Physics of Semiconductor Devices

Exam March 2007, Problem 2

Consider a silicon pn junction.

(a) Draw the band diagram indicating the valence band, the conduction band, the Fermi energy, and the built-in potential, V_bi assuming that no voltage is applied across the junction. Indicate on this diagram approximately where the depletion region would be.

(b) The doping is N_d = 2×10¹⁶ 1/cm³ and N_a = 8×10¹⁶ 1/cm³. At 300 K, what is the concentration of holes on the p-side and the concentration of holes (minority carriers) on the n-side? For silicon, n_i = 1.5×10¹⁰ 1/cm³.

(c) Explain what the diffusion current is and which way it is flowing for forward bias and reverse bias.