I want to know what will be the effect on current if we change(increase or decrease) the area of the emitter, base, and collector. For example, if we double the emitter area what will be the effect on emitter current, base current, and collector current?

A standard method to manipulate the behaviour of bipolar is to include INTEGER scale factors on the schematic used by the simulator and in defining the silicon layout.

Doubling the emitter-area, such as laying down TWO devices adjacent (for thermal tracking), while operating at the same total current, reduces the current density and thus reduces the emitter-base voltage by 18 milliVolts. Also, the allowed peak current will have doubled, should you expect heating issues or beta-collapse at higher currents.

Bandgap voltage references often use 2x or 4x or even 100x. I've reverse-engineered some older ICs that used 100:1 collection of diodes, where the 100:1 allows

log2(100) * 18 milliVolts

as the working delta_Voltage for the bias-generator design.

If you see the internal structure of a transistor, you can see that the area of emitter base and collector are different. See the simplified schematic below. The actual structure is little more complex than this.

enter image description here

The region directly below the emitter region is called the internal transistor. The remaining region is the external transistor. Most of the carrier transport happens in the internal transistor and hence the current will be directly proportional to the emitter area.

The external part of the transistor mostly contributes to the parasitic (resistance and capacitance). So increasing the area of base and collector won't change the current much.

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