Q4. A red LED and a blue LED are each joined in turn, in forward bias, to a variable supply. Which starts to conduct at the lower voltage, and why?
The red LED. Its photons carry less energy, about 1.9 eV against about 2.7 eV for blue, so its band gap, and with it its threshold voltage, is smaller (reasoned; thresholds from Wikipedia, “Light-emitting diode”).
Q5. Assertion: an LED connected in reverse across a 9 V battery may be damaged. Reason: for an LED rated at a maximum reverse voltage of 5 V, 9 V exceeds its permitted operating limit. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: 9 V exceeds the 5 V reverse rating, and a typical LED breaks down near 5 V in reverse, above which a large current flows and damages it (Wikipedia, “Light-emitting diode”). Strictly, the rating is a limit not to be exceeded, not the measured breakdown voltage (Kingbright WP7113ID datasheet).
Q6. A solar cell gives a short-circuit current of 2.4 A in full sunlight. Estimate its short-circuit current when the light is half as intense at the same spectrum and temperature. What happens to its open-circuit voltage?
About 1.2 A, since ISC is proportional to the intensity. VOC falls only slightly, because it grows with the logarithm of the light-generated current (Wikipedia, “Theory of solar cells”).
Q7. How many solar cells, each of open-circuit voltage 0.55 V, must be joined in series to give an open-circuit voltage of 13.2 V?
13.2/0.55 = 24 cells, since voltages in series add.
Q8. Light of wavelength 1000 nm falls on a silicon photodiode (Eg = 1.1 eV). Is a photocurrent produced?
Yes. E ≈ 1240/1000 = 1.24 eV, which is more than 1.1 eV, so each photon can make an electron-hole pair; 1000 nm is inside silicon’s cut-off of about 1130 nm.
Q9. Name the device: (a) gives out light when forward biased; (b) is reverse biased, and its reverse current rises with the light falling on it; (c) needs no external bias source and delivers power from light.
(a) LED; (b) photodiode; (c) solar cell.