1. What is the dependence of the transition capacitance on relative permittivity?
a) Inversely proportional to relative permittivity
b) Directly proportional to relative permittivity
c) Independent of relative permittivity
d) Directly proportional to relative permittivity with a degree of 2
Explanation: Transition capacitance is the capacitance exhibited by a diode due to the p-side and n-side of the diode is separated by a distance of depletion width same as in an electrolytic capacitor.
The equation of transition capacitance = ƸA/W
Where Ƹ = permittivity of the material of diode, W = depletion width
A = area of cross section
Relative permittivity is directly proportional to the permittivity of the material.
2. For a diode the transition capacitance was 10pF. The depletion width changed from 1µm to 10 µm. All other conditions remain unchanged. The new diode capacitance is __________
a) 5pF
b) 1.414pF
c) 1pF
d) 10pF
Explanation: The equation of transition capacitance = ƸA/W
Where Ƹ = permittivity of the material of diode, W = depletion width
A = area of cross section
Since depletion width increased 10 times and all other quantities are the same, the capacitance decrease by 10 times.
3. For a diode the transition capacitance was 15pF. The diode is replaced with another diode of same material with twice cross sectional area. Terminal voltage remains unchanged. The capacitance of new diode is __________
a) 15pF
b) 30pF
c) 60pF
d) 7.5pF
Explanation: The equation of transition capacitance = ƸA/W
Where Ƹ = permittivity of the material of diode, W = depletion width
A = area of cross section
Since A becomes 2A capacitance also doubles.
4. A diode is replaced with another diode of different material. The ratio of relative permittivity of new material to old is 0.5. The initial capacitance was 20pF, then final capacitance will be __________
a) 15pF
b) 20pF
c) 10pF
d) 2pF
Explanation: The equation of transition capacitance = ƸA/W
Where Ƹ = permittivity of the material of diode, W = depletion width
A = area of cross section
Since permittivity becomes half capacitance also halves.
5. A diode had a transition capacitance of 1pF and depletion width of 1 µm. The capacitance changes to 10 pF when the depletion width changes. The final depletion width is __________
a) 10 µm
b) 0.1 µm
c) 1 µm
d) 100 µm
Explanation: The equation of transition capacitance = ƸA/W
Where Ƹ = permittivity of the material of diode, W = depletion width
A = area of cross section
Since depletion width and capacitance are inversely proportional
Depletion width decreases to 0.1 µm
6. Which of these is the odd one in the choices?
a) Transition capacitance
b) Diffusion capacitance
c) Space charge capacitance
d) Depletion layer capacitance
Explanation: Transition capacitance is the junction capacitance in a reverse biased diode. The rate of change of immobile charges wrt a change in reverse bias voltage is called transition capacitance, or space charge or depletion layer capacitance.
7. Consider two diodes, A is step graded, B is linear graded. Find the ratio of the capacitance of A to B, when the applied voltage in reverse bias is 64V.
a) 0.2
b) 2
c) 0.5
d) 5
Explanation: In A, CA ∝ \(\frac{1}{\sqrt{V}}\)
CB ∝ \(\frac{1}{\sqrt[3]{V}}\)
\(\frac{C_A}{C_B} = \frac{4}{8} = \frac{1}{2}\) = 0.5.
8. Consider 2 reverse biased diodes. If the ratio of applied reverse bias voltages is 0.5, find the ratio of transition capacitances of the 2 diodes.
a) 2
b) 4
c) 1.31
d) 2.6
Explanation: \(\frac{C_{T1}}{C_{T2}} = (\frac{V2}{V1})^{\frac{1}{2.5}} \)
\(\frac{C_{T1}}{C_{T2}} = 2^{\frac{1}{2.5}}\) = 1.31.
9. The transition capacitance depends on the forward current of the diode.
a) True
b) False
Explanation: Diffusion capacitance is present in the forward bias and transition capacitance is present in reverse bias. Hence it is diffusion capacitance which is proportional to the forward current of diode, not transition capacitance.
10. Capacitance per unit area at no reverse bias is 2 pF/cm2. For a step graded diode of area 5cm2, what is net capacitance at 99 V reverse bias voltage?
a) 2pF
b) 5pF
c) 0.1pF
d) 1pF
Explanation: Co=2pf/cm2
C = CO*A/(1 + 99)0.5 = 2pF x 5/10 = 1pF.