# Electronic Devices and Circuits Questions and Answers – Diode Capacitances

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

1. Compared to a PN junction with NA=1014/CM3, which one of the following is true for NA=ND= 1020/CM3?
a) depletion capacitance decreases
b) depletion capacitance increases
c) depletion capacitance remains same
d) depletion capacitance can’t be predicted

Explanation: We know, CT=Aε/W and
W ∝ (1/NA+1/ND) 1/2. So, CT ∝ (1/NA+1/ND)-1/2
So when NA and ND increases, depletion capacitance CT increases.

2. If CT is the transition capacitance, which of the following are true?
1) in forward bias, CT dominates
2) in reverse bias, CT 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

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/cm2 at zero bias condition.If the built in voltage, Vbi is 1V, the capacitance at reverse bias of 99V is?
a) 0.1nF/cm2
b) 1nF/cm2
c) 1.5nF/cm2
d) 2nF/cm2

Explanation: Cjo is the capacitance at zero bias, that is VR=0V, Cjo=Cj for VR=0V. We know, Cj = Cjo/(1+(VR/Vbi))m , m=1/2 for abrupt. So, putting Cj=0.1nF/cm2 where, VR=99V and Vbi=1V we get, Cjo= 0.1(1+99)1/2 = 0.1nF/cm2.

4. The built in capacitance V0 for a step graded PN junction is 0.75V. Junction capacitance Cj at reverse bias when VR=1.25V is 5pF. The value of Cj when VR=7.25V is?
a) 0.1pF
b) 1.7pF
c) 1pF
d) 2.5Pf

Explanation: We know, Cj1/ Cj2=[(V0+VR2)/(V0+VR2)]1/2
So, Cj2=Cj1/ {(0.75+7.25)/(0.75+1.25)}1/2 we get Cj2=Cj1 /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 Cj is 1pF for V0+VR =1V, then for V0+VR =4V what will be the value of Cj?
a) 0.1pF
b) 1.7pF
c) 1pF
d) 0.5Pf

Explanation: We know, Cj1/ Cj2=[(V0+VR2)/(V0+VR1)]1/2
Cj2=Cj1(1/4)1/2=1/2 .
We get Cj2=1/2=0.5pF.
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6. A silicon PN junction diode under revers bias has depletion width of 10µm, relative permittivity is 11.7 and permittivity, ε0 =8.85×10-12F/m. Then depletion capacitance /m2 =?
a) 0.1µF/m2
b) 1.7µF/m2
c) 10µF/m2
d) 0.5µF/m2

Explanation: We know, CT =Aε0εr /W
CT/A= (8.85×10-12)(11.7)/10
=10
By putting the values we get 10µF/m2.

7. The transition capacitance, CT 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

Explanation: CT = K/(V0+VB)1/2
As it’s having uniform doping on both sides, the voltage V0 will be zero. So, CT=K/(VB)1/2. The variation of transition capacitance with built in capacitance is (VB )-1/2.

8. The CT for an abrupt PN junction diode is ________
a) CT = K/(V0+VB)1/2
b) CT = K/(V0+VB)-1/2
c) CT = K/(V0+VB)1/3
d) CT = K/(V0+VB)-1/3

Explanation: For an abrupt PN junction diode, CT = K/(V0+VB)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

Explanation: CD =τ I /n0 VT
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

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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