Thermal runaway in a transistor biased in the active region is due to 1. heating of the transistor. 2. change in β due to an increase in temperature. 3. change in reverse collector saturation current due to a rise in temperature. 4. base-emitter voltage VBE which decreases with rise in temperature. Which of the above statements is/are correct?

Option 2 : 2 and 3

Explanation:

Thermal runaway

The excess heat produced at the collector-base junction may even burn and destroy the transistor. This situation is called the “Thermal Runaway” of the transistor.

The collector current is defined as

I_C  = β I_B + (1 + β) I_C0

I_C0: leakage current and this depends on temperature.

• If the temperature is increased then I_C0 increases.
• If I_C0 increases then collector current also increases

As T ↑ ⇒ I_co ↑ ⇒ I_c ↑ ⇒ P_T ↑ ⇒ T ↑

This is a Cumulative process.

Statement 3 is correct

β depends on various parameters like

• Temperature
• Frequency
• Process variation etc.

∴ Thermal runaway is also possible for a change in β with temperature. Statement 2 is correct.

For Silicon transistor, this temperature is in the range 150^° to 225^° C and for Germanium, it is between 60^° to 100^° C.