AIIMS 2006 Physics Transistors MCQ Question

To understand the minimum potential difference between the base and emitter required to switch a silicon transistor ON, we need to delve into the basic operation of bipolar junction transistors (BJTs), specifically NPN or PNP types.

Explanation of the Correct Answer

A silicon transistor is a type of BJT that requires a certain threshold voltage to conduct. This threshold is often referred to as the base-emitter voltage ($V_{BE}$). For silicon transistors, this minimum voltage typically ranges around 0.6 V to 0.7 V.

Thus, the minimum potential difference between the base and emitter required to switch a silicon transistor ON is approximately 1 V. This value accounts for slight variations and losses in practical applications, but for ideal conditions, 0.7 V is a good rule of thumb.

Therefore, the correct answer is:

A) 1 V

Clarification of Other Options

Let’s examine why the other options are incorrect:

• B) 3 V: This value is significantly higher than the typical threshold voltage for silicon transistors. Such a high base-emitter voltage would not be necessary to turn the transistor ON and could lead to excessive power dissipation and potential damage to the transistor.

• C) 5 V: Similar to option B, a base-emitter voltage of 5 V is much greater than what is typically needed. While a transistor can handle higher voltages, it does not require them to be switched ON, making this option impractical for standard operation.

• D) 4.2 V: Again, this value is well above the range needed to turn ON a silicon transistor. Like options B and C, using such a high voltage could risk damaging the transistor or lead to inefficient operation.

Relevant Concepts and Formulas

The operation of a silicon transistor in the active region is primarily described by the Shockley equation, which gives the relationship between the collector current ($I_C$), the base-emitter voltage ($V_{BE}$), and the saturation current ($I_S$):

$$I_C = I_S \left( e^{\frac{V_{BE}}{V_T}} - 1 \right)$$

Where:

• $I_C$ is the collector current,
• $I_S$ is the saturation current,
• $V_T$ is the thermal voltage (approximately 26 mV at room temperature).

To ensure that the transistor is in the active region, it is necessary to apply a base-emitter voltage ($V_{BE}$) of at least around 0.7 V for silicon. The exponential nature of the equation means that even small changes in $V_{BE}$ lead to significant changes in $I_C$, reinforcing the importance of reaching that minimum threshold.

Conclusion

In summary, the minimum potential difference between the base and emitter required to switch a silicon transistor ON is approximately 1 V. This understanding is crucial for those working with BJTs in electronics and circuit design, as it helps ensure proper functioning and prevents damage to the components.