p-n Junction & Diode — MH-CET Physics MCQs with Solutions
Free MH-CET Physics p-n Junction & Diode MCQs with step-by-step solutions (21 questions). Part of Semiconductor Devices. Practise online on Prepizo — no login needed.
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Questions with solutions
Q1 — p-n Junction & Diode · medium · theory
The depletion region of an unbiased p-n junction contains:
A. Only free electrons
B. Only fixed immobile ions ✓ Correct
C. Free electrons and holes in equal numbers
D. Only protons
Solution: Diffusion sweeps the mobile carriers out, leaving the charged donor and acceptor ions behind.
Q2 — p-n Junction & Diode · medium · theory
The potential barrier of a silicon and a germanium p-n junction is approximately:
A. $0.7\text{ V}$ and $0.3\text{ V}$ ✓ Correct
B. $0.7\text{ V}$ and $0.7\text{ V}$
C. $1.1\text{ V}$ and $0.7\text{ V}$
D. $0.3\text{ V}$ and $0.7\text{ V}$
Solution: This is the forward voltage that must be exceeded before appreciable conduction begins.
Q3 — p-n Junction & Diode · easy · theory
When a p-n junction is forward biased:
A. Only minority carriers move
B. The potential barrier is reduced and a large current flows ✓ Correct
C. The potential barrier is increased and no current flows
D. The depletion region widens
Solution: The external supply opposes the built-in field, letting majority carriers cross freely.
Q4 — p-n Junction & Diode · easy · theory
When a p-n junction is reverse biased:
A. The barrier increases and only a small leakage current flows ✓ Correct
B. The junction behaves as a perfect conductor
C. The barrier decreases and a large current flows
D. The depletion region disappears
Solution: The small reverse current is carried by thermally generated minority carriers.
Q5 — p-n Junction & Diode · easy · theory
A junction diode is described as a unidirectional device because it:
A. Conducts only at high frequency
B. Conducts appreciably in one direction only ✓ Correct
C. Never conducts at all
D. Conducts equally in both directions
Solution: This asymmetry is what makes rectification possible.
Q6 — p-n Junction & Diode · medium · theory
The dynamic (AC) resistance of a junction diode is defined as:
A. $\dfrac{\Delta I}{\Delta V}$
B. $\dfrac{I}{V}$
C. $\dfrac{V}{I}$
D. $\dfrac{\Delta V}{\Delta I}$ ✓ Correct
Solution: It is the slope of the characteristic curve at the operating point, and differs from the static resistance $\dfrac{V}{I}$.
Q7 — p-n Junction & Diode · medium · theory
On applying reverse bias to a p-n junction, the width of the depletion region:
A. Remains unchanged
B. Becomes zero
C. Decreases
D. Increases ✓ Correct
Solution: The applied field reinforces the built-in field, pulling the carriers further apart.
Q8 — p-n Junction & Diode · medium · theory
In the circuit symbol of a junction diode, the arrowhead indicates:
A. The direction of conventional current in forward bias ✓ Correct
B. The position of the depletion region
C. The direction of the reverse leakage current
D. The direction of electron flow in forward bias
Solution: The arrow points from the p-side (anode) to the n-side (cathode).
Q9 — p-n Junction & Diode · medium · numerical
A change of $0.05\text{ V}$ across a diode produces a current change of $5\text{ mA}$. Its dynamic resistance is:
A. $250\,\Omega$
B. $100\,\Omega$
C. $0.25\,\Omega$
D. $10\,\Omega$ ✓ Correct
Solution: $r_d = \dfrac{\Delta V}{\Delta I} = \dfrac{0.05}{5 \times 10^{-3}} = 10\,\Omega$.
Q10 — p-n Junction & Diode · medium · numerical
A change of $0.1\text{ V}$ across a diode produces a current change of $2\text{ mA}$. Its dynamic resistance is:
A. $500\,\Omega$
B. $50\,\Omega$ ✓ Correct
C. $20\,\Omega$
D. $0.2\,\Omega$
Solution: $r_d = \dfrac{0.1}{2 \times 10^{-3}} = 50\,\Omega$.
Q11 — p-n Junction & Diode · hard · numerical
A silicon diode in series with a $430\,\Omega$ resistor is connected to a $5\text{ V}$ supply in forward bias. The current is:
A. $100\text{ mA}$
B. $10\text{ mA}$ ✓ Correct
C. $1\text{ mA}$
D. $11.6\text{ mA}$
Solution: The diode drops $0.7\text{ V}$, so $I = \dfrac{5 - 0.7}{430} = \dfrac{4.3}{430} = 10\text{ mA}$.
Q12 — p-n Junction & Diode · hard · numerical
A silicon diode in series with a $1\text{ k}\Omega$ resistor is connected to a $10\text{ V}$ supply in forward bias. The current is:
A. $0.7\text{ mA}$
B. $9.3\text{ mA}$ ✓ Correct
C. $14.3\text{ mA}$
D. $10\text{ mA}$
Solution: $I = \dfrac{10 - 0.7}{1000} = 9.3\text{ mA}$.
Q13 — p-n Junction & Diode · medium · numerical
A silicon diode carries $10\text{ mA}$ at a forward voltage of $0.7\text{ V}$. Its static resistance is:
A. $70\,\Omega$ ✓ Correct
B. $0.07\,\Omega$
C. $700\,\Omega$
D. $7\,\Omega$
Solution: Static resistance $= \dfrac{V}{I} = \dfrac{0.7}{0.01} = 70\,\Omega$.
Q14 — p-n Junction & Diode · medium · numerical
A change of $0.2\text{ V}$ across a diode produces a current change of $10\text{ mA}$. Its dynamic resistance is:
A. $50\,\Omega$
B. $2\,\Omega$
C. $200\,\Omega$
D. $20\,\Omega$ ✓ Correct
Solution: $r_d = \dfrac{0.2}{10 \times 10^{-3}} = 20\,\Omega$.
Q15 — p-n Junction & Diode · medium · numerical
Two identical silicon diodes are connected in series and forward biased. The total voltage drop across them is approximately:
A. $0.7\text{ V}$
B. $0.35\text{ V}$
C. $2.1\text{ V}$
D. $1.4\text{ V}$ ✓ Correct
Solution: Each diode drops about $0.7\text{ V}$, and the drops add in series.
Q16 — p-n Junction & Diode · medium · numerical
The reverse saturation current of a junction diode is typically of the order of:
A. A few milliampere
B. Zero exactly
C. A few microampere ✓ Correct
D. A few ampere
Solution: It is carried by thermally generated minority carriers and is almost independent of the reverse voltage.
Q17 — p-n Junction & Diode · easy · numerical
The forward and reverse resistances of a junction diode are respectively:
A. Both very high
B. Both very low
C. Very high and very low
D. Very low and very high ✓ Correct
Solution: This large ratio, often $10^5$ or more, is what makes the diode an effective one-way valve.
Q18 — p-n Junction & Diode · hard · numerical
A diode has a forward resistance of $20\,\Omega$ and a reverse resistance of $200\text{ k}\Omega$. The ratio of reverse to forward resistance is:
A. $10^5$
B. $10^3$
C. $10^2$
D. $10^4$ ✓ Correct
Solution: $\dfrac{2 \times 10^5}{20} = 10^4$.
Q19 — p-n Junction & Diode · hard · numerical
The width of the depletion region of a typical p-n junction is of the order of:
A. $1\text{ m}$
B. $1\,\mu\text{m}$ ✓ Correct
C. $1\text{ cm}$
D. $1\text{ mm}$
Solution: It is a very thin layer, which is why even modest voltages produce enormous fields across it.
Q20 — p-n Junction & Diode · hard · numerical
A germanium diode in series with a $470\,\Omega$ resistor is connected to a $5\text{ V}$ supply in forward bias. The current is approximately:
A. $11\text{ mA}$
B. $0.6\text{ mA}$
C. $10\text{ mA}$ ✓ Correct
D. $9.1\text{ mA}$
Solution: Germanium drops about $0.3\text{ V}$, so $I = \dfrac{5 - 0.3}{470} = \dfrac{4.7}{470} = 10\text{ mA}$.
Q21 — p-n Junction & Diode · medium · numerical
In forward bias, the current through a junction diode is carried mainly by:
A. Photons
B. Minority carriers only
C. Immobile ions
D. Majority carriers crossing the junction ✓ Correct
Solution: The reduced barrier allows the plentiful majority carriers on each side to diffuse across.