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How to get junction resistance

Discussion in 'Electronic Design' started by Paul, Nov 19, 2008.

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  1. Paul

    Paul Guest

    Does anyone know how to calculate the junction resistance for a n-Si
    Schottky diode from the fundamental properties. I know the effective
    depletion width, the junction area, the doped resistance, and of
    course the intrinsic resistance of silicon. Obviously I cannot use the
    doped resistance because the depletion region is just that, depleted.
    Yet it doesn't seem correct to use the intrinsic resistance because
    it's doped silicon, no? Any ideas?

    I appreciate any help,
  2. neon


    Oct 21, 2006
    Try boltzman constant for a change.
  3. Paul

    Paul Guest

    Given the above info, I guess it's impossible to calculate Rj. I
    calculated the bandgap. Now is it possible to calculate Rj?

  4. First, you need to define the junction resistance. A Schottky diode,
    like all rectifying diodes, has an extremely nonlinear I(V)
    characteristic. If what you are looking for is the small-signal
    equivalent resistance, it is the inverse of the small-signal
    conductance dI/dV. The full I(V) curve contains all the circuit-
    relevant information about the diode at dc. If you are looking for
    an equivalent-circuit model element that has an effect at higher
    frequencies, then its definition depends entirely on the (never
    unique) equivalent-circuit model.

    - Bill Frensley
  5. Paul

    Paul Guest

    That requires current and voltage measurements to obtain the
    resistance. I know about the diode modeling equations. I'm talking
    about calculating the resistance without I & V measurements. Since
    most of the resistance is in the depletion region, I would settle for
    Rd. At zero bias, it's often written as Ro. The depletion region is
    void of mobile carriers, even at zero bias. I'm uncertain if the
    semiconductors effective resistance while void of mobile carriers is
    equal to the intrinsic resistance. If not, then how is it calculate?

  6. For V=0, the quantity you're looking for is 0.258 V / Is. Calculating
    Is is the problem. The conventional model is Richardson's thermionic
    emission theory, which for room temperature gives 1.1E7 A/cm2 (times)
    exp(-Schottky barrier height/0.258 eV) (times) (diode area). To get
    one significant figure of accuracy, you need to know the Schottky barrier
    height to about 0.01 eV. You probably won't do much better than just
    guessing that Is is 1E-10 A.

    By the way, this problem has ->nothing<- to do with Ohmic conduction.

    - Bill Frensley
  7. Paul

    Paul Guest

    Thanks a whole bunch Bill! Your post on the Richardson's thermionic
    emission theory led to all of the exact mathematical equations for
    schottky diodes -->

    Again, thanks so much!
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