, we need replace I CO by the current in J C acting as a pn diode i.e. we now processed to obtain a generalized expression for I C which is valid not only for reverse bias J C but for any voltage across J C. Thus, for active region operation, I C is almost independent of the collector voltage but depends only on the emitter current I E. Generalized Expression for Collector CurrentĮquation 6 giving I C in terms of I CO, and I E is valid for operation in the active region. 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 COC and I E have opposite signs in both pnp and npn transistors. Term is called the large signal current gain of a common base transistor. Thus, from equation 6, may be defined as the ratio of collector current increment from cutoff value (I C = I CO) to the emitter current increment from cutoff (I E = 0). However, equation 6 permits us to define in an alternative manner. Total diffusion hole current across J C from the base into the collector is, In a pnp transistor, I e is positive while both I C and I CO are negative since the current in the collector lead actually flows in the direction opposite to that indicated by the arrow on I C in figure 1. Where is the fraction of the total current I E which constitutes I pC. Under the active region operation with J E forward biased, the collector current is given by, I PCO results exclusively from the holes generated thermally within the base while I nco results from the electrons generated thermally within the collector. In equation 4, minus sign has been added to I CO intentionally so that I C and I CO may have the same assigned direction of flow. This I CO is sum of two components: (i) current I nco caused by electrons diffused across J C from n-type collector to n-type base and (ii) current I PCO caused by holes diffusion across J C from n-type base to p-type collector. In addition 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. Forward bias at J E results in injection of holes across J E into the base region. ![]()
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