Physics
Suitable impurities are added to a semiconductor depending on its use. This is done in order to

increase its electrical resistivity
increase its electrical conductivity
increase its life
enable it to withstand higher voltages

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Physics
RADAR is used for:

receiving a signals in a radio receiver.
detecting and locating the position of objects such as Aeroplanes.
locating geostationary satellites.
locating submerged submarines.

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