Doping is the deliberate introduction of impurity atoms into pure silicon to control its electrical conductivity — the fundamental process that transforms insulating silicon into the precisely controlled P-type and N-type semiconductors needed to build transistors, diodes, and every active device on a chip.

What Is Doping?

• Definition: Adding controlled amounts of specific atoms (dopants) into a silicon crystal lattice to create free charge carriers — either electrons (N-type) or holes (P-type).
• P-Type Doping: Boron (Group III) atoms replace silicon atoms, creating "holes" — missing electrons that act as positive charge carriers.
• N-Type Doping: Phosphorus or Arsenic (Group V) atoms add extra electrons as negative charge carriers.
• Concentration: Dopant levels range from 10¹⁴ to 10²¹ atoms/cm³, precisely controlling resistivity from kΩ·cm to mΩ·cm.

Why Doping Matters

• Transistor Formation: Every transistor requires precisely doped source, drain, channel, and well regions — doping defines how transistors switch.
• Junction Creation: P-N junctions (where P-type meets N-type silicon) are the building blocks of diodes, transistors, and solar cells.
• Threshold Voltage Control: Channel doping concentration sets the voltage at which a transistor turns on.
• Resistivity Tuning: Interconnect contacts, resistors, and capacitors all require specific doping profiles.

Doping Methods

• Ion Implantation: The primary method in modern fabs — ionized dopant atoms are accelerated (1-500 keV) and shot into the wafer surface with precise dose and depth control.
• Diffusion: Older method — wafers are heated in a dopant-containing gas atmosphere, and atoms diffuse into silicon. Still used for deep wells and some specialty processes.
• In-Situ Doping: Dopants are introduced during epitaxial silicon growth — used for uniformly doped layers.
• Plasma Doping (PLAD): Low-energy, high-dose implantation for ultra-shallow junctions at advanced nodes.

Ion Implantation Parameters

Common Dopants

• Boron (B): Standard P-type dopant, lightweight, used for channels and wells.
• Phosphorus (P): Standard N-type dopant, moderate mass, used for wells and deep junctions.
• Arsenic (As): Heavy N-type dopant, creates shallow junctions due to low diffusivity.
• BF₂: Boron difluoride — heavier molecule creates ultra-shallow P-type junctions.

Equipment Vendors

• Applied Materials (Varian): VIISta series — industry-leading high-current and medium-current implanters.
• Axcelis Technologies: Purion series — single-wafer high-energy and high-current platforms.
• AIBT (formerly Nissin Ion): Specialty implanters for advanced applications.

Doping is the process that gives silicon its superpowers — without precise dopant control at the atomic level, modern transistors operating at 3nm and below would be impossible to manufacture.