Assertion & Reason Test: Semiconductor Electronics - Grade 12 MCQ

Conductors – They are materials that have a lot of free electrons and as the flow of current is essentially the flow of electrons in a material, thus, we say that conductors have very less resistance and can conduct more current. Example – Metals, tap water, etc.

Insulators - They are materials that have no free electrons, thus, we say that insulators have very high resistance and do not conduct electricity. Example - Wood, plastic, etc.

Semiconductors – Semiconductors lie somewhere in between conductors and insulators. They have few free electrons that can lead to the flow of current. Semiconductors are essentially a crystal lattice in which the atoms are grouped close together. Example – Silicon, Gallium, etc.

Semiconductors are very unique, their ability of conducting electricity can change when impurities are introduced in the crystal lattice (generally a small percentage of another element is added to the semiconductor).
The free electrons that can conduct electricity are said to be in conduction band, basically all the electrons in the conduction band have enough energy to move away from their atoms and conduct electricity.
Now, increasing the temperature will increase the energy of more electrons, increasing the total number of free electrons in the conduction band, this will result in increase in conductivity and a decrease in resistivity.
Thus, both the assertion and the reasons are false as increase in temperature leads to a decrease in resistivity.

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.