NEET Properties of Semiconductors MCQs & PYQs

NPN transistors are widely preferred because their majority charge carriers, electrons, have significantly higher mobility than holes. Higher electron mobility allows for faster switching speeds and better high-frequency performance, making NPN transistors generally more efficient and widely used. Therefore, Assertion (A) is true, Reason (R) is true, and R is the correct explanation for A.

Concept: Semiconductor conductivity.
Intrinsic semiconductors have fewer free charge carriers than extrinsic (doped) semiconductors, so (A) is true. Increasing temperature generates more carriers in semiconductors, increasing conductivity, so (R) is true. However, (R) describes temperature dependence, not the difference between intrinsic and extrinsic. Thus, (R) is not the correct explanation for (A).

Concept: Ohm's Law and current definition.
Semiconductors are non-ohmic devices, so (A) is true. Electric current is indeed the rate of flow of charge, \(I = \frac{dQ}{dt}\), so (R) is true. However, (R) is a definition of current and does not explain why semiconductors are non-ohmic. Thus, (R) is not the correct explanation of (A).

Concept: Temperature dependence of resistance.
Metals have a positive temperature coefficient of resistance, while semiconductors have a negative one, so (A) is true. In metals, drift velocity *decreases* with increasing temperature due to increased scattering. In semiconductors, carrier concentration increases, so (R) is false.

All semiconductors (p-type or n-type) have a negative temperature coefficient of resistance, so (A) is false. In p-type semiconductors, majority carriers are holes, and in n-type, they are electrons. Reason (R) swaps these, so it is also false. Thus, both (A) and (R) are false.

Assertion (A) is true: The probability of finding electrons in the conduction band is proportional to \(e^{-E_g / (2kT)}\), decreasing exponentially with band gap \(E_g\).

Reason (R) is true: A larger band gap means more energy is required for electrons to jump. Reason (R) correctly explains Assertion (A).

Germanium is a tetravalent semiconductor. To create a p-type semiconductor, it must be doped with a trivalent impurity like Boron.

At absolute zero (0 K), electrons in a semiconductor like Silicon do not have any thermal energy to cross the band gap. The conduction band remains completely empty, making it act as a perfect insulator.

For both insulators and semiconductors, the number density of charge carriers increases exponentially with temperature. This causes their resistance to decrease, meaning they have a negative temperature coefficient of resistance.

Carbon in the form of diamond is an insulator with a very large band gap of around 5.4 eV. Silicon and germanium are semiconductors with much smaller band gaps of 1.1 eV and 0.7 eV respectively. Therefore, $(E_g)_C > (E_g)_{Si}$.