# act25R - ans - ANSWERS PHYS-1200 PHYSICS II Class 25...

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ANSWERS PHYS-1200 PHYSICS II SPRING 2005 Class 25 Activity: p and n Semiconductors Part 1. Doping Silicon -- Making semiconductor devices 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 5 silicon atoms is replaced by a phosphorus atom. 1. What is the valence of silicon? What is the valence of phosphorous? Silicon is valence 4. Phosphorous is valence 5 2. Which type will the doped semiconductor be, n or p ? Si doped with P will be n-type. 3. If silicon has a molar mass of 28.1 g/mol and a density of 2.33 g/cm 3 , how many silicon atoms would there be in a unit volume (1 cubic meter) ? Si atoms = 6.02 x 10 22 Si atoms x 1 mol Si x 2.33 grams Si x 100 3 cm m 3 mol Si 28.1 grams Si 1 cm3 1 m 3 There are 5 x 10 28 atoms of Si in 1 m 3 4. So, given that one of every 10 5 silicon atoms is replaced by a phosphorus atom, what charge carrier number density will the phosphorus add? 5 x 10 28 / 1 x 10 5 = 5 x 10 23 atoms added by phosphorous. Each atom adds 1 electron, so an equal number of charge carriers are added. 5. 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? holes Si electrons conduction Si electrons P Added holes Si electrons conduction Si PureSi DopedSi . . . . . . . . . + + + = = (5 x 10 15 + 5 x 10 15 + 5 x 10 23 ) / (5 x 10 15 + 5 x 10 15 ) = 5 x 10 7 It is important to include both Si electrons and holes in the numerator AND denominator 1

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act25R - ans - ANSWERS PHYS-1200 PHYSICS II Class 25...

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