The electrical properties of germanium can be altered drastically by the addition of impurity. This process is referred to as

Step 1: Analyze the question The question asks for the name of the process in which impurities are added to a semiconductor such as germanium to change its electrical properties.

Step 2: Explain the concept Germanium and silicon are intrinsic semiconductors. In their pure form they conduct poorly because they have few free charge carriers at room temperature. Their conductivity can be increased greatly by adding a very small amount of a different element. This process is called doping. (The option is spelled "dopping", but it means doping.)

There are two types of doping:

• n-type: add a pentavalent impurity (5 valence electrons) such as phosphorus, arsenic or antimony. This gives extra free electrons, so the electrons are the majority carriers.
• p-type: add a trivalent impurity (3 valence electrons) such as boron, aluminium, gallium or indium. This creates "holes", so the holes are the majority carriers.

Step 3: Eliminate the wrong options

• A. Saturation: this describes a current or magnetization reaching its maximum value. It is not a way of adding impurities.
• B. Bonding: this is how atoms join together, such as covalent bonding in germanium. It is not the process of adding impurities.
• C. Amplification: this means increasing the strength of a signal, which transistors do. It has nothing to do with adding impurities.
• D. Doping: this is the deliberate addition of impurity atoms to a semiconductor to change its conductivity. This matches the question.

Step 4: Answer The correct option is D (doping).

Exam tip: Remember "Doping = Adding impurities to a semiconductor". Pentavalent gives n-type (negative electrons) and trivalent gives p-type (positive holes). JAMB often asks about this, so learn the examples of each type of impurity.