Saturation region of transistor

In summary, a transistor is an electronic component that controls the flow of electricity between two points. It has two junctions, a base junction and an emitter junction. The base junction is forward biased, while the emitter junction is reverse biased. The knee voltage of the base-collector junction is less than the voltage of the base-emitter junction when both junctions are forward biased. This is because the base-collector junction has a smaller voltage than the base-emitter junction.
  • #1
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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  • #2
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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  • #3
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?
 
  • #4
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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  • #5
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..
 
  • #6
I have no idea about good books about transistors, sorry.
 
  • #7
Atleast for the physics behind them?
 

1. What is the saturation region of a transistor?

The saturation region of a transistor is a state in which the transistor is fully conducting and acts as a closed switch. In this state, the collector current remains constant regardless of changes in the base-emitter voltage.

2. How does the transistor enter the saturation region?

The transistor enters the saturation region when the base-emitter voltage is high enough to forward-bias the base-emitter junction. This causes a large number of minority carriers to flow from the base to the emitter, resulting in a high collector current.

3. What is the significance of the saturation region in transistor operation?

The saturation region is important because it allows the transistor to function as an amplifier. By controlling the base-emitter voltage, the collector current can be varied and amplified. Additionally, in digital circuits, the saturation region allows the transistor to act as a switch between the on and off states.

4. What is the difference between the active region and the saturation region?

The active region of a transistor is the state in which the transistor is partially conducting, and the collector current is directly proportional to the base-emitter voltage. In the saturation region, the collector current remains constant, regardless of changes in the base-emitter voltage. This is due to the presence of excess minority carriers in the base region.

5. How does the saturation region affect the gain of a transistor?

The saturation region can have a negative impact on the gain of a transistor. In this region, the collector current remains constant, even if the base current increases. This results in a decreased current gain, which can affect the overall performance of the transistor in amplifying signals.

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