Q.Why is the emitter, the base and the collector of a BJT doped differently?

[!TLDR] The emitter, base, and collector need different doping levels and areas so that most carriers the emitter supplies pass through the thin base with minimal recombination and are efficiently collected -- essential for transistor action (current gain). [!ANSWER] Emitter: heavily doped, to supply a large number of majority carriers. Base: thin and lightly doped, so most injected carriers pass through with minimal recombination (keeping $I_B$ small). …

The three regions of a BJT are deliberately doped and shaped differently, and this asymmetry is not incidental -- it is what makes genuine transistor (current-amplifying) action possible in the first place, as distinct from simply two back-to-back diodes (which would NOT amplify). The EMITTER is made a THICK, HEAVILY doped layer specifically to supply a LARGE number of majority carriers into the device -- a heavily doped emitter has plenty of majority carriers available to inject into the base when forward biased, which is essential since the emitter current $I_E$ is the largest current in the device and the ultimate source of everything the collector eventually collects. The BASE is made THIN and LIGHTLY doped (compared to the emitter) so that the carriers injected from the emitter have only a short distance to diffuse across, and few base majority carriers are available for recombination along the way -- both factors together ensure that the great majority (about 95%) of injected carriers survive to reach the collector rather than being lost to recombination in the base, keeping the base current $I_B$ small. The COLLECTOR is on the far side of the base, ALSO lightly doped (roughly ten times more lightly than the base) but with a LARGER AREA than either the emitter or base -- its light doping and larger area let it efficiently sweep in and collect the majority of carriers arriving from across the base (which spread out somewhat as they diffuse), and its larger area also helps it dissipate whatever small amount of heat is generated during collection, since the collector typically handles the most power in the circuit. If all three regions were doped and sized identically (essentially two symmetric back-to-back diodes), there would b …

Explain the specific doping/geometry role of each of the three regions (emitter: heavy doping for carrier supply; base: thin + light doping for low-loss transit; collector: light doping + larger area for efficient collection/heat dissip …

Describing only ONE region's doping (commonly just the base) without covering all three, or omitting WHY the differing doping matters functionally (i.e. that it is precisely this asymmetry -- not just 'diffe …