In a diamond semiconductor, why is the room-temperature electron density of a phosphorus-doped n-type layer far lower than the phosphorus concentration?

Answer: B. The phosphorus donor level lies about 0.57 eV below the conduction band, so it hardly ionizes at room temperature

For a donor to release an electron, the electron must be thermally lifted from the donor level to the conduction band. In silicon the phosphorus level is shallow, about 0.045 eV below the conduction band, so nearly all of it ionizes at room temperature. In a diamond semiconductor the phosphorus level is deep at about 0.57 eV, far more than the room-temperature thermal energy (about 0.026 eV), so most phosphorus atoms release no electrons. The boron acceptor is also fairly deep at about 0.37 eV, making doping a major challenge in diamond device development.

• A. Whether the gap is indirect affects light emission efficiency, not carrier density.
• B. Correct. The deep level is the cause.
• C. Phosphorus acts as a donor in diamond too.