Sunday, May 20, 2012

Carrier Concentrations in Extrinsic Semiconductors

        We can obtain the concentration of minority and majority carriers in n-type and p-type materials using law of mass action.
n-type material :
For n-type material it is seen that, nn = ND.
At any fixed temperature, according to law of mass action,
                    nn x pn = ni2         
       where nn is electrons i.e. majority carriers concentration while pn is hole i.e. minority carriers concentration. Using n ≈   ND, we can write minority carrier concentration as,
                       ND pn = ni2         
...                     pn   = ni2/ND                                         .....................(1)
Note : Knowing ni and ND, the number of holes in n-type material i.e. minority carrier concentration can be obtained.
p-type material :
       For p-type material it is seen that, pp  = NA.
According to law of mass action,
              np x pp = ni2
       where np is electrons i.e. minority carrier concentration while is holes i.e. majority carrier concentration. Using pp ≈ NA we can write.
...             np = ni2/NA                               ............... (2)
Note : Knowing ni and NA, the number of electrons in p-type material i.e. minority carrier concentration can be obtained.

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