Question

3. a) Account for two kinds of semiconductor and use schemes to explain charge flow between n-type and p-type semiconductors in the dark condition without a bias.

b) A substance known to have a cubic unit cell gives reflections with Cu Karadiation (wavelength 154 pm) at the glancing angle of 32.6 degrees

The reflection is known to be due to the (220) plane. Index the reflection at the glancing angle of 39.4 degrees

Answer 1

a) There are two types of semiconductor. Ptype and N type.

P type semiconductor

It is obtained when the trivalent impurity is added to the intrinsic semiconductor. Trivalent impurities are boron, aluminium gallium etc. They are called acceptor impurities.Silicon has four electrons in the outermost shell. When they combine with trivalent impurities, a vacant space is created. This is known as hole. Holes are the majority carriers in p type semiconductor.

N type semiconductor

It is obtained when the pentavalent impurity is added to the intrinsic semiconductor. pentavalent impurities are phosphurus, arsenic antimony etc. They are called donor impurities.Silicon has four electrons in the outermost shell. When they combine with pentavalent impurities, an extra electron is created. These electrons are the majority carriers in n type semiconductor.

Charge flow between p type and n type semiconductors

If we join two semiconductors together, we will get a PN junction. When the N-type semiconductor and P-type semiconductor materials are joined together , some of the free electrons from the donor impurity atoms move towards the junction to fill up the holes in the P-type material producing negative ions. This movement of electrons towards the junction creates a positively charged ions in the n- region. The region near the junction with immobile ions is known as depletion region or space charge region.