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# act25ans - Do NOT write on these sheets or take them with...

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Do NOT write on these sheets or take them with you! The next class needs them too! PHYS-1200 PHYSICS II SPRING 2005 Class 25 Activity: p and n Semiconductors A. Pure silicon at room temperature has an electron number density in the conduction band of about 5 ×10 15 m -3 and an equal density of holes in the valence band. Suppose that one of every 10 6 silicon atoms is replaced by a phosphorus atom. 1. Which type will the doped semiconductor be, n or p ? Phosphorous is valence 5, so the material will be n-type 2. What charge carrier number density will the phosphorus add? (Hint: See sample problem 41-6 on page 1154.) There are 5 ×10 28 atoms per m 3 in silicon, so there are 5 ×10 28 /1 ×10 6 = 5 ×10 22 phosphorous atoms per m 3 added, and an equal number of electrons. 3. What is the ratio of the charge carrier number density (electrons in the conduction band and holes in the valence band) in the doped silicon to that in pure silicon? Doped/Pure = (5 ×10 22 + 5 ×10 15 + 5 ×10 15 )/( 5 ×10 15 + 5 ×10 15 ) = 5 ×10 6 = 5 million It is important to include both electrons and holes in the denominator of this expression.

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