# Suppose a ‘n’-type wafer is created by doping Si crystal having 5 × 10^28 atoms/m^3 with 1ppm concentration of As.

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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.

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(i) In ‘n’ region; number of e– is due to As:  Similarly, when Boron is implanted a ‘p’ type is created with holes

nh = NA = 200 × 10–6 × 5 × 1028

= 1 × 1025/m

This is far greater than e– that existed in ‘n’ type wafer on which Boron was diffused.

Therefore, minority carriers in created ‘p’ region (ii) Thus, when reverse biased 0.45 × 1010/m3, holes of ‘n’ region would contribute more to the reverse saturation current than 2.25× 107/m3 minority e– of p type region.