Ideal Diode Characteristics

Discussion in 'General Electronics Chat' started by jrv9090, Feb 10, 2015.

  1. jrv9090

    Thread Starter New Member

    Nov 26, 2014
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    I am trying to understand the workings of a diode. On one side, I have the ideal diode equation which says, I = Isat * [exp(V/Vt) -1 ]. Say, the diode reverse saturation current to be approximated to 10^15 Amps, vt = 25 mV at room temp, I have plotted the V-I curve for the equation as below. (Another question is from where to obtain the values of reverse saturation current. I cannot find it in the diode datasheet say for 1n4001. Hence I have taken an approximation only).

    Now I look at the datasheet of diode 1N4001, and I find the V-I curves to be not similar especially after 1 volt. I am referring to the instantaneous voltage current curve in the datasheet(attached). This curve does look like an exponential curve till 1 volt, beyond that the curve is not linear.

    My question is, why are the graphs not similar? Or is there any fault with the values I have chosen or any fault in my comparison? View attachment 80248
     
    Last edited: Feb 10, 2015
  2. MrChips

    Moderator

    Oct 2, 2009
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    Do you know the meaning of 10^15 Amps?
     
  3. jrv9090

    Thread Starter New Member

    Nov 26, 2014
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    I understand its too low. But the values of my graph and the one on datasheet correspond somewhat with this value of Isat. And also, I have asked where to get the value of Isat from datasheet.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    It is given in the data sheet as IRM = 5 x 10^-6 A

    Note the negative sign.
     
  5. jrv9090

    Thread Starter New Member

    Nov 26, 2014
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    But correct me if I am wrong. I feel the value to be too high for Isat and its given to be the maximum reverse current as in maximum current in the breakdown region. If that's right, then still I don't have Isat value from the datasheet.
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    You do not find the Isat current directly in data sheet. But you can can easily derive it from a forward transfer characteristic.
    Also as you already notice the real diodes does not behave exactly the same as a ideal diodes. For real diodes Vt is larger than 26mV and real diodes also have some bulk resistance and this is why transfer characteristic are not matched.
    http://forum.allaboutcircuits.com/blog/fun-with-the-diode-equation.589/
     
    Last edited: Feb 10, 2015
  7. jrv9090

    Thread Starter New Member

    Nov 26, 2014
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    Thanks for the feedback. Is there any equation or theory combining both the ideal characteristics and bulk characteristics. If I understanding is right, the diode acts in the ideal range up to certain voltage and then behaves as a resistor. I am studying at a semiconductor device level from Millman Halkias. Though I have a somewhat understanding of the diode in its ideal region, I am unable to get any material on what happens in the device to voltages beyond the ideal behavior.
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I always thought that ideal diode is defined such that V=0 when I>0 and I=0 when V<0.
     
  9. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    No this is not like this, Shockley equation describe the voltage-current relation of the pn junction only, not the part we called diode.
    As for the model please read this
    http://www.allaboutcircuits.com/vol_3/chpt_3/14.html
    http://www.youspice.com/ys/diodefromdatasheet.3sp
     
  10. cabraham

    Member

    Oct 29, 2011
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    That is truly an ideal diode. Maybe clarification is needed. The Shockley equation is a good model for a real diode device. Your point is valid though. What you describe is a perfect diode.

    Claude
     
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