PN junctions and diodes

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)

  1. AAbout 260 Ω
  2. BAbout 26 Ω
  3. CAbout 2.6 Ω
Show answer and explanation

Answer: B. About 26 Ω

The dynamic resistance of a diode is the reciprocal of the slope of its current-voltage curve, rd = nkT/(qI). With n = 1 and I = 1 mA, rd = 26 mV ÷ 1 mA = 26 Ω. The larger the current, the smaller the dynamic resistance.

  • A. About 260 Ω is the value at a current of 0.1 mA.
  • B. Correct. 26 mV ÷ 1 mA = 26 Ω.
  • C. About 2.6 Ω is the value at a current of 10 mA.
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