Q.

Which impurity is added to gallium phosphide to make it an efficient light emitter?

A. silicon
B. hydrogen
C. nitrogen
D. phosphorus
Answer» C. nitrogen
Explanation: an indirect band-gap semiconductor may be made into an electro- luminescent material by the addition of impurity centers which will convert it into a direct band-gap material. the introduction of nitrogen as an impurity into gallium phosphide makes it an effective emitter of light. such conversion is only achieved in materials where the direct and indirect band- gaps have a small energy difference.
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