PN junctions and diodes

How is a photodiode generally used to detect light?

  1. AA large forward current is passed to make it emit light
  2. BReverse bias is applied, and carriers generated by light in the depletion layer are collected as current
  3. CAC voltage is applied to make it resonate
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Answer: B. Reverse bias is applied, and carriers generated by light in the depletion layer are collected as current

Reverse-biasing a photodiode widens the depletion layer; electrons and holes generated there by light are separated by the field, and a current (photocurrent) proportional to light intensity flows. Reverse bias also reduces junction capacitance, giving a faster response.

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More PN junctions and diodes questions

  1. How does an LED emit light?
  2. Which is a characteristic of a PIN diode, which has a lightly doped i-layer between the p and n layers?
  3. Which is a characteristic of an avalanche photodiode (APD)?
  4. 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)?
  5. What is the purpose of a diode (flyback diode) connected in parallel with an inductive load such as a relay coil?