Question

Very pure silicon is an insulator, but becomes a p-type or an \(\mathrm{n}\) -type semiconductor when doped with a: (a) Group 12 and a group 16 element respectively (b) Group 4 and group 6 element respectively (c) Group 1 and a group 12 element respectively (d) Group 13 and a group 15 element respectively

Short answer

(d) Group 13 and a group 15 element respectively.

Step-by-step solution

Understanding Pure Silicon Properties

Silicon is a group 14 element known to be a semiconductor. In its pure form, it has a crystal structure where each silicon atom shares electrons with four neighboring silicon atoms, resulting in no free electrons or holes to conduct electricity, making it an insulator.

Definition of P-type and N-type Doping

Doping modifies pure silicon to change its electrical properties. **P-type doping** adds elements with fewer valence electrons than silicon, creating 'holes', while **N-type doping** adds elements with more valence electrons than silicon, providing extra electrons for conduction.

Identifying Suitable Group Elements for Doping

For **P-type semiconductors**, elements from Group 13 (three valence electrons, e.g., Boron) create holes. For **N-type semiconductors**, elements from Group 15 (five valence electrons, e.g., Phosphorus) provide extra electrons. These dopants match perfectly with the silicon lattice to alter its conductive properties.

Evaluating Given Options

Analyze each option: (a) Group 12 (e.g., Zinc) and Group 16 (e.g., Sulfur) don't match the properties required for p-type or n-type formation. (b) Group 4 (e.g., Carbon) and Group 6 (e.g., Oxygen) also don't provide the necessary electron or hole characteristics. (c) Group 1 (e.g., Sodium) and Group 12 are unsuitable for semiconductor applications. (d) Group 13 (e.g., Boron for p-type) and Group 15 (e.g., Phosphorus for n-type) match the doping requirements for silicon.

Conclusion

Based on the doping criteria and element group characteristics, the correct answer is option (d): Group 13 for p-type and Group 15 for n-type semiconductors.

Key concepts

P-type doping

P-type doping is a process where a semiconductor is intentionally contaminated with impurities to improve its electrical conductivity.
When silicon, a Group 14 element, is doped with elements from Group 13, it becomes a p-type semiconductor.
Group 13 elements, like Boron, have three valence electrons. When introduced into the silicon lattice, they create 'holes' or spaces where electrons are absent. This absence results because only three electrons are available for bonding with silicon's four bonds. These 'holes' act as positive charge carriers and facilitate the movement of electricity through the semiconductor.
This is because they attract electrons from neighboring atoms, allowing an electric current to flow as electrons fill these holes.
The term 'p-type' reflects the positive nature of the charge carriers in this doping process.

N-type doping

N-type doping refers to the introduction of impurities into a semiconductor to enhance its conductivity by increasing the number of free electrons.
In the case of silicon, doping it with Group 15 elements, like Phosphorus, results in an n-type semiconductor.
Group 15 elements have five valence electrons, which is one more electron than silicon's four. This extra electron becomes available as a free carrier, providing a negative charge to the material. The presence of these free electrons allows the semiconductor to conduct electricity more efficiently.
N-type semiconductors are so named because the charge carriers, in this case, are negative.
The movement of these extra electrons ensures that an electric current can easily pass through the material.

Group 14 element

Group 14 elements, also known as the carbon group, possess four valence electrons, making them versatile in bonding with other elements.
Silicon, a prominent Group 14 element, is widely used in semiconductors due to its stable crystalline structure and ability to form covalent bonds with other atoms.
This structure provides a perfect foundation for doping processes, where its electrical conductivity is adjusted through the addition of specific elements. In its pure form, silicon acts as an insulator because there are no free electrons or holes for electrical conduction. However, with the correct doping, silicon becomes a capable semiconductor, controlling and improving the flow of electricity.
Understanding silicon's abilities as a Group 14 element is essential in the electronics and semiconductor industries.

Doping in silicon

Doping in silicon is the introduction of specific impurities to drastically change and control its conductive properties.
Silicon naturally acts as an insulator, but with the precise incorporation of atoms from either Group 13 or Group 15, it can transition into either a p-type or n-type semiconductor. This controlled modification allows silicon to form the backbone of modern electronic devices, like transistors and integrated circuits.
The flexibility of silicon to switch between an insulating and a conducting state through doping makes it invaluable in computer chips and solar cells.
As technology has advanced, strategies for doping have become more sophisticated, resulting in more efficient and powerful electronic components.