Suppose a ‘n’-type wafer is created by doping Si crystal having 5 × 1028 atoms/m3 with 1ppm concentration of As. On the surface 200 ppm Boron is added to create ‘P’ region in this wafer. Considering ni = 1.5 × 1016 m–3, (i) Calculate the densities of the charge carriers in the n & p regions. (ii) Comment which charge carriers would contribute largely for the reverse saturation current when diode is reverse biased.

i) Each arsenic atom will provide on electron. Hence, density of electrons will be




We also know that



So density of holes in this side will be,



When boron is added to the other side then each atom will contribute to one hole. So, density of holes will be,




Now the concentration of electrons will be,




ii) Density of holes in n region is



Density of electron in p region is



So, holes in n region will contribute more to the reverse saturation current.


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