LED, photodiode and solar cell — NEET & JEE Questions

Question 1 · difficulty L1 · recall

An LED emits light when it is

• A. reverse biased far below breakdown only
• B. unbiased
• C. forward biased
• D. connected as an open circuit

Question 2 · difficulty L1 · understanding

A photodiode is commonly operated in

• A. strong forward bias as a heater
• B. zero bias only in every application
• C. AC breakdown
• D. reverse bias

Question 3 · difficulty L2 · understanding

A solar cell converts

• A. light energy directly into electrical energy
• B. electrical energy into nuclear energy
• C. heat into sound only
• D. DC into AC without a circuit

Question 4 · difficulty L2 · application

The colour of an LED is primarily related to

• A. the resistor colour code
• B. the semiconductor band gap
• C. the wire length only
• D. ambient magnetic declination

Question 5 · difficulty L2 · understanding

A light-emitting diode (LED) is operated in

• A. reverse bias, so that minority carriers recombine
• B. forward bias, so that carriers injected across the junction recombine and emit photons
• C. zero bias, like a solar cell
• D. reverse breakdown, like a Zener diode

Question 6 · difficulty L3 · application

Increasing illumination on a reverse-biased photodiode generally

• A. decreases it to zero
• B. changes only the diode capacitance with no current effect
• C. increases photocurrent magnitude
• D. reverses majority-carrier types permanently

Question 7 · difficulty L3 · application

A solar cell can deliver power without an external bias because

• A. it is a chemical battery hidden in the package
• B. light eliminates the p-n junction
• C. its resistance becomes negative at all points
• D. its built-in junction field separates photogenerated carriers

Question 8 · difficulty L3 · application

An LED is made from a semiconductor with band gap 2.0 eV. Taking hc = 1240 eV nm, the wavelength of the light it emits is about

• A. 310 nm
• B. 1240 nm
• C. 620 nm
• D. 2480 nm

Question 9 · difficulty L3 · application

A photodiode is fabricated from a semiconductor with band gap 2.8 eV. The longest wavelength of light it can detect is approximately

• A. 620 nm
• B. 880 nm
• C. 280 nm
• D. 440 nm

Question 10 · difficulty L4 · analysis

Why are LEDs generally more energy-efficient than incandescent lamps for producing visible light?

• A. A larger fraction of electrical energy can be converted directly into photons rather than heat
• B. LEDs contain no resistance whatsoever
• C. incandescent filaments emit no visible light
• D. LEDs violate energy conservation