PN junctions and diodes

For an ideal diode (ideality factor n = 1) at room temperature, roughly how much must the forward voltage increase to raise the forward current tenfold?

  1. AAbout 6 mV
  2. BAbout 60 mV
  3. CAbout 600 mV
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Answer: B. About 60 mV

The forward current is proportional to exp(qV/kT), so increasing the current tenfold requires increasing V by kT/q × ln 10 ≈ 26 mV × 2.3 ≈ 60 mV. This is the origin of the rule of thumb of about 60 mV per decade.

  • A. About 6 mV only raises the current by a factor of about 1.3.
  • B. Correct. kT/q × ln 10 ≈ 60 mV.
  • C. About 600 mV would raise the current by a factor of about 10¹⁰.
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More PN junctions and diodes questions

  1. Which expression gives the depletion (junction) capacitance C of a reverse-biased PN junction in terms of junction area A, depletion width W and semiconductor permittivity ε?
  2. Why does no current flow externally in a PN junction in thermal equilibrium?
  3. Which is the correct definition of the fill factor (FF) of a solar cell?
  4. Roughly what is the small-signal (dynamic) resistance of an ideal silicon diode carrying a forward current of 1 mA at room temperature? (kT/q ≈ 26 mV)
  5. How does the breakdown voltage of a PN junction generally change when the impurity concentration on one side is lowered?