PN junctions and diodes

Learn about the depletion layer of PN junctions, forward and reverse bias, breakdown and diode applications through multiple-choice questions.

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PN junctions and diodes: key points

When p-type and n-type semiconductors are joined, carriers diffuse and recombine near the boundary, forming a depletion layer with almost no carriers. A built-in potential develops across the depletion layer, and this is what allows current to flow in only one direction (rectification).

Under forward bias the barrier is lowered and current flows; under reverse bias the depletion layer widens and almost no current flows. If the reverse voltage is increased further, breakdown occurs, a phenomenon used in voltage-regulator (Zener) diodes.

All PN junctions and diodes questions (50)

  1. How is voltage applied to forward-bias a PN junction diode?
  2. What happens to the width of the depletion layer when the reverse bias applied to a PN junction is increased?
  3. Which phenomenon does a voltage-regulator (Zener) diode mainly use?
  4. Which correctly describes the relationship between forward current and forward voltage of a PN junction diode?
  5. What is the forward voltage (the approximate voltage at which current starts to flow) of a typical silicon PN junction diode?
  6. How does the reverse saturation current of a PN junction diode depend on temperature?
  7. Which is a characteristic of a Schottky barrier diode?
  8. Which property does a variable-capacitance (varactor) diode use?
  9. In a PN junction where the p side is far more heavily doped than the n side (a p+n junction), how does the depletion layer spread?
  10. Why does a reverse current flow briefly (a reverse recovery time exists) when a PN junction diode is switched from forward to reverse?
  11. How does the built-in (diffusion) potential of a PN junction relate to the doping on each side?
  12. Which correctly describes the mechanism of avalanche breakdown?
  13. In a very heavily doped PN junction, what is the main mechanism of breakdown at low reverse voltage?
  14. What does a bridge rectifier made of four diodes do?
  15. How is a photodiode generally used to detect light?
  16. How does an LED emit light?
  17. Which is a characteristic of a PIN diode, which has a lightly doped i-layer between the p and n layers?
  18. Which is a characteristic of an avalanche photodiode (APD)?
  19. In the diode equation I = Is {exp(qV / nkT) − 1}, what current flows when a large reverse voltage is applied (within the range where no breakdown occurs)?
  20. What is the purpose of a diode (flyback diode) connected in parallel with an inductive load such as a relay coil?
  21. In a forward-biased PN junction, which capacitance arises from the storage of injected minority carriers?
  22. How does the depletion width of a one-sided abrupt junction depend on the reverse bias voltage V (when V is much larger than the built-in potential)?
  23. Why are termination structures such as guard rings placed at the edge (periphery) of a high-voltage PN junction?
  24. In a simple voltage regulator using a Zener diode, what is the role of the resistor placed in series with the diode?
  25. Which diode, made from a PN junction very heavily doped on both sides, shows a negative-resistance region where current decreases as forward voltage increases?
  26. What is the typical open-circuit voltage of a single crystalline silicon solar cell?
  27. Which value is closest to the theoretical maximum efficiency of a single-junction solar cell (the Shockley-Queisser limit)?
  28. How does the avalanche breakdown voltage generally change as temperature rises?
  29. What is the phenomenon in which forward current through a PIN diode injects many carriers into the i-layer and greatly reduces its resistance?
  30. What can be evaluated by measuring the capacitance of a reverse-biased PN junction while varying the voltage (C-V measurement)?
  31. Of the two terminals of a PN junction diode, what is the terminal on the p-type side called?
  32. What is the property of a diode that lets current flow easily in one direction but hardly at all in the other called?
  33. Which statement correctly describes the depletion layer of a PN junction in thermal equilibrium?
  34. In which direction is a voltage regulator (Zener) diode normally used in a voltage regulator circuit?
  35. When a sinusoidal AC signal is applied to a half-wave rectifier made of one diode and a resistor, what is the output?
  36. When an LED is connected to a power supply to light it, what is the main purpose of a resistor in series with the LED?
  37. What is the main purpose of placing a capacitor in parallel with the output of a rectifier?
  38. How does the forward voltage of a silicon diode carrying a constant current change as the temperature rises?
  39. 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?
  40. 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?
  41. Why does no current flow externally in a PN junction in thermal equilibrium?
  42. Which is the correct definition of the fill factor (FF) of a solar cell?
  43. 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)
  44. How does the breakdown voltage of a PN junction generally change when the impurity concentration on one side is lowered?
  45. What is the main role of ESD protection diodes connected to the input/output pins of an IC?
  46. Zener diodes with a Zener voltage of about 5 to 6 V show little change in voltage with temperature. Why?
  47. What is a common way to make a metal-semiconductor contact ohmic (resistive)?
  48. In the diode current expression I ∝ exp(qV / nkT), when is the ideality factor n close to 2?
  49. When a reverse bias is applied to an abrupt PN junction, where is the electric field strongest?
  50. Which correctly describes the depletion approximation used in analyzing PN junctions?

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