Impurity Variation Method for Transistor Construction

The impurity variation method is a technique used in the construction of transistors, specifically in the formation of the base region. This method involves varying the concentration of impurities in the base region to optimize the transistor's performance.

Among the given options, the correct answer is option 'B' - grown junction type. The grown junction type transistor construction method utilizes the impurity variation technique to create the base region.

Explanation:
The construction of a transistor involves three regions - emitter, base, and collector. The impurity variation method focuses on the base region, which plays a crucial role in the transistor's overall performance.

Grown Junction Type Transistor Construction:
The grown junction type transistor construction method involves the following steps:

1. Substrate Selection:
- A suitable substrate material, such as silicon, is selected for the transistor construction.

2. Epitaxial Growth:
- An epitaxial layer is grown on the substrate using a technique called epitaxy.
- Epitaxy involves the deposition of a single-crystal layer on the substrate, maintaining the crystal orientation of the substrate.

3. Impurity Doping:
- The base region is formed by introducing impurities into the epitaxial layer.
- The impurities are introduced in a controlled manner, with the concentration varying across the base region.
- This impurity variation is achieved by controlling the doping process.

4. Diffusion and Annealing:
- The impurities introduced into the base region diffuse and spread through the epitaxial layer.
- Annealing is performed to activate the impurities and remove any defects caused by the diffusion process.

5. Masking and Etching:
- Masking techniques are used to define the emitter and collector regions.
- Etching is performed to remove the unwanted material, leaving behind the desired transistor structure.

6. Metalization and Contact Formation:
- Metalization is done to provide electrical connections to the various regions of the transistor.
- Contacts are formed to establish electrical connections between the metalization layer and the transistor regions.

This construction method allows for precise control of the impurity concentration in the base region, which is crucial for achieving the desired transistor characteristics. The impurity variation method helps optimize the transistor's performance by tailoring the base region's doping profile.

Therefore, the grown junction type transistor construction method is the correct answer as it involves the impurity variation technique for transistor construction.