Question

1. Draw the schematics of forward-biased and negative-biased diodes. Show the polarity of voltage
source (positive and negative terminal of the source), the position of Fermi levels and the current
direction. Explain why there is a small current flow when a p-n junction is under reverse bias.

2. A p-n junction can be made by diffusing acceptor atoms into an n-type semiconductor. Suppose
that boron is diffused into a silicon wafer doped with arsenic at 1015 cm-3 such that the boron
concentration in the p region is 6 x 1015 cm-3. What are the hole and electron concentration in the p
region? What is the hole concentration in the n region, that is, the undoped region beneath the
diffused region? The intrinsic carrier density, ni, is equal to 1.45 x 1010 cm-3 and assume that the
diffused dopant profile resembles a “box” shape.

3. Electronic grade silicon (EGS) is the starting point for the Czochralski crystal growth process.
Briefly describe the three important steps in the process of extracting and purifying silicon starting
from raw SiO2 (sand) and ending with single crystal Si..

4. In optical lithography, a smaller wavelength results in better resolution. However, in an x-ray lithography experiment, it is observed that when the wavelength of the x-ray is too small, the resolution is actually worse. Explain this phenomenon.

Answer 1