Free PN Junction MCQs with Answers

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.