Problem 7 (5 points). Pure, undoped silicon has a free electron concentration of
ID: 695379 • Letter: P
Question
Problem 7 (5 points). Pure, undoped silicon has a free electron concentration of 1.5×100 cm', an electron mobility of 1400 cm/(V s) and a hole mobility of 450 cm/(V-s) at room temperature. The doping density dependence of charge carrier mobility in silicon at room temperature is shown in the diagram below. (a) What is the room-temperature electrical conductivity of pure silicon? (2 points) (b) If silicon is doped with 1018 cm3 of phosphorus atoms, what is the semiconductor type (n-type or p-type), and what is its room-temperature electrical conductivity? (3 points) 1600 1 1400 1200 © Bart Van Zedbroeck 2007 Electrons E 1000 600 400 200 loi4 1016 101s 1020 Doping density (em)Explanation / Answer
The general formula of conductivity is
Conductivity=q(n*Mn +p*Mp) where
q is charge
n is concentration of electrons
p is hole concentration and
Mp is hole mobility.
1) conductivity=1.6*10^-19(1.5*100*1400+1.5*100*450)
=4.44*10^-14
2)as phosphorus is have 5 valence electrons,4 will make bond with silicon and there will remain an extra ELECTRON ,thus it is n type semiconductor.
The conductivity due to holes in this case will be negligible wrt conductivity of electrons.so we have
Conductivity=q*n*Mn
Conductivity=1.6*19^-19(1018)^3*1400
Conductivity=2.363*10^-7.