Saturation region of transistor

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Discussion Overview

The discussion revolves around the operation of an npn transistor in saturation mode, specifically examining the voltage relationships between the collector, emitter, and base. Participants explore the implications of forward biasing both the base-emitter and base-collector junctions, and the resulting voltage conditions.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that in saturation mode, both the base-emitter and base-collector junctions are forward biased, questioning how the collector can be at a higher potential than the emitter with Vce at 0.2 V.
  • Another participant asserts that the base has the highest potential and the emitter the lowest, suggesting a simple inequality among the three voltages involved.
  • A further contribution discusses the ideal diode model, proposing that if Vbe is 0.7 V and Vce is 0.2 V, then Vcb must be -0.5 V, leading to the conclusion that the base is at a higher potential than the collector.
  • One participant questions the relationship between the knee voltage of the base-collector junction and Vce, seeking to understand the mechanisms at play when both junctions are forward biased.
  • Another participant emphasizes that stating both junctions are forward biased does not imply a direct causal relationship between their voltages.
  • Several participants request recommendations for books that provide detailed explanations of diodes and transistors, indicating a desire for further learning on the topic.

Areas of Agreement / Disagreement

Participants express differing views on the implications of voltage relationships in saturation mode, with no consensus reached on the mechanisms involved or the interpretation of the knee voltages.

Contextual Notes

Participants reference specific voltage values and conditions, but there are unresolved assumptions regarding the behavior of the transistor in saturation mode and the implications of forward biasing both junctions.

Who May Find This Useful

This discussion may be of interest to students and individuals seeking to understand the operational principles of transistors, particularly in the context of saturation mode and junction biasing.

Alpharup
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Let us consider an npn transistor. The emitter is grounded and the transistor is operating in CE mode.
Consider that the region of operation is saturation mode.
My teacher asks us to consider that Voltage between collector and emitter is 0.2 V, that is, Vce=0.2 V.
I have read a few posts in some websites which say that in saturation region, both base-emitter junction and base collector junction are forward biased. They also say that Vce varies from 0.1 volt to 0.5 V depending on the transistor(My teacher asks us to take 0.2 V, as I mentioned before).
My question is; How is it that the collector is at higher potential compared to emitter when both the base-collector junction and base-emitter junction are forward biased( say, Vce is 0.2 V)?
 
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Where is the problem? The highest potential is at the base, the lowest potential at the emitter.
This is a simple inequality with 3 different values.
 
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mfb said:
Where is the problem? The highest potential is at the base, the lowest potential at the emitter.
This is a simple inequality with 3 different values.

My problem is over here...
1..Let us consider that both the base-emitter junction and base-collector junction are forward biased. By ideal diode model, knee voltage of base-emitter junction, Vbe be 0.7 V. Let us take that when Vce is above 0.2 V, the transistor is in active region. So, at 0.2V, it is in so-called "saturation region ", where both junctions are foward biased.
2..by using sum of voltages rule, Vce=Vcb+Vbe.
ie... 0.2=Vcb+0.7
So, Vcb=-0.5V.
ie...Vbc=0.5V.
ie...base is at higher potential compared to collector or base-collector junction is forward biased..So, there is no contradiction..
Hence, Vbc<Vbe...
3...But here is my question put in another way:
Why is the knee voltage of base-collector junction(Vbc) less than Vce when both junctions are forward biased? What is the mechanism happening in both junctions?
 
Why is the knee voltage of base-collector junction(Vbc) less than Vce when both junctions are forward biased?
"Both junctions are forward biased" is not sufficient to have different voltages there. Both are true in this case, but there is no causal relationship between them.
What is the mechanism happening in both junctions?
A transistor is more than two junctions. Otherwise we could just use two diodes.
 
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We just got an intro into the working of diodes/ transistors in this semester. Could you please name some good books which give the detailed explanation of diode, transistor, JFET, MOSFET and similar devices working? We presntly use electronic principles by malvino..
 
I have no idea about good books about transistors, sorry.
 
Atleast for the physics behind them?
 

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