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Suppose a 'n'-type wafer is created by dopin Si crystal having 5xx10^(28) atoms/m^(3) with 1 ppm concentration of As. On the surface 200 ppm Boron is added to create 'P' region in this wafer. Considering n_(i)=1.5xx10^(16)m^(-3)(i) Calculate the denisties of the charge carrier in the n & p regions. (ii) Comment which charge carriers would contribute largely for the reverse saturation current when diode is reverse biased. |
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Answer» Solution :Here 1 ppm = 1 part per million = 1 part in 10 lakh (i) For n-type semiconductor : (no. of Si atoms in 1 `m^(3)` volume) `to` (no. of As atoms in 1 `m^(3)` volume) `10^(6) to 1` `therefore 5xx10^(28) to `(?) `rArr` NUMBER density of PENTAVALENT As atoms (or) free electron number density). `n_(e )=(5xx10^(28))/(10^(6))=5xx10^(22)("free electron")/(m^(3))` Now `n_(e )n_(h)=n_(i)^(2)` `therefore n_(h)=(n_(i)^(2))/(n_(e ))` `=((1.5xx10^(16))^(2))/(5xx10^(22))` `therefore n_(h)=4.5xx10^(9)"hole"//m^(3)""...(1)` (ii) For p-type semiconductor: (no. of Si atoms in 1 `m^(3)` volume) `to` (no. of B atoms in 1 `m^(3)` volume) `10^(6) to 200` `therefore 5xx10^(28)to` (?) `rArr` Number density of trivalent B atoms (or hole number density). ` n_(h)=(200xx5xx10^(28))/(10^(6))` `therefore n_(h)=10^(25)("hole")/(m^(3))` Now `n_(e )n_(h)=n_(i)^(2)` `therefore n_(e )=(n_(i)^(2))/(n_(h))` `therefore n_(e )=((1.5xx10^(16))^(2))/(1xx10^(25))` `therefore n_(e )=2.25xx10^(7)("free electrons")/(m^(3))""...(2)` EQUATIONS (1) and (2) indicate that `n_(h) gt n_(e )` `rArr` In the reverse bias condition of given pn-junction, holes will contribute majority in producing reverse saturation current. |
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