In a BJT, largest current flow occurs (A) in the emitter (B) in the collector (C) in the base (D) through CB junction

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Concept understanding — Bipolar Junction Transistor (BJT)

A bipolar junction transistor (BJT) is a semiconductor device with two junctions and three terminals -- emitter (E), base (B), and collector (C) -- in which current is carried by both electrons and holes, formed as two structural types: n-p-n (a thin p-type base sandwiched between two n-type layers) and p-n-p (a thin n-type base sandwiched between two p-type layers). Their circuit symbols are distinguished by the direction of the emitter's arrowhead, pointing outward (away from the base) for n-p-n and inward (towards the base) for p-n-p, which also indicates the direction of conventional current flow in the emitter. Geometrically, a transistor resembles two p-n junction diodes joined back to back at a shared base, but this 'two-diode' picture alone does NOT reproduce genuine transistor action, since real transistor operation depends critically on the base being a single, extremely thin, lightly-doped region shared between both junctions.

The three regions are deliberately doped and shaped asymmetrically: the emitter is thick and heavily doped, supplying a large number of majority carriers; the base is thin and lightly doped (relative to the emitter), so injected carriers mostly diffuse straight across with minimal recombination; and the collector is lightly doped (about ten times more lightly than the base) with a larger area, letting it efficiently collect the arriving carriers and dissipate heat. In the standard biasing scheme for normal (active-region) operation, the emitter-base junction is forward biased and the base-collector junction is reverse biased, allowing an easy flow of majority carriers from emitter, through the thin base, to the collector -- so that the collector current $I_C$ ends up close to (about 95% of) the emitter current $I_E$ , while the base current $I_B$ (the small remainder that recombines in or is supplied by the base) is only a few percent of $I_E$ , with $I_E=I_B+I_C$ always holding by current conservation.

In a BJT, the three currents are related by $I_E=I_B+I_C$ (Kirchhoff's current law applied at the base node). Since $I_C\approx0.95I_E$ and $I_B\approx0.05I_E$ for a typical transistor, the collector current is close to but always slightly LESS than the emitter current, and the base current is always the smallest of the three (only a few percent of $I_E$ ). Because $I_E$ is literally the SUM of the other two currents, it must be the largest of the three at every instant, regardless of the exact value of $\beta_{DC}$ or $\alpha_{DC}$ for the particular transistor. This is a direct structural consequence of current conservation at the base node, not something that depends on operating point or transistor type (n-p-n or p-n-p). [!ANSWER] (A) in the emitter

Q.In a BJT, largest current flow occurs (A) in the emitter (B) in the collector (C) in the base (D) through CB junction

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