Free Electronics MCQs with Answers

11. A Zener diode is normally operated

• A. in forward bias as a rectifier
• B. in reverse bias at its breakdown voltage, as a voltage regulator
• C. with no bias at all
• D. only at very high frequency

Explanation: Beyond the breakdown voltage a Zener conducts heavily while the voltage across it stays almost constant, which makes it a simple and effective voltage reference. Unlike an ordinary diode this breakdown is non destructive provided a series resistor limits the current. In forward bias it behaves like any other silicon diode.

12. A light emitting diode produces light when

• A. it is reverse biased and breaks down
• B. it is forward biased and electrons recombine with holes, releasing energy as photons
• C. it is heated
• D. current passes through a filament inside it

Explanation: Each recombination across the junction releases an energy equal to the band gap of the semiconductor, and in materials such as gallium arsenide phosphide that energy falls in the visible range. The colour therefore depends on the band gap of the material rather than on any coating. Because no filament is heated, an LED wastes far less energy than an incandescent lamp.

13. A photodiode is used to detect light because incident photons

• A. heat the junction
• B. create extra electron hole pairs, increasing the reverse current
• C. reduce the leakage current to zero
• D. change the doping of the material

Explanation: Photons with enough energy free electrons across the band gap in the depletion region, so the reverse current rises in proportion to the light intensity and gives a measurable signal. The device is therefore operated in reverse bias, the opposite of an LED. Solar cells work on the same principle but are designed to deliver power rather than a signal.

14. Compared with a conductor, a pure semiconductor at room temperature has

• A. far fewer free charge carriers, and a conductivity that rises with temperature
• B. more free carriers
• C. the same number of carriers
• D. no carriers at any temperature

Explanation: A pure semiconductor has a small energy gap, so only a few electrons have enough thermal energy to cross it at room temperature, but heating promotes many more and conductivity climbs sharply. In a metal the carrier number is fixed and heating only increases scattering, so resistance rises. Doping raises the carrier concentration enormously without needing heat.

15. The energy gap between the valence band and the conduction band is largest in

• A. a conductor
• B. a semiconductor
• C. an insulator
• D. all three equally

Explanation: An insulator has a gap of several electron volts, far more than thermal energy can supply, so almost no electrons reach the conduction band. A semiconductor's gap is about 1 eV, small enough for a useful number to cross, and in a conductor the bands overlap so there is effectively no gap at all. This single picture explains the huge range of electrical behaviour in solids.

16. In an n type semiconductor, the minority carriers are

• A. electrons
• B. holes
• C. protons
• D. positive ions

Explanation: Doping supplies a large number of electrons, but thermal generation still creates a small population of holes, and these minority carriers are what produce the leakage current in a reverse biased junction. Their number rises steeply with temperature, which is why semiconductor devices become unreliable when they overheat. In p type material the roles are exactly reversed.

17. The ripple frequency at the output of a full wave rectifier fed from a 50 Hz supply is

• A. 25 Hz
• B. 50 Hz
• C. 100 Hz
• D. 200 Hz

Explanation: Both halves of each cycle produce an output pulse, so there are two pulses per input cycle and the ripple appears at twice the supply frequency. A half wave rectifier would give ripple at 50 Hz instead. The higher ripple frequency is one reason full wave output is easier to smooth.

18. Silicon is preferred to germanium in most modern devices because silicon

• A. has a lower barrier potential
• B. tolerates higher temperatures and has a much smaller leakage current
• C. is a better conductor
• D. requires no doping

Explanation: Silicon's larger band gap means far fewer thermally generated carriers, so leakage is low and devices work reliably at higher temperatures, and it also forms a stable oxide that makes integrated circuits possible. Germanium's lower barrier of 0.3 V is an advantage in some low voltage detector circuits, which is why it has not vanished entirely. Silicon is also abundant and cheap.

19. A transistor is used as an amplifier because a small change in the base current produces

• A. no change in the collector current
• B. a much larger change in the collector current
• C. a smaller change in the collector current
• D. a change only in the emitter voltage

Explanation: The current gain of a common emitter transistor is typically between 50 and 300, so a weak input signal at the base controls a much larger collector current and the extra energy comes from the power supply. The transistor does not create energy; it controls the flow of it. Used in saturation and cut off instead, the same device acts as a switch.

20. In a common emitter transistor circuit, the emitter base junction and the collector base junction are respectively

• A. forward biased and reverse biased
• B. both forward biased
• C. both reverse biased
• D. reverse biased and forward biased

Explanation: Forward bias on the emitter base junction injects carriers into the thin lightly doped base, and the reverse biased collector base junction then sweeps almost all of them across, which is what produces the current gain. Biasing both junctions forward drives the transistor into saturation, the fully on switch state, and both reverse gives cut off. Correct biasing is the first requirement of any amplifier design.