ELEC421/6221
Submission deadline October 01, 2014
Assignment 2
1 Consider an ideal silicon pn junction diode with the following parameters: τn0 = τp0 = 0.1 x 10-6
s, Dn = 25 cm2/s, Dp = 10 cm2/s. What must be the ratio of Na / Nd so that 95% of the current in the
depletion region is carried by electrons?
2 (8.17) Consider the ideal long silicon pn junction. T = 300 K. The n region is doped with 10 16
donor atoms per cm3 and the p region is doped with 5 x 1016 acceptor atoms per cm3. The minority
carrier lifetimes are τn0 = 0.05 µs and τp0 = 0.01µs. The minority carrier diffusion coefficients are
Dn = 23 cm2/s and Dp = 8 cm2/s. The forward-bias voltage is Va = 0.610 V. Calculate (a) the excess
hole concentration as a function of x for x ≥ 0, (b) the hole diffusion current density at x = 3 x
10-4 cm, and (c) the electron current density at x = 3 x 10-4 cm.
3 (8.29) Consider Example 8.6 (8.7 in 3rd edition) of your textbook. Calculate the forward bias
voltage at which the ideal diffusion current is equal to the recombination current.
4 (8.35) Consider a uniformly doped silicon pn junction at T = 300 K with impurity doping
concentrations Na = Nd = 5 x 1015 cm-3. and minority carrier lifetime τn0 = τp0 = τ0 = 10-7 s. A reverse
bias voltage of VR = 10 V is applied. A light source incident only on the space charge region,
producing an excess carrier generation rate of g’ = 4 x 1019 cm-3s-1. Calculate the generation current
density.