Why Silicon is of no Use to VLSI Engineers

The world of semiconductors, vital to modern electronics, often revolves around silicon, a material abundant and stable in its pure form. However, understanding why pure silicon isn't a semiconductor is crucial.

Semiconductors are fundamental in modern electronics, enabling the creation of transistors, diodes, and other crucial components. Silicon is often at the heart of these technologies, but pure silicon itself isn’t a semiconductor. To comprehend why, we need to delve into band theory and the behavior of electrons in materials.

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Band Theory and Pure Silicon

Pure materials, such as silicon in its intrinsic state, do not function as semiconductors due to their fully occupied energy bands. The principles of band theory, governed by the Pauli Exclusion Principle, reveal that electrons in pure silicon lack the necessary energy levels to transition between bands, rendering it insulating at room temperature.

When we consider the probability of exciting a particle between two energy states with a typical energy difference of about 3 eV the resulting value is remarkably small, often on the order of 10−51. This minuscule probability emphasizes that at room temperature, the energy alone is usually insufficient to excite an electron from the valence band to the conduction band.

However, to enhance a material’s conductivity, especially in the context of semiconductors, intentional doping is employed. Doping introduces new energy levels within the material that fall between the valence and conduction bands, effectively promoting conductivity by providing alternate pathways for electron movement.

Intrinsic vs. Extrinsic Semiconductors

Intrinsic Semiconductors

When silicon is in its pure, undoped state, it’s termed as an intrinsic semiconductor. However, as mentioned earlier, it doesn’t exhibit conductivity under normal circumstances due to its filled energy bands.

Extrinsic Semiconductors

Extrinsic semiconductors are created by intentionally introducing impurities (doping) into the material to alter its electrical properties.

  • N-type Semiconductors:
  • Doped with pentavalent impurities (e.g., phosphorus, arsenic), introducing extra electrons, creating an excess of negative charge carriers (electrons), hence the term “n-type” (negative).

  • P-type Semiconductors:
  • Doped with trivalent impurities (e.g., aluminum, boron), creating holes due to the missing electrons, resulting in an excess of positive charge carriers (holes), giving rise to the term “p-type” (positive).

Properties of N-type and P-type Semiconductors

  • N-type Semiconductor:
  • Abundance of free electrons.
  • Conducts electricity by movement of electrons.
  • Negative charge carriers dominate.

  • P-type Semiconductor:
  • Abundance of holes (missing electrons).
  • Conducts electricity through movement of holes.
  • Positive charge carriers dominate.

Read more: What happens in a Semiconductor Manufacturing Fab

Why do we use Silicons then?

Doping is a fundamental technique in semiconductor technology, and while it is primarily associated with silicon, it’s indeed used with other materials as well. The choice of material for semiconductor applications depends on a variety of factors including its electronic properties, stability, availability, cost, and suitability for specific applications. Let’s delve into why silicon is often the focus and why other materials are also doped for semiconductor applications.

Silicon’s Prevalence:

  • Abundance and Purity: Silicon is the second most abundant element on Earth’s crust and can be extracted with high purity, making it readily available for large-scale semiconductor production.
  • Stability and Reliability: Silicon exhibits stable electrical properties over a wide range of temperatures, making it reliable for various electronic applications.

Compatibility with Modern Electronics:

  • Mature Technology: Silicon has been extensively researched and developed over decades, leading to a highly mature and sophisticated semiconductor technology.
  • Compatibility with Existing Infrastructure: The entire semiconductor industry has been built around silicon. Changing to a different material would require a significant overhaul of existing infrastructure, making it economically and practically challenging.

Electronic Properties:

  • Ideal Band Gap: Silicon possesses a band gap of approximately 1.1 eV, making it suitable for a broad range of applications including transistors, diodes, and solar cells.
  • Thermal Stability: Silicon’s properties remain stable at high temperatures, allowing for reliable performance even under demanding conditions.

Other Doped Materials:

  • Gallium Arsenide (GaAs): Used in high-frequency applications like microwave integrated circuits due to its higher electron mobility compared to silicon.
  • Gallium Nitride (GaN): Suitable for high-power, high-frequency applications such as LEDs and power electronics due to its wide bandgap.
  • Indium Phosphide (InP): Used in optoelectronics, lasers, and high-speed transistors due to its high electron mobility.

Why Doping Other Materials?

  • Customization: Doping other materials allows for tailoring their electrical properties to specific applications. Different materials have varying band gaps and carrier mobilities, making them suitable for different functions.
  • Optimized Performance: Doping enables the creation of materials that perform optimally in specific conditions, enhancing device efficiency and functionality.

In essence, silicon is popular due to its abundance, stability, and compatibility with existing tech. However, engineers and researchers deliberately dope other materials to utilize their distinct advantages over silicon, tailoring the choice of material and doping strategy based on the specific requirements of the application at hand.

Conclusion

Pure silicon lacks semiconductor properties due to its filled energy bands. However, intentional doping with specific impurities enables us to create n-type and p-type semiconductors. This precise control of silicon’s electrical behavior is fundamental to the development of transformative electronic devices.

Kumar Priyadarshi
Kumar Priyadarshi

Kumar Joined IISER Pune after qualifying IIT-JEE in 2012. In his 5th year, he travelled to Singapore for his master’s thesis which yielded a Research Paper in ACS Nano. Kumar Joined Global Foundries as a process Engineer in Singapore working at 40 nm Process node. Working as a scientist at IIT Bombay as Senior Scientist, Kumar Led the team which built India’s 1st Memory Chip with Semiconductor Lab (SCL).

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