**Assertion (A): FET does not suffer from thermal breakdown.**
**Reason (R): FET has a positive temperature coefficient for resistance.**

The correct answer is option 'A', i.e., Both A and R are true and R is the correct explanation of A.

**Explanation:**

**Field Effect Transistor (FET):**
A Field Effect Transistor (FET) is a three-terminal semiconductor device that can amplify or switch electronic signals and electrical power. FETs are widely used in various electronic devices due to their high input impedance, low noise, and low power consumption.

**Thermal Breakdown:**
Thermal breakdown refers to a phenomenon where excessive heat causes a device to malfunction or fail. In the case of transistors, thermal breakdown can occur when the temperature rise exceeds the safe operating limits, leading to reduced performance or permanent damage.

**Positive Temperature Coefficient:**
The temperature coefficient of resistance (TCR) indicates how the resistance of a material changes with temperature. A positive temperature coefficient means that the resistance increases with an increase in temperature.

**Explanation of Assertion (A):**
FET does not suffer from thermal breakdown. This statement is true because of the following reasons:
- FETs are constructed using a different technology compared to Bipolar Junction Transistors (BJTs). FETs are based on the principle of field effect, where the output current is controlled by an electric field, whereas BJTs are based on the principle of current control.
- Unlike BJTs, FETs do not have a PN junction, which is more susceptible to thermal breakdown due to temperature-related effects.
- FETs are made of materials like silicon or gallium arsenide, which have a high melting point and can withstand higher temperatures without significant degradation.
- FETs have a higher safe operating temperature range compared to BJTs, reducing the chances of thermal breakdown.

**Explanation of Reason (R):**
FET has a positive temperature coefficient for resistance. This statement is true because of the following reasons:
- The resistance of a FET increases as the temperature increases. This positive temperature coefficient helps in stabilizing the operating point of the device.
- When the temperature rises, the mobility of charge carriers (electrons or holes) decreases, leading to an increase in resistance.
- This positive temperature coefficient helps in reducing the chances of thermal runaway, where an increase in temperature leads to an increase in current, further increasing the temperature and causing failure.

**Conclusion:**
Both Assertion (A) and Reason (R) are true, and Reason (R) correctly explains Assertion (A). FETs are less prone to thermal breakdown due to their construction and material properties, and the positive temperature coefficient of resistance helps in stabilizing their performance.