Question

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 n_i = 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.

Answer 1

(i) In ‘n’ region; number of e– is due to As:

Similarly, when Boron is implanted a ‘p’ type is created with holes

n_h = N_A = 200 × 10^–6 × 5 × 10²⁸

= 1 × 10²⁵/m³  

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.