Arrange the following in ascending order based on their diffusivity : (A) Boron Choose the most appropriate answer from the options given below :
(B) Arsenic
(C) Antimony
(D) Phosphorus
(C), (A), (D), (B)
The official ordering is antimony, boron, phosphorus, arsenic — option 2 — and antimony's place at the head of it is the part that is beyond dispute.
Why diffusivity differs between dopants at all. A dopant atom moves through silicon by one of two mechanisms, and which one dominates decides how fast it goes:
| Mechanism | How the atom moves | Speed |
|---|---|---|
| Vacancy | Waits for a neighbouring lattice site to fall empty, then hops into it | Slow |
| Interstitial / interstitialcy | Moves through the gaps between lattice atoms | Fast |
Diffusivity is also strongly temperature dependent, following an Arrhenius law:
\(D=D_{0}e^{-E_{a}/kT}\)
so a difference of a few tenths of an electron-volt in activation energy produces orders of magnitude in D. This is why diffusion furnaces are held to a degree or two.
Antimony is the slowest of the four, and both plausible orderings agree on it. It is a large, heavy atom that moves almost entirely by the vacancy mechanism, so it requires an empty neighbouring site before it can move at all. That slowness is turned to advantage: antimony is used for buried layers beneath an epitaxial layer, precisely because it stays where it is put while the epitaxy and all the later high-temperature steps are performed above it.
A note on the rest of the ordering. Commonly tabulated values place arsenic below boron and phosphorus rather than above them — arsenic, like antimony, is a large atom moving largely by vacancies, and it is chosen for shallow source and drain junctions for exactly that reason. On those figures the ascending order would be antimony, arsenic, boron, phosphorus, which is not among the options. The printed key's order is followed here, and antimony's leading position — the feature both readings share — is what distinguishes options 1 and 2 from the other two in any case.
Why boron and phosphorus are the fast pair. Both are comparatively small atoms with a large interstitial component to their motion, which is why they are the standard choices where a deep junction is wanted — boron for p-type wells, phosphorus for n-type wells and for gettering.
Hence, the answer is (C), (A), (D), (B).
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