PN Junction Diode MCQs

Practice PN Junction Diode MCQs with four options, answers and explanations for focused Electronics & Telecommunication Engineering revision.

Question 1

Electronic Devices & CircuitsPN Junction Diode

The depletion region of a PN junction mainly contains?

AOnly freely moving electrons
BOnly freely moving holes
CImmobile ionized donor and acceptor atoms
DNo electric field

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Correct Answer: C. Immobile ionized donor and acceptor atoms

Explanation: Carrier diffusion near the junction leaves uncovered fixed ionized donors and acceptors, forming the space-charge or depletion region.

Reference: Standard semiconductor electronics: PN junction

Question 2

Electronic Devices & CircuitsPN Junction Diode

Forward biasing a PN junction generally causes the depletion width to?

AIncrease
BDecrease
CRemain exactly unchanged
DBecome infinite

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Correct Answer: B. Decrease

Explanation: Forward bias opposes the built-in junction field, reducing the barrier potential and narrowing the depletion region.

Reference: Standard semiconductor electronics: diode biasing

Question 3

Electronic Devices & CircuitsPN Junction Diode

The approximate forward voltage drop of a conducting silicon diode at ordinary current levels is commonly taken as?

A0.1 V
B0.3 V
C0.7 V
D5 V

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Correct Answer: C. 0.7 V

Explanation: For basic circuit analysis, a forward-biased silicon PN diode is commonly approximated as having about a 0.7 V drop, although the actual value depends on current and temperature.

Reference: Standard electronics: silicon diode approximation

Question 4

Electronic Devices & CircuitsPN Junction Diode

For a silicon PN diode carrying current in forward bias, an increase in junction temperature generally causes the forward voltage required for the same current to?

AIncrease
BDecrease
CRemain exactly constant
DBecome equal to the reverse breakdown voltage

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Correct Answer: B. Decrease

Explanation: For a silicon diode at a given forward current, the forward voltage typically decreases as junction temperature rises; a commonly used approximate coefficient is about -2 mV/°C near ordinary operating conditions.

Reference: Standard semiconductor electronics: diode temperature behavior

Question 5

Electronic Devices & CircuitsPN Junction Diode

In the ideal-diode approximation, the forward-biased diode is modeled as?

AAn open circuit
BA short circuit
CA current source only
DA capacitor only

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Correct Answer: B. A short circuit

Explanation: An ideal forward-biased diode has zero voltage drop and zero resistance, so it is represented as a short circuit.

Reference: Standard circuit analysis: ideal diode model

Question 6

Electronic Devices & CircuitsPN Junction Diode

The dynamic (small-signal) resistance of a forward-biased diode at room temperature is approximately VT/ID. If ID = 1 mA and VT = 25 mV, rd is?

A2.5 ohm
B25 ohm
C250 ohm
D25 kOhm

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Correct Answer: B. 25 ohm

Explanation: Using rd ≈ VT/ID gives 25 mV / 1 mA = 25 ohm.

Reference: Standard semiconductor electronics: diode small-signal resistance

Question 7

Electronic Devices & CircuitsPN Junction Diode

A silicon diode with a 0.7 V constant-drop model is connected in series with a 1 kOhm resistor to a 5 V DC source in forward bias. The diode current is approximately?

A0.7 mA
B4.3 mA
C5 mA
D5.7 mA

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Correct Answer: B. 4.3 mA

Explanation: The resistor sees approximately 5 - 0.7 = 4.3 V, so I = 4.3 V / 1 kOhm = 4.3 mA.

Reference: Standard diode circuit calculation

Question 8

Electronic Devices & CircuitsPN Junction Diode

At room temperature, using n=1 and ID=1 mA, a diode’s approximate small-signal resistance with VT≈26 mV is?

A2.6 ohm
B26 ohm
C260 ohm
D26 kiloohm

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Correct Answer: B. 26 ohm

Explanation: rd≈VT/ID=26 mV/1 mA=26 ohm.

Reference: Standard semiconductor electronics: diode incremental resistance

Question 9

Electronic Devices & CircuitsPN Junction Diode

The junction capacitance of a reverse-biased PN diode generally does what as reverse-bias magnitude increases?

ADecreases
BIncreases without limit
CRemains exactly constant
DBecomes negative

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Correct Answer: A. Decreases

Explanation: Increasing reverse bias widens the depletion region. Since junction capacitance is inversely related to depletion width, the capacitance decreases.

Reference: Standard semiconductor electronics: transition capacitance

Question 10

Electronic Devices & CircuitsPN Junction Diode

Avalanche breakdown in a PN junction is mainly associated with?

AImpact ionization and carrier multiplication
BForward-bias recombination only
CMechanical contact bounce
DZero electric field

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Correct Answer: A. Impact ionization and carrier multiplication

Explanation: In avalanche breakdown, carriers accelerated by a strong electric field create additional electron-hole pairs through impact ionization.

Reference: Standard semiconductor electronics: avalanche breakdown

Question 11

Electronic Devices & CircuitsPN Junction Diode

Compared with avalanche breakdown, Zener breakdown is most strongly associated with?

AQuantum-mechanical tunneling in a heavily doped junction
BImpact ionization in a lightly doped junction only
CThermal radiation from an LED
DTransistor saturation

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Correct Answer: A. Quantum-mechanical tunneling in a heavily doped junction

Explanation: Zener breakdown occurs in heavily doped junctions with narrow depletion regions, where a strong field enables tunneling.

Reference: Standard semiconductor electronics: Zener breakdown mechanism

Question 12

Electronic Devices & CircuitsPN Junction Diode

For an abrupt PN junction under increased reverse bias, the depletion-region width generally?

AIncreases
BDecreases to zero
CRemains exactly constant
DChanges sign

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Correct Answer: A. Increases

Explanation: Reverse bias increases the junction electric field and expands the depletion region.

Reference: Standard semiconductor electronics: depletion width

Question 13

Electronic Devices & CircuitsPN Junction Diode

The small-signal resistance of a forward-biased diode at a fixed operating point is approximately proportional to?

AThermal voltage divided by diode current
BDiode current divided by thermal voltage
CSquare of reverse voltage only
DZero for every current

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Correct Answer: A. Thermal voltage divided by diode current

Explanation: From the exponential diode equation, the incremental resistance is approximately rd = nVT/ID at the operating point.

Reference: Standard semiconductor electronics: diode small-signal resistance

Question 14

Electronic Devices & CircuitsPN Junction Diode

A silicon PN junction has a reverse saturation current of 10 nA. Using the ideal diode equation with n=1 and VT=25 mV, approximately by what factor does the forward current increase when the forward voltage rises by 50 mV in the exponential region?

A2
B4
C7.4
D100

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Correct Answer: C. 7.4

Explanation: The current ratio is exp(ΔV/VT)=exp(50/25)=e^2≈7.4, assuming the exponential term dominates.

Reference: Standard semiconductor electronics: diode exponential characteristic

Question 15

Electronic Devices & CircuitsPN Junction Diode

Reverse recovery time of a PN diode is mainly associated with?

ARemoval of stored charge when switching from forward conduction to reverse blocking
BTime required to heat the package
CForward voltage becoming exactly zero
DZener voltage regulation only

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Correct Answer: A. Removal of stored charge when switching from forward conduction to reverse blocking

Explanation: After forward conduction, stored minority-carrier charge must be removed before the diode fully blocks reverse current, producing reverse recovery time.

Reference: Standard semiconductor electronics: diode reverse recovery