Question

Elemental silicon to be used as a semiconductor is purified by

• A. heating under vacuum
• B. floatation
• C. zone refining
• D. electrolysis

Hint:

High-purity silicon is required for the semiconductor industry. The needed levels of purity cannot be reached by chemical methods alone.

Answer 1

The Correct Option is C

To purify elemental silicon for use as a semiconductor, the method known as zone refining is commonly used. Let's delve into why zone refining is the correct method and why the other options are not suitable for this particular application:

1. Zone Refining:
   • This method is based on the principle that the impurities in a molten state are more soluble than in a solid state.
   • During zone refining, a narrow region of a silicon rod is melted, usually by moving a heat source along the length of the rod.
   • As the molten zone moves, it carries impurities with it due to the higher solubility of impurities in the liquid state compared to the solid state.
   • This process is repeated multiple times to achieve a high level of purity of silicon, which is crucial for semiconductor applications.
   • Therefore, zone refining is particularly suitable for the purification of semiconductor-grade silicon.
2. Heating Under Vacuum:
   • This process involves removing impurities by vaporizing them under low pressure. However, it is not effective for all types of impurities common in silicon.
   • It is more suitable for volatile impurities, but many impurity elements in silicon are not volatile enough to be removed efficiently this way.
3. Floatation:
   • This method is primarily used in ore beneficiation to separate minerals based on differences in their hydrophobicity.
   • It is not applicable for purifying silicon as it deals with solid-liquid separation and not with direct impurity removal from a semiconductor material.
4. Electrolysis:
   • While electrolysis is an effective method for refining metals such as copper and aluminum, it is not used for the purification of silicon.
   • Silicon is a non-metal and does not conduct electricity well enough to be refined via electrolysis in its elemental state.

In conclusion, zone refining is the most effective method for purifying elemental silicon for semiconductor use due to its ability to selectively remove impurities more soluble in the molten phase. Hence, the correct answer is zone refining.