When a silicon is doped with phosphorus, there is an extra electron in the semiconductor. Where would this extra electron be located in the band diagram?

Question

BlackTom AI · Accepted Answer

Question restatement: When a silicon crystal is doped with phosphorus, there is an extra electron in the semiconductor. Where would this extra electron be located in the band diagram?

Note on data: The provided answer is listed as "e. Donor state" but no other answer options were included to analyze against.

Analysis of the concept: In silicon, phosphorus adds a donor level just below the conduction band. This donor state corresponds to an energy level where the extra electron is loosely bound to the phosphorus nucleus. In the band diagram, this donor level lies within the band gap, closer to the conduction band than to the valence band.

Why the donor state is the correct location: The extra electron from phosphorus is not initially part of the valence band; instead it occupies the donor level, which is energetically favorable compared to placing the electron directly into the conduction band. This donor state acts as a shallow energy well that can supply electrons to the conduction band when thermally excited, thereby increasing conductivity.

Why the conduction band location would be incorrect: If the electron were in the conduction band, it would imply it is already freely conducting rather than being bound as a donor electron. The donor state explicitly describes the bound level just below the conduction band, not a full free state in the conduction band.

Why the valence band location would be incorrect: The valence band is much lower in energy; an extra electron from a donor would not reside in the valence band because that would require crossing a large band gap energy in the wrong direction for donor doping. Donor electrons are associated with energy levels within the gap, near the conduction band, not in the valence band.

Why the core concept matters: This placement explains how n-type doping increases carrier concentration: electrons are provided by donors, occupying shallow donor levels and then thermally ionizing into the conduction band as needed for conduction. The donor state is thus the correct and standard description for the extra electron’s location in the band diagram following phosphorus doping.

When a silicon is doped with phosphorus, there is an extra electron in the semiconductor. Where would this extra electron be located in the band diagram?单项选择题

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