Question 31:
easyAssertion (A): Output frequency of time varying DC voltage in a full wave rectifier is twice of input frequency.
Reason (R): A center tap transformer increases the frequency of input.
A full-wave rectifier converts both halves of an AC input into a pulsating DC output, effectively doubling the input frequency. Thus, Assertion (A) is true. A transformer only steps up or steps down voltage and current; it does not alter the frequency of the AC signal. Hence, Reason (R) is false.
Question 32:
easyAssertion (A): Light emitting diode (LED) emits self radiation.
Reason (R): LED are reverse biased p-n junctions.
Concept: LED operation.
LEDs are \(p-n\) junctions that emit light when forward biased due to electron-hole recombination. Reverse biasing does not cause light emission. Thus, Assertion (A) is true, but Reason (R) is false.
Question 33:
easyAssertion (A): Avalanche breakdown dominates when the doping concentration is high and depletion layer is thin.
Reason (R): Zener breakdown occurs due to the collision of minority charge carrier.
Concept: \(p-n\) junction breakdown.
Avalanche breakdown occurs in lightly doped junctions with wider depletion regions. Zener breakdown occurs in heavily doped junctions due to quantum tunneling, not minority carrier collisions. Thus, both (A) and (R) are false.
Question 34:
easyAssertion (A): The logic gate NOT can not be built using diode.
Reason (R): The output voltage and the input voltage of the diode does not have \(180^{\circ}\) phase difference.
Assertion (A) is true because a NOT gate requires active components to provide inversion and gain, which a passive diode cannot do. Reason (R) is true because a diode circuit does not inherently provide the \(180^{\circ}\) phase shift characteristic of logic inversion. Thus, (R) correctly explains (A).
Question 35:
easyIn a reverse biased diode when the applied voltage changes by \(5 \text{V}\), the current is found to change by \(2 \mu\text{A}\). The reverse bias resistance of the diode is:
Reverse resistance is given by \(R = \frac{\Delta V}{\Delta I}\). Substituting the values, \(R = \frac{5 \text{V}}{2 \times 10^{-6} \text{A}} = 2.5 \times 10^6 \Omega\).
Question 36:
easyIn the following questions, a statement of Assertion (A) is followed by a statement of Reason (R)
Assertion (A): The photo-diode can be used as a photodetector to detect optical signals.
Reason (R): It is easier to observe the change in the current with change in the light intensity, if a forward bias is applied to photodiode.
Photo-diodes are operated in reverse bias because the fractional change in reverse current upon illumination is far larger and more easily measured than in forward bias.
Question 37:
easyA p-n photodiode is fabricated from a semiconductor with a band gap of 2.5 eV. It can detect a signal of wavelength
(2009)
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.
Question 38:
easyA p-n photodiode is made of a material with a band gap of 2.0 eV. The minimum frequency of the radiation that can be absorbed by the material is nearly
(2008)
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.
Question 39:
easyChoose the only false statement from the following
(2005)
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.
Question 40:
easyThe electron concentration in an n-type semiconductor is the same as hole concentration in a p-type semiconductor. An external field (electric) is applied across each of them. Compare the currents in them.
(2021)
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.