Adding an impurity to silicon can change its electrical properties, turning it into a semiconductor. In the case of n-type semiconductors, the impurity donates extra electrons, creating an excess of negative charge carriers. One common impurity used to create n-type semiconductors is phosphorous.

Phosphorous is a group 5 element and has five valence electrons. When it is added as an impurity to silicon, it replaces some of the silicon atoms in the crystal lattice. However, phosphorous has one extra valence electron compared to silicon, creating a surplus of electrons in the crystal structure.

Explanation:
- Silicon as a Semiconductor
Silicon is a group 4 element and has four valence electrons. In its pure form, silicon acts as an intrinsic semiconductor, meaning it has a balanced number of positive and negative charge carriers. At room temperature, a small number of electrons can be excited to the conduction band, creating electron-hole pairs.

- Impurity Doping
By adding impurities to silicon, the electrical properties of the material can be modified. This process is called doping. Doping can be done with either group 3 or group 5 elements, depending on the desired type of semiconductor.

- N-Type Semiconductors
In n-type semiconductors, the impurity used is from group 5 of the periodic table. Group 5 elements, such as phosphorous, have one extra valence electron compared to silicon. When phosphorous is added as an impurity to silicon, it replaces some of the silicon atoms in the crystal lattice. The extra electron from the phosphorous atom is not tightly bound and becomes a charge carrier.

- Excess Electrons
The presence of phosphorous atoms in the silicon lattice introduces extra electrons into the crystal structure. These extra electrons act as negative charge carriers. They are free to move through the lattice, conducting electricity. This excess of negative charge carriers is what characterizes n-type semiconductors.

- Summary
Adding phosphorous as an impurity to silicon creates an n-type semiconductor. Phosphorous donates an extra electron, creating an excess of negative charge carriers in the silicon crystal structure. This excess of electrons allows the material to conduct electricity and exhibit the properties of an n-type semiconductor.