The GATE Grind

GATE 2020 EC – Question 17

Electronic Devices · P-N Junction and Semiconductor Devices · 1 mark · Multiple choice

Consider the recombination process via bulk traps in a forward biased $pn$ homojunction diode. The maximum recombination rate is $U_{max}$. If the electron and the hole capture cross-sections are equal, which one of the following is FALSE?

  1. With all other parameters unchanged, $U_{max}$ decreases if the intrinsic carrier density is reduced.
  2. $U_{max}$ occurs at the edges of the depletion region in the device.
  3. $U_{max}$ depends exponentially on the applied bias.
  4. With all other parameters unchanged, $U_{max}$ increases if the thermal velocity of the carriers increases.

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Show answer and explanation

Correct answer: (B) $U_{max}$ occurs at the edges of the depletion region in the device.

Explanation

In the depletion region the recombination rate via traps is largest where $n=p$, which is the middle of the depletion region, not at its edges. So B is false. $U_{max}\propto n_i\,e^{qV/2kT}\sigma v_{th}N_t$, which supports A, C and D.