N Type Dopant

N-type dopants are donor elements from Group V of the periodic table — phosphorus, arsenic, and antimony — that each contribute one extra electron to the silicon crystal lattice when they occupy a substitutional lattice site, creating the negative-charge-carrier regions that form NMOS transistors, n-wells, and n-type junctions throughout a chip. Each of these three elements occupies a genuinely different practical role despite sharing the same basic donor mechanism, and choosing between them is a real engineering decision driven by mass-dependent diffusion and depth-control behavior, not an arbitrary preference.

Phosphorus's moderate atomic mass of 31 amu gives it genuinely balanced depth control along with high solid solubility, reaching roughly 1×10²¹ per cubic centimeter at 1000°C, and relatively fast diffusion — a combination that makes it the natural choice specifically for deep well and channel implants where that faster diffusion actually helps dopants spread to the intended depth during subsequent thermal processing. This fast-diffusion property, which would be a liability for a shallow-junction application needing a sharply controlled profile, is instead a genuine asset for well formation, where the goal is a deep, relatively broad doped region rather than a sharply defined shallow one — exactly the application phosphorus's specific mass and diffusion behavior naturally suits.

Arsenic's considerably heavier 75 amu mass limits implant depth at any given energy and substantially reduces channeling relative to lighter dopants, while its slow diffusion keeps the resulting profile tightly controlled during subsequent thermal steps — precisely the combination that makes arsenic the preferred choice for NMOS source/drain extensions at advanced nodes where junction depth has to stay extremely shallow and well controlled. This slow-diffusion characteristic is exactly the opposite property that makes phosphorus well-suited to well formation — arsenic's resistance to post-implant spreading is specifically what lets a shallow, sharply defined junction survive subsequent anneal steps without diffusing deeper than intended, which is exactly the behavior advanced-node source/drain extension requirements depend on.

Antimony, the heaviest common n-type dopant at 122 amu, diffuses more slowly than either phosphorus or arsenic, making it the preferred choice specifically for buried layers in bipolar transistor structures where minimal dopant redistribution during all of the subsequent thermal processing that bipolar fabrication requires is the dominant practical requirement. Bipolar process flows typically involve multiple high-temperature steps after the buried layer is formed, and antimony's resistance to diffusion during all of that subsequent thermal budget is exactly what keeps the buried layer's intended depth and profile from degrading by the time the device is finished — a property phosphorus or arsenic couldn't match given their comparatively faster diffusion.

Implant energies for n-type dopants span an enormous practical range — from roughly 0.2 keV for ultra-shallow extensions up to 2 MeV for deep retrograde wells — and doses span from roughly 1×10¹² per square centimeter for threshold-voltage adjustment up to 5×10¹⁵ for heavy source/drain implants, meaning the same three donor elements get deployed across essentially the entire available range of implant conditions depending on which specific device structure is being formed. This enormous range across both energy and dose for the identical three elements is exactly why n-type dopant selection can't be reduced to "pick one element for n-type doping" — the specific combination of element, energy, and dose has to match the specific structure being implanted, with each of the three elements' distinct mass and diffusion properties suited to a genuinely different corner of that overall design space.

Thermal annealing after implantation is what actually activates an n-type dopant electrically, moving it from wherever the implant physically deposited it onto a proper substitutional lattice site where it can finally contribute its extra electron to the conduction band — meaning an implanted but unannealed wafer contains dopant atoms that are physically present but not yet electrically active. This activation step is a necessary completion of the doping process rather than an optional refinement: an n-type dopant sitting in a non-substitutional position inside the lattice contributes essentially nothing to the region's electrical conductivity, which is exactly why implant dose and energy alone don't fully determine a region's final electrical behavior — the subsequent anneal's effectiveness at achieving substitutional activation matters just as much.