This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Diode Capacitances”.

1. Compared to a PN junction with N_{A}=10^{14}/CM^{3}, which one of the following is true for N_{A}=N_{D}= 10^{20}/CM^{3}?

a) depletion capacitance decreases

b) depletion capacitance increases

c) depletion capacitance remains same

d) depletion capacitance can’t be predicted

View Answer

Explanation: We know, C

_{T}=Aε/W and

W ∝ (1/N

_{A}+1/N

_{D})

^{ 1/2}. So, C

_{T}∝ (1/N

_{A}+1/N

_{D})

^{-1/2}

So when N

_{A}and N

_{D}increases, depletion capacitance C

_{T}increases.

2. If C_{T} is the transition capacitance, which of the following are true?

1) in forward bias, C_{T} dominates

2) in reverse bias, C_{T} dominates

3) in forward bias, diffusion capacitance dominates

4) in reverse bias, diffusion capacitance dominates

a) 1 only

b) 2only

c) 2 and 3

d) 3 only

View Answer

Explanation: In reverse bias condition, depletion region increases and acts as an insulator or dielectric medium. So, the transition capacitance increases. In forward bias condition, due to stored charge of minority carriers, diffusion capacitance increases.

3. For an abrupt PN junction diode, small signal capacitance is 1nF/cm^{2} at zero bias condition.If the built in voltage, V_{bi} is 1V, the capacitance at reverse bias of 99V is?

a) 0.1nF/cm^{2}

b) 1nF/cm^{2}

c) 1.5nF/cm^{2}

d) 2nF/cm^{2}

View Answer

Explanation: C

_{jo}is the capacitance at zero bias, that is V

_{R}=0V, C

_{jo}=C

_{j}for V

_{R}=0V. We know, C

_{j}= C

_{jo}/(1+(V

_{R}/V

_{bi}))m , m=1/2 for abrupt. So, putting C

_{j}=0.1nF/cm

^{2}where, V

_{R}=99V and V

_{bi}=1V we get, C

_{jo}= 0.1(1+99)

^{1/2}= 0.1nF/cm

^{2}.

4. The built in capacitance V_{0} for a step graded PN junction is 0.75V. Junction capacitance C_{j} at reverse bias when V_{R}=1.25V is 5pF. The value of C_{j} when V_{R}=7.25V is?

a) 0.1pF

b) 1.7pF

c) 1pF

d) 2.5Pf

View Answer

Explanation: We know, C

_{j1}/ C

_{j2}=[(V

_{0}+V

_{R2})/(V

_{0}+V

_{R2})]

^{1/2}

So, C

_{j2}=C

_{j1}/ {(0.75+7.25)/(0.75+1.25)}

^{1/2}we get C

_{j2}=C

_{j1}/2 =5/2=2.5Pf.

5. Consider an abrupt PN junction. Let V0 be the built in potential of this junction and VR be the reverse bias voltage applied. If the junction capacitance C_{j} is 1pF for V_{0}+V_{R} =1V, then for V_{0}+V_{R} =4V what will be the value of C_{j}?

a) 0.1pF

b) 1.7pF

c) 1pF

d) 0.5Pf

View Answer

Explanation: We know, C

_{j1}/ C

_{j2}=[(V

_{0}+V

_{R2})/(V

_{0}+V

_{R1})]

^{1/2}

C

_{j2}=C

_{j1}(1/4)

^{1/2}=1/2 .

We get C

_{j2}=1/2=0.5pF.

6. A silicon PN junction diode under revers bias has depletion width of 10µm, relative permittivity is 11.7 and permittivity, ε0 =8.85×10^{-12}F/m. Then depletion capacitance /m^{2} =?

a) 0.1µF/m^{2}

b) 1.7µF/m^{2}

c) 10µF/m^{2}

d) 0.5µF/m^{2}

View Answer

Explanation: We know, C

_{T}=Aε

_{0}ε

_{r}/W

C

_{T}/A= (8.85×10

^{-12})(11.7)/10

=10

By putting the values we get 10µF/m

^{2}.

7. The transition capacitance, C_{T} of a PN junction having uniform doping in both sides, varies with junction voltage as ________

a) (VB )^{1/2}

b) (VB )^{-1/2}

c) (VB )^{1/4}

d) (VB )^{-1/4}

View Answer

Explanation: C

_{T}= K/(V

_{0}+V

_{B})

^{1/2}

As it’s having uniform doping on both sides, the voltage V

_{0}will be zero. So, C

_{T}=K/(VB)

^{1/2}. The variation of transition capacitance with built in capacitance is (V

_{B})

^{-1/2}.

8. The C_{T} for an abrupt PN junction diode is ________

a) C_{T} = K/(V_{0}+V_{B})^{1/2}

b) C_{T} = K/(V_{0}+V_{B})^{-1/2}

c) C_{T} = K/(V_{0}+V_{B})^{1/3}

d) C_{T} = K/(V_{0}+V_{B})^{-1/3}

View Answer

Explanation: For an abrupt PN junction diode, C

_{T}= K/(V

_{0}+V

_{B})

^{n}. Here, n=1/2 for abrupt PN junction diode and 1/3 for linear PN junction diode. When the doping concentration of a diode varies within a small scale of area, then the diode is called as an abrupt diode.

9. The diffusion capacitance of a PN junction _______

a) decreases with increasing current and increasing temperature

b) decreases with decreasing current and increasing temperature

c) increasing with increasing current and increasing temperature

d) doesnot depend on current and temperature

View Answer

Explanation: C

_{D}=τ I /n

_{0}V

_{T}

Where, I is the current and VT is temperature factor. The diffusion capacitance is directly proportional to current and indirectly proportional to the temperature.

10. Transition capacitance is also called as _______

a) diffusion capacitance

b) depletion capacitance

c) conductance capacitance

d) resistive capacitance

View Answer

Explanation: Transition capacitance occurs in reverse bias. We obtain a depletion layer in that case. Hence it’s also called as depletion capacitance. The diffusion capacitance occurs in forward bias.

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