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GATE 2016 EC – Question 21

Electronic Devices · Energy Bands and Carrier Concentration · 1 mark · Multiple choice

A small percentage of impurity is added to an intrinsic semiconductor at 300 K. Which one of the following statements is true for the energy band diagram shown in the following figure?

An energy band diagram with the conduction band edge $E_c$ at the top and the valence band edge $E_v$ at the bottom. A new energy level is drawn 0.01 eV below $E_c$, close to the conduction band.
  1. Intrinsic semiconductor doped with pentavalent atoms to form $n$-type semiconductor
  2. Intrinsic semiconductor doped with trivalent atoms to form $n$-type semiconductor
  3. Intrinsic semiconductor doped with pentavalent atoms to form $p$-type semiconductor
  4. Intrinsic semiconductor doped with trivalent atoms to form $p$-type semiconductor

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Correct answer: (A) Intrinsic semiconductor doped with pentavalent atoms to form $n$-type semiconductor

Explanation

A new level just below the conduction band, 0.01 eV from $E_c$, is a donor level, because its electron is easily excited into the conduction band. Donors come from pentavalent atoms such as phosphorus, and they make the semiconductor $n$-type.