The impurity added to dope gate polysilicon in an n-MOSFET is

The correct option is 2

Explanation:

In an n-MOSFET, the gate polysilicon needs to be doped to make it conductive. For an n-MOSFET, the channel is n-type, and the gate polysilicon is typically doped with an n-type impurity to lower its resistance and to help set the threshold voltage.

Common n-type impurities used for doping silicon are Group V elements. Let's look at the options:

1. Boron: Boron is a Group III element and is a p-type dopant. It creates holes in silicon.
2. Antimony: Antimony (Sb) is a Group V element and is an n-type dopant. It contributes free electrons to silicon.
3. Gallium: Gallium (Ga) is a Group III element and is a p-type dopant.
4. Carbon: Carbon (C) is a Group IV element, like silicon. It's typically used to modify silicon's lattice properties, not as a primary dopant to create n-type or p-type conductivity.

Therefore, for doping gate polysilicon in an n-MOSFET to make it n-type, Antimony would be a suitable impurity. Other common n-type dopants for silicon include Phosphorus and Arsenic.