In solid state and semiconductor physics, which of the following substances is not classified as a semiconductor material?

Introduction / Context:
Semiconductors are materials with electrical conductivity between that of conductors and insulators. They are the foundation of modern electronics and are used in diodes, transistors, and integrated circuits. Common semiconductor materials include silicon, germanium, and certain compound semiconductors like cadmium sulfide. This question asks you to identify which listed substance is not a semiconductor. Recognising typical semiconductors and insulators is important for understanding basic electronics.

Given Data / Assumptions:
- The substances listed are CdS, NaCl, Si, and Ge. - We must choose the one that is not a semiconductor. - We assume standard classification used in physics and electronics.

Concept / Approach:
Silicon and germanium are classic elemental semiconductors used in most electronic devices. Cadmium sulfide CdS is a compound semiconductor often used in photoresistors and solar cells. Sodium chloride NaCl, on the other hand, is an ionic compound and behaves as an insulator in solid state, with very low electrical conductivity. While molten or dissolved NaCl can conduct due to mobile ions, solid NaCl is not considered a semiconductor. Therefore, NaCl is the correct choice as the non semiconductor among the options.

Step-by-Step Solution:
Step 1: Recall that silicon Si is the most widely used semiconductor in electronic chips and solar panels. Step 2: Recall that germanium Ge is another elemental semiconductor used historically in early transistors. Step 3: Recognise that cadmium sulfide CdS is a compound semiconductor used in light dependent resistors and some optoelectronic devices. Step 4: Note that sodium chloride NaCl is a common ionic salt, usually an electrical insulator in solid form. Step 5: Conclude that NaCl is not classified as a semiconductor, making it the correct answer.

Verification / Alternative check:
Consulting basic electronics textbooks shows that silicon and germanium are standard semiconductors, while compound semiconductors include materials like CdS, GaAs, and others. NaCl does not appear in lists of semiconductor materials. Instead, it is discussed in the context of ionic bonding and the structure of common salt crystals. Experimental observations also show that a piece of solid NaCl does not conduct electricity, whereas doped silicon and CdS do under appropriate conditions, confirming their semiconductor nature.

Why Other Options Are Wrong:
Option A (CdS) is wrong as a choice for non semiconductor because cadmium sulfide is widely used as a semiconductor material in photoconductive cells and solar devices. Option C (Si) is incorrect because silicon is the primary semiconductor used in integrated circuits and power devices. Option D (Ge) is incorrect because germanium is a well known semiconductor used in some specialised applications and historically in early transistors.

Common Pitfalls:
Students sometimes assume that any crystalline solid might be a semiconductor, which is not true. Another pitfall is focusing on the fact that NaCl solutions conduct electricity and forgetting that the question concerns the material as a semiconductor in solid state. Remember that semiconductor behaviour refers to solids whose conductivity can be controlled by doping and temperature, which is not the case for ionic salts like NaCl.

Final Answer:
The correct answer is: Sodium chloride NaCl.