NEET P-N Junction Diode and its applications MCQs & PYQs

A Zener diode regulates voltage when reverse-biased in the breakdown region (A is correct). The potential barrier for a Silicon p-n junction is around 0.7 V, which is outside the 0.1 V to 0.3 V range (B is incorrect).

Dynamic resistance is defined as \( r_d = \frac{\Delta V}{\Delta I} \). Here, \( \Delta V = 1.2 \text{ V} - 1 \text{ V} = 0.2 \text{ V} \) and \( \Delta I = 20 \text{ mA} - 10 \text{ mA} = 10 \times 10^{-3} \text{ A} \). Thus, \( r_d = \frac{0.2}{10^{-2}} = 20 \ \Omega \).

A photocell is indeed commonly referred to as an 'electric eye' due to its light-sensing ability. So, (A) is true. However, a photocell merely detects the presence or intensity of light; it does not 'see' or form images of objects in front of it in the way a biological eye does. Therefore, (R) is false.

For visible light emission, the photon energy must be within the visible spectrum, which corresponds to energies from approximately \(1.8\) to \(3.1 \text{ eV}\). Thus, Assertion (A) is true. Reason (R) states the range from \(0.4\) to \(1.8 \text{ eV}\), which is incorrect for visible light.

In forward bias, the external voltage opposes the built-in potential barrier, effectively reducing the internal electric field across the depletion region. This reduction in the electric field causes the depletion region to narrow. Hence, both Assertion (A) and Reason (R) are true, and (R) correctly explains (A).

Bridge full-wave rectifiers are more popular because they do not require a costly center-tapped transformer and offer higher output voltage. So Assertion (A) is true. Reason (R) is also true as a bridge rectifier uses four diodes. However, the number of diodes is not the primary reason for its preference over a center-tap rectifier.

Assertion (A) is false; photodiodes rely on \(p-n\) junction semiconductor physics, while photocells (e.g., photoemissive cells) often rely on the photoelectric effect. Reason (R) is also false as their biasing circuits differ significantly (e.g., photodiode often reverse biased, photocell can be forward biased or unbiased).

Assertion (A) is true in terms of performance; GaAs solar cells offer very high efficiency. Reason (R) is also true; the bandgap of GaAs (approx. \(1.42\text{ eV}\,) is near \(1.5\text{ eV}\), which matches well with the peak intensity of the solar spectrum. (R) correctly explains (A) as this optimal bandgap is key to its high efficiency.

Assertion (A) is true; LEDs emit photons when electrons and holes recombine. Reason (R) is false; while carriers cross the depletion region, significant recombination leading to light emission primarily occurs in the quasi-neutral regions (or active layer) under forward bias, not mainly within the depletion region itself.

Assertion (A) is true; in an N-type semiconductor, donor doping increases electron concentration, which, by mass action law (\(np=n_i^2\)), reduces the equilibrium hole concentration. Reason (R) is true; the increased number of electrons in an N-type semiconductor leads to a higher rate of recombination with the minority holes. (R) correctly explains (A) as this increased recombination helps establish and maintain the lower equilibrium hole concentration.