Question

Question 8 Pure silicon at room temperature has an electron number density of about 5 × 1015 m3 and an equal density of holes In the valence band. Suppose that one of every 10° silicon atoms is replaced by a phosphorus atom. (a) Which type will the doped semiconductor be, n or p? (b) What charge carrier number density will the phosphorus add? (c) 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? The density of silicon is 2.33 g/cm and its molar mass is 28.086 g/mol (b) Number (c) Number Click if you would like to Show Work for this question: Units Open Show Work

Answer 1

a) Phosphorus has 5 electrons in the valence shell as compared to 4 for Si. Therefore the doped semiconductor is n type.

b) No. of Si atoms per cm³ = 2.33/28.086 * 6.022*10²³ = 4.996*10²²m^-3. Therefore no. of doped electron carriers =

4.996*10²²/10⁶ = 4.996*10¹⁶ m^-3

c) No. of electrons = 4.996*10¹⁶ + 5*10¹⁵ = 5.496*10¹⁶ m^-3 , No. of Holes = 5*10¹⁵ m^-3

Ratio = 5.496*10¹⁶ /5*10¹⁵ = 10.992