NEET Semiconductor Physics MCQs & PYQs

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.

The maximum wavelength that can be detected is given by $\lambda = \frac{hc}{E_g}$. Substituting the values gives $\lambda = \frac{12400}{2.5} = 4960 \AA$. The photodiode can detect wavelengths less than this maximum limit, so $4000 \AA$ is the correct answer.

The minimum frequency is $\nu = \frac{E_g}{h}$. Converting energy to Joules gives $E_g = 2.0 \times 1.6 \times 10^{-19} J$. Thus $\nu = \frac{3.2 \times 10^{-19}}{6.63 \times 10^{-34}} \approx 4.8 \times 10^{14}$ Hz, which is nearly $5 \times 10^{14}$ Hz.

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, $(E_g)_C > (E_g)_{Si}$.

In semiconductors, as temperature increases, more electrons acquire enough thermal energy to jump to the conduction band. This increases conductivity and decreases resistivity. Therefore, the statement that resistivity increases with temperature is false.

At absolute zero, the valence band is completely full and the conduction band is completely empty. At room temperature, thermal energy excites a fraction of electrons from the valence band to the conduction band, leaving holes behind. Thus both bands are partially filled.

Copper is a metal (conductor), so its resistance decreases when cooled due to decreased lattice scattering. Germanium is a semiconductor, so its resistance increases when cooled because fewer charge carriers are thermally excited across the band gap.

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.

Current is directly proportional to the mobility of the charge carriers. Since free electrons have a significantly higher mobility than holes in the crystal lattice, the drift velocity and hence the current in the n-type semiconductor will be greater.

In an n-type semiconductor, electrons are the majority charge carriers and holes are the minority charge carriers. Therefore, the statement that majority carriers are holes is false.[cite: 1]