How Does Charge Injection Affect NPN Transistor Operation?

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SUMMARY

The discussion focuses on the charge injection in NPN transistors, specifically addressing the behavior of charge carriers in the base-emitter and base-collector junctions. It is established that while the base-emitter junction is forward biased, allowing charge carriers to enter the base, the thin nature of the base allows most carriers to flow to the collector instead of contributing to base current. The collector-base junction exhibits a resistance of approximately 100 kΩ, which influences the flow of charges, but once charges are injected into the depletion region, they are attracted towards the collector due to the strong potential gradient, effectively overcoming the resistance.

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  • Understanding of NPN transistor operation
  • Familiarity with semiconductor physics
  • Knowledge of junction biasing (forward and reverse)
  • Concept of charge carrier dynamics in semiconductor materials
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  • Learn about the role of potential gradients in semiconductor devices
  • Explore the characteristics of the depletion region in NPN transistors
  • Investigate the impact of base width on transistor performance
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Electrical engineers, semiconductor physicists, and students studying transistor operation and charge dynamics in electronic circuits will benefit from this discussion.

Pushoam
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In a transistor, the base-emitter junction is forward biased while the base-collector junction is reverse biased.
The emitter emits charge carriers.These charge carriers go to the base.
My question is: why don't all of these charges flow as base current since this junction provides low resistance path?
In the book, it is said that since the base is thin, most of the charge carriers( which have come to the base) go to the collector.
Then, my question is why doesn't collector remain negatively charged as the collector junction is highly resistive? Why do these charges flow as a collector current?
 
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Once charges are in the narrow base region, they can be attracted towards the battery positive suppying the base, or to the greater battery positive supplying the collector.

I wouldn't say the collector is of highly resistive material. Maybe some tens of ohms.
 
NascentOxygen said:
Maybe some tens of ohms.
In my book, it is said that the collector-base junction has a resistance of the order of 100 k##\Omega##.
 
Pushoam said:
In my book, it is said that the collector-base junction has a resistance of the order of 100 k##\Omega##.
Ah, that's the resistance (a measure of the difficulty of getting through the reverse-biased region) seen by charges originating from the external collector-base circuit, they see a high resistance. But if you arrange some other means to inject charge into the depletion region, then those introduced charges once they're in the depletion region get swept along by the strong potential gradient and to them it's an easy route out (i.e., they see low resistance).
 

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