Suppose a pure Si crystal has 5×1028 atoms m−3. it is doped by 1 ppm concentration of pentavalent As. Calculate the number of electrons and holes. Given that n1=1.5×1016m−3.
1 atom of Si doped out of 106 atoms (1 ppm of As)
⇒ In 5×1028 atoms net doped =5×1028106=5×1022 atoms
1 As atom creates 1e⊝ excen
so, no. of excen electron =5×1022=ne
also, nenn=n21
⇒nn=n21ne=(1.5×1016)25×1022=4.5×109
∴ No. of holes formed =4.5×109.
as ne>nn→ It is a N - type semiconductor formed.