Class 12 Physics, Chapter 14 (NCERT). 15 questions by concept, 2 with pictures. Open “Show answer” for the answer and why.
How to teach this chapter · Common misconceptions & worked visual for this chapter
Diamond, silicon and germanium each have four valence electrons per atom. Which band-gap ordering is correct near room temperature?
(a) Eg(diamond)>Eg(Si)>Eg(Ge)
Equal valence count does not determine equal crystal band gaps: approximately 5.4,1.1 and 0.7 eV respectively.
A student says diamond must conduct as well as silicon because both atoms are tetravalent. Which response is correct?
(d) Their crystal band gaps differ greatly, so thermal carrier populations differ
Band structure depends on bonding in the solid, not valence count alone. Diamond has a much wider gap.
In an intrinsic semiconductor, electrons drift left and holes drift right under an electric field. How do their conventional-current contributions combine?
(b) Both point right and add
Conventional electron current is opposite electron velocity; hole current follows hole velocity.
What effective charge is assigned to a hole in a silicon valence band?
(b) +e
A hole is the vacancy left by an electron and behaves as an effective positive charge.
An n-type crystal contains far more conduction electrons than holes. What is its overall charge in the usual neutral bulk condition?
(c) Zero; donor ion cores balance the extra electron charge
Mobile carrier imbalance is compensated by the fixed ionised dopant cores.
Which dopant type produces p-type silicon with holes as majority carriers?
(c) Trivalent acceptors such as boron
A trivalent substitution leaves an available acceptor state and an associated valence-band hole.
Why can a depletion region create an electric field even though it lacks most mobile majority carriers?
(b) Immobile positive donor and negative acceptor ions remain
Carrier depletion leaves fixed space charge: positive on n and negative on p.
In an unbiased p-left, n-right junction, which fixed-charge arrangement is present near the junction?
(c) Negative ions on p; positive ions on n
Ionised acceptors remain on p and ionised donors on n after mobile carriers diffuse and recombine.
What happens to barrier height and depletion width when forward bias is increased within the usual operating range?
(b) Both decrease
Forward bias opposes the built-in potential and reduces the depleted barrier.
A junction has built-in barrier 0.7 V. A forward bias 0.2 V is applied. In the elementary barrier model, what is the remaining barrier?
(b) 0.5 V
Forward bias subtracts from the built-in barrier:0.7−0.2=0.5 V.
The graph gives a locally straight part of a diode’s forward characteristic. What is the approximate dynamic resistance near 15 mA?
(c) 10 Ω
Use the local changes:ΔV=0.1 V and ΔI=10 mA=0.010 A, giving 10 Ω. The dc ratio at 0.75 V and 15 mA is 50 Ω and is a different quantity.
A diode’s reverse voltage reaches breakdown while an external resistor limits current safely. What happens to reverse current?
(d) It rises sharply
The reverse-open approximation fails at breakdown; the current-limiting circuit controls the resulting large reverse current.
Why must an ordinary diode rectifier have a reverse-voltage rating above the applied reverse peak?
(d) Otherwise reverse breakdown may cause large current and overheating
A sufficiently large reverse voltage causes breakdown, so ignoring reverse current indefinitely is unsafe circuit reasoning.
A student places the only smoothing capacitor in series between rectifier output and load. Which correction permits the capacitor to supply the load between charging peaks?
(c) Move the capacitor across the load terminals
The smoothing capacitor should be connected in parallel across the load terminals, with the correct polarity if it is electrolytic.
Near a rectified-voltage peak, the source charges the capacitor. Between peaks, the capacitor discharges through the load and helps maintain the load voltage.
A capacitor in series would block steady direct current instead of serving as a reservoir across the load. The correction is to move it across the load terminals.
Where should a smoothing capacitor be connected in a rectifier with a resistive load?
(b) In parallel with the load
Across the load it charges near peaks and supplies load current between peaks.
Wrong
A silicon diode behaves like a short circuit at any forward voltage.
Why it feels right. The circuit symbol suggests a simple one-way switch.
Right
Real forward current grows strongly with forward voltage. The ideal switch is an approximation.
Worked example
