Assertion & Reason Test: Semiconductor Electronics - Grade 12 MCQ

To solve the given question, we will analyze both the assertion and the reason provided.

Step 1: Understand the Assertion
The assertion states that "The resistivity of a semiconductor decreases with temperature."

- Explanation: In semiconductors, as the temperature increases, more charge carriers (electrons and holes) are generated due to the breaking of covalent bonds. This increase in charge carriers enhances the conductivity of the semiconductor. Since resistivity (ρ) is inversely related to conductivity (σ), we can conclude that as conductivity increases, resistivity decreases.

Step 2: Understand the Reason
The reason states that "The atoms of a semiconductor vibrate with larger amplitude at higher temperature thereby increasing its resistivity."

- Explanation: While it is true that atoms in a semiconductor vibrate more at higher temperatures, this does not lead to an increase in resistivity. Instead, the increased vibration can lead to more collisions among charge carriers, but the primary effect at higher temperatures is the generation of more charge carriers. Therefore, this reasoning is incorrect because it contradicts the assertion.

Silicon is preferred over Germanium for making semiconductor devices. The assertion is true.

The energy gap of Germanium is about 0.7 eV, where as the energy gap of Silicon is 1.1 eV. Hence, the reason is false.

In semiconductors the energy gap between conduction band and valence band is small. Due to this, the electrons in the conduction band can gain sufficient thermal energy with temperature rise and can easily jump across the small energy gap to reach the conduction band. Thus, conductivity increases and resistance decreases.

In reverse bias condition, the depletion region of p-n junction diode expands and the majority charge carriers cannot cross the junction. So, the resistance increases (ideally ∞).

So, the assertion and reason both are true and reason explains the assertion.

In p-side there are few electrons as minority charge carrier and in n-side, there are few holes as minority charge carriers. In reverse bias condition, the holes at n-side feel a pull exerted by the negative polarity of the voltage source connected to the p-side. Similarly, the electrons at p-side feel a pull exerted by the positive polarity of the voltage source connected to the n-side. So, these minority carries now can cross the junction and give rise to a feeble current in the opposite direction. Hence, the reason is also true and it explains the assertion.