Saturation region of transistor

  1. 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)?
  2. jcsd
  3. mfb

    Staff: Mentor

    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.
    1 person likes this.
  4. 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. using sum of voltages rule, Vce=Vcb+Vbe.
    ie... 0.2=Vcb+0.7
    So, Vcb=-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?
  5. mfb

    Staff: Mentor

    "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.
    A transistor is more than two junctions. Otherwise we could just use two diodes.
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  6. 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 eletronic principles by malvino..
  7. mfb

    Staff: Mentor

    I have no idea about good books about transistors, sorry.
  8. Atleast for the physics behind them?
  9. dlgoff

    dlgoff 3,158
    Science Advisor
    Gold Member
    2014 Award

    1 person likes this.
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