Introduction:
In transistor construction, impurity variation methods are used to create different regions within the transistor structure with varying impurity concentrations. These impurity variations are important for controlling the transistor's electrical characteristics, such as its current gain or voltage handling capabilities. Among the given options, the grown junction type is the method that utilizes impurity variation for transistor construction.

Grown Junction Type:
The grown junction type transistor construction method involves growing a layer of semiconductor material with a different impurity concentration on top of an existing semiconductor substrate. This grown layer is known as the epitaxial layer. The epitaxial layer is formed by depositing a controlled amount of impurities onto the substrate surface and allowing them to diffuse into the substrate during the growth process.

Impurity Variation:
The impurity concentration in the grown junction type transistor varies between the epitaxial layer and the substrate. This impurity variation is achieved by carefully controlling the deposition process and the diffusion of impurities during growth. The epitaxial layer is typically doped with impurities that have a higher concentration compared to the substrate, resulting in a higher impurity concentration in the epitaxial layer.

Significance:
The impurity variation in the grown junction type transistor construction has several advantages:

1. Improved Electrical Characteristics: The impurity variation helps in creating regions with different electrical characteristics within the transistor structure. This allows for better control of the transistor's performance parameters such as current gain, frequency response, and breakdown voltage.

2. Reduced Surface Contamination: The epitaxial layer acts as a protective barrier, preventing surface contamination on the substrate. This is particularly important in high-frequency applications where surface contamination can degrade the transistor's performance.

3. Reduced Substrate Resistance: The impurity variation helps in reducing the resistance of the substrate. The higher impurity concentration in the epitaxial layer allows for better conductivity, resulting in reduced resistance and improved overall transistor performance.

Conclusion:
Among the given options, the grown junction type transistor construction method utilizes impurity variation to create different regions within the transistor structure. This impurity variation is achieved by growing an epitaxial layer with a different impurity concentration on top of the substrate. The impurity variation in the grown junction type transistor construction provides improved electrical characteristics, reduced surface contamination, and reduced substrate resistance.