diodes

Discussion in 'General Electronics Chat' started by seyibucho, Apr 14, 2007.

  1. seyibucho

    Thread Starter New Member

    Apr 14, 2007
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    i have been having doubts about forward biased diodes allowing small current flow until the voltage passes 0.7volts and after larger currents flow. could anybody explain why a forward biased diode behaves in this way?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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  3. Dave

    Retired Moderator

    Nov 17, 2003
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    The answer is essentially a semiconductor physics one. When you create the diode structure by placing the p-type and n-type doped materials together a process of diffusion occurs across the junction of the p- and n-type materials. As a result a region develops where no mobile charge is supported - this is the depletion region (a region depleted by mobile charge). As you apply a forward voltage across the diode, electrons are attracted toward the positive terminal, and 'holes' are attracted to the negative terminal, the net results is a reduction or narrowing of the depletion region. Eventually at 0.7V for silicon the depletion region disappears and the current capabilities of the diode increase in a manner consistent with the Eber's-Moll characteristic. Conversely, if you apply a reverse bias voltage the opposite happens and the depletion region increases in size - this explains why diodes display the small current flow in reverse-bias condition (of course until the onset of avalanche breakdown).

    Dave
     
  4. emad

    New Member

    Mar 26, 2007
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    hi
    there is no doubt for this fact....that the diode have no current across it
    untel it arrives to the point which 0.7v overcome to the internal voltage
    barrier.
    so if you want to confirm my idea look to the slope charactristic of diode in any book you essentialy see there is no current until the voltage value be come 0.7 after that large current pass through the biode
    for this reason we put a resistor in sieres with the diode.
     
  5. Dave

    Retired Moderator

    Nov 17, 2003
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    You will find that up to 0.7V there is a small current (pico-nano amps) due to a tiny amount of mobile charge present in the depletion region due to thermal effects. Take the junction down (or at least close to) absolute zero and you mitigate this factor. As it happens this small current has very little impact on the macro-characteristics of the diode.

    Dave
     
  6. emad

    New Member

    Mar 26, 2007
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    i will ask you Mr Dave
    is there in our live absulate zero
    for example i want to use diode in any analog circuit
    for sure i will not to care about ( pico-nano amps) so the one thing that
    i must consider ( this diode will be in cut-off ) untel the potential applied
    become more than 0.7v
    thanks
    emad
     
  7. Dave

    Retired Moderator

    Nov 17, 2003
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    For all intents purposes you can consider a silicon diode is cut-off up to around 0.7V (0.3V for a germanium diode). The point I was making is that on the low-level physics there is still a tiny and insignificant current due to thermal effects promoting electrons into the conduction band. In fact at absolute zero, there are issues with the energy band allocation due to the fact you can only have electrons of certain spins in an energy band at once - its a long time since I looked at this so I can't accurately remember the details, but it presents another quirk of quantum theory.

    Dave
     
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