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Name: Kegan
Status: other
Grade: other
Location: FL
Country: USA
Date: Summer 2013


Question:
To my understanding, photoemissive materials are typically metals since they have electrons that are easily manipulated (knocked loose). Why then is silicon used? I understand it is a metalloid and therefore has similar properties of metals, but is there a property of silicon that I am unaware of that makes it more attractive than metals and other metalloids?

Replies:
One useful feature of silicon is that it can be implanted or doped with impurities to develop the electrical characteristics you desire. That, and its source is plentiful in nature (sand).

Thanks for using NEWTON!'

Ric Rupnik


Kegan,

It is not pure silicon that is attractive for creating photoemitters, such as Light Emitting Diodes (LEDs), but doped silicon. By doping silicon , or adding the right impurities (such as phosphorous or boron), you can change the properties of the material to create a layered material with different conduction properties. In a typical LED, you have two layers that create an energy barrier across the interface or junction. An electron crossing the junction loses energy and radiates a photon of light.

Kyle J. Bunch, PhD, PE


Hi Kegan,

First, it would be useful to discuss the photoelectric effect vs. photovoltaic effect. Photoelectric effect is when materials absorb photon they release electrons, and usually metals have threshold frequency of illuminated light so only above the (minimum) frequency of the light can release electrons which are photoelectrons.

http://en.wikipedia.org/wiki/Photoelectric_effect

Second, silicon is a semiconductor, which has a band gap (~1.1eV for bulk) so it requires to have more energy than that band gap energy to excite the electrons from the ground state (valence band) to the excited state (conduction band). This excited electrons in the conduction band and holes in the valence band can be separated to generate electric current or voltage in a material which is photovoltaic effect. In this case, electrons are not escape from the materials - which is different from the photoelectric effect where electrons are released from the materials.

http://en.wikipedia.org/wiki/Photovoltaic_effect

Basically photovoltaic effect (from silicon and other semiconductors) is useful for solar cells and related applications, while photoelectric effect is useful for photomultipliers, image sensors, etc. The former involves in excitation of electrons into the conduction band (no electrons released, mostly semiconductors), while the latter involves in physical escape of electrons from the surface of materials (mostly metals).

Best, Weonkyu from Los Alamos


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