Given below are two statements : One is labelled as Assertion A and the other is labelled as Reason R : Assertion (A) : Power MOSFETs have a positive temperature coefficient, which combats the possibility of thermal runaway...

90) Given below are two statements : One is labelled as Assertion A and the other is labelled as Reason R :

Assertion (A) : Power MOSFETs have a positive temperature coefficient, which combats the possibility of thermal runaway

Reason (R) : Negative temperature coefficient results in decreased level of resistance with increase in temperature and causes thermal runaway

In the light of the above statements, choose the correct answer from the options given below :

(1) Both (A) and (R) are true and (R) is the correct explanation of (A)

(2) Both (A) and (R) are true but (R) is not the correct explanation of (A)

(3) (A) is true but (R) is false

(4) (A) is false but (R) is true

Answer: A

Solution

Assertion (A) is True: Power MOSFETs exhibit a positive temperature coefficient of resistance. As the temperature of a MOSFET increases, the mobility of the majority carriers decreases, which causes the "ON" resistance (
```
 $R_{D S (o n)}$ 
```
) to increase. This increase in resistance naturally limits the current flow, thereby preventing the uncontrolled heating cycle known as thermal runaway. This makes MOSFETs easy to connect in parallel.
Reason (R) is True: A negative temperature coefficient (found in devices like BJTs) means that as temperature increases, resistance decreases. This leads to higher current, more heat generation, and even lower resistance, creating a self-destructive loop called thermal runaway.
Connection: Reason (R) correctly explains why the positive coefficient mentioned in (A) is a beneficial property for avoiding the phenomenon described in (R). It provides the scientific context for why the behavior in Assertion (A) is desirable.

Conclusion:
Both (A) and (R) are true, and (R) is the correct explanation of (A).