Semiconductors and Electronic Devices Test - 11

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.

Zener diode is used as a voltage regulator in reverse biased condition. So, both assertion and reason are true, but the reason does not explain the assertion.

When a diode is used as a rectifier, it has to face both positive and negative halves of the alternating voltage.

In p-type semiconductor, electron is the minority charge carrier. So, number of electrons is less than the number of electrons in intrinsic semiconductor. So the assertion is true. According to the law of mass action = n²_i = n_e × n_h . In intrinsic semiconductor, n_e = n_h . So in p-type semiconductor, ne

As temperature rises, the electrons of valence band sufficient energy and jump to the conduction band. Thus, the resistivity decreases. So the assertion is true.

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.

Semiconductors do not obey Ohm's law. So the assertion is true. V-I characteristic of a semiconductor is non-linear. Hence the reason is false.

In forward bias condition, the depletion region of a p-n junction diode contracts and the majority charge carriers can cross the junction very easily. So, the resistance becomes low (ideally 0).

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.

When a p-n junction is reverse biased, then the majority charge carriers cannot cross the junction. So, no forward current flows. But in reverse direction, a feeble current flows which is known as reverse saturation current. So, the assertion is true.

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.

Electrons experience many collisions. The mobility of electrons is more because they require less energy to move, not because they are lighter.