The correct answer is option 'C' - Thyristor is not made using germanium because it has high leakage current.

Explanation:

1. Composition of Thyristor:
A thyristor is a type of semiconductor device that consists of alternating layers of P-type and N-type materials. The most commonly used material for the thyristor is silicon, but other materials like gallium arsenide and silicon carbide can also be used.

2. Leakage Current:
Leakage current refers to the small amount of current that flows through a device even when it is supposed to be in the off state. In the case of thyristors, leakage current occurs due to the imperfect insulation between the different layers of the device.

3. Germanium and Leakage Current:
Germanium is an element that was widely used in early semiconductor devices before silicon became the preferred material. However, germanium has a higher intrinsic carrier concentration compared to silicon, which results in higher leakage current. This means that germanium-based devices are more prone to leakage current compared to silicon-based devices.

4. Advantages of Silicon over Germanium:
Silicon has several advantages over germanium when it comes to semiconductor devices like thyristors:

- Higher Breakdown Voltage: Silicon has a higher breakdown voltage compared to germanium, which means it can handle higher voltages without breaking down.
- Lower Leakage Current: As mentioned earlier, silicon has a lower intrinsic carrier concentration compared to germanium, resulting in lower leakage current.
- Higher Thermal Stability: Silicon devices are more thermally stable compared to germanium devices. This means that they can handle higher temperatures without experiencing significant changes in their electrical characteristics.

5. Importance of Low Leakage Current in Thyristors:
Thyristors are used in various applications where precise control of electrical current is required. High leakage current can lead to inaccuracies in these applications and can result in unwanted power losses. Therefore, it is essential to have thyristors with low leakage current to ensure their proper functioning.

Conclusion:
In conclusion, thyristors are not made using germanium because germanium has a higher leakage current compared to silicon. Silicon-based thyristors offer better performance and reliability due to their lower leakage current, higher breakdown voltage, and higher thermal stability.