Electronics The Twelfth Lecture Tenth week 15/ 1/ 1437 هـ أ / سمر السلمي

Outline for today Bipolar junction transistor What happens inside transistor Transistor parameters How amplification occurs in the transistor

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 Bipolarjunction transistor  Bipolar junction transistor  What happens inside transistor At the beginning, we deal with Active mode and base common configuration to npn. We notice that in forward bias between base & emitter junction, the length of the depletion region W 1 is small unlike the length of the depletion region between base & collector junction W 2 is big. Also, we care about the thickness of base W B is small. At base & emitter junction (np), the diffusion of electrons from emitter to base and opposite for the diffusion of hole from base to emitter. If the thickness of base W B is small, the diffusion of electrons from emitter to base complete its way to collector = W1W1 W2W2 - +

 What happens inside transistor At the beginning,electrons inject in emitter then diffuse to base and part of electrons recombine with holes. In return, holes inject in base then diffuse to emitter which know base current. However, we must remember that the emitter and base junction n + p has more impurities in emitter therefore most junction current in forward bias will be from electrons (electronic current). As we mentioned earlier, if the thickness of base is small, electronic current will not be able to recombine with all majority carrier (holes) in base. =

 What happens inside transistor Thus, most electronic current will withdraw or diffused to base and collector junction pn (also, reverse voltage effects in diffusion process which lead to the fall of electrons in energy well). This is followed by the appearance of equivalent region inside base as we move away from two junctions region toward the center, if concentration of impurities’ injection regular, the base region will be free of electronic field and charge carrier will be driven by diffusion power. Also, base current I B creates from recombination some of electrons which inject with holes in base region (I B considers of most important current). In addition, there are small weak currents such as reverse leakage current I cp of hoes in base and collector junction. =

 what happen inside transistor Also, from the figure we should know I En Total electronic current emit from emitter I Cn Residual part of total electronic current emit from emitter and collect in collector I En - I cn Residual part of total electronic current emit from emitter and flow in base as recombination current I Ep hole current at base current and it creates from holes inject in base to emitter I cp hole current as reverse leakage current direction from collector to base =

 Transistor parameters Previously we mentioned that W B the thickness of base plays very important role in efficiency transistor ( two possibilities) 1- W B ≈ L n or L p (The diffusion length of electron and hole depending on the type of transistor npn or pnp, respectively ) is small, as we mentioned early, so a few amount of charge carrier recombine with other type of carrier. Thus, efficiency transistor increases, in addition to injection of emitter from a number of charge carrier to base so it requires(high doping of emitter as n + or p + ) =

Transistor parameters 2 - W B >>L n or L p (The diffusion length of electron and hole depending on the type of transistor npn or pnp, respectively ), the thickness of base is big. Therefore, any charge carrier which inject from emitter to base will recombine with other type of carrier in base before going to collector. Thus, the only current in base and collector junction is reverse leakage current or reverse saturation current I CBO and no passing of other current in base and collector junction and become two open circles

 How amplification occurs in the transistor Here also, we deal with npn type and active mode but we now use emitter common configuration. When hole current enters from base, potential reduce between emitter and base (forward bias). If W B the thickness of base is small, we will assume that % 1 of electronic current recombine with hole current in base and 99% of electronic current go toward collector Therefore, output current from collector I C almost 99% higher from base current I B Thus, we can say when a small amount of current enters of I B, it will create high current of I E also of I C because most electronic current go from emitter to Collector. We can say I E ≈ I C =