Electronics The Twelfth Lecture

Tenth weekهـ 1437/ 1/ 15

السلمي / سمر أ

Outline for today

Bipolar junction transistor

What happens inside transistor

Transistor parameters

How amplification occurs in the transistor

Wednesday from 2 to 3, the other time is not specified yet

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Time of Periodic Exams The Second periodic exam in / 2 / 1437 - 1110هـ , Please everyone attend in her

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Bipolar junction transistorWhat 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 W1 is small unlike the length of the depletion region between base & collector junction W2 is big. Also, we care about the thickness of base WB 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 WB is small, the diffusion of electrons from emitter to base complete its way to collector

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W1 W2

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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.

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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 IB creates from recombination some of electrons which inject with holes in base region (IB considers of most important current). In addition, there are small weak currents such as reverse leakage current Icp of hoes in base and collector junction .

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what happen inside transistor

Also, from the figure we should know

IEn Total electronic current emit from emitter

ICn Residual part of total electronic current emit from emitter and collect in collector

IEn - Icn Residual part of total electronic current emit from emitter and flow in base as

recombination current

IEp hole current at base current and it creates from holes inject in base to emitter

Icp hole current as reverse leakage current direction from collector to base

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Transistor parameters

Previously we mentioned that WB the thickness of base plays very important role

in efficiency transistor ( two possibilities)

1- WB ≈ Ln or Lp (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+ )

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Transistor parameters

2 - WB >>Ln or Lp (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 ICBO 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 WB 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 IC

almost 99% higher from base current IB

Thus, we can say when a small amount of

current enters of IB , it will create high

current of IE also of IC because most

electronic current go from emitter to

Collector. We can say IE ≈ IC

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