LED, photodiode and solar cell — practice 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 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 6 · 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 7 · 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

Question 8 · difficulty L4 · analysis

A photodiode is placed in darkness and then illuminated more strongly at fixed reverse bias. Its I-V operating point shifts mainly by

• A. a large decrease in reverse current
• B. an increase in reverse current magnitude
• C. a change from reverse to forward bias automatically
• D. zeroing the junction field

Question 9 · difficulty L5 · analysis

A solar cell's output power is zero at both

• A. every finite load
• B. maximum-power point only
• C. short circuit and open circuit
• D. one-quarter of open-circuit voltage only

Question 10 · difficulty L5 · analysis

An LED connected directly to a voltage source without current limiting may be damaged because

• A. its reverse current is always infinite
• B. light emission requires zero current
• C. its band gap becomes zero at any voltage
• D. its forward I-V curve can allow a rapid current increase