Analog Circuits Questions and Answers Part-7

1. Is the statement “Diffusion current produces Drift current” true?
a) Yes
b) No
c) Cannot Say
d) Insufficient Data

Answer: a
Explanation: The movement of charge carriers is diffusion current and this movement produces an electric field which is the root cause of drift current. This is also proved by the fact that both drift and diffusion current are equal and opposite in an unbiased semi-conductor.

2. What is the average net velocity in the direction of the electric field?
a) Velocity of electrons
b) Velocity of holes
c) Drift velocity
d) Collision velocity

Answer: c
Explanation: The carriers accelerate in the direction of electric field between collisions but for each time interval τc (collision time) there is a collision which randomises the velocity of the carrier. This average net velocity in the direction of electric field is Drift velocity.

3. What is mobility?
a) Ease of carrier drift
b) Ease of current flow
c) Ease of access to the junction
d) Ease of movement

Answer: a
Explanation: Mobility is the ease with which carriers can drift. The higher the collision time, the greater is the mobility also the lighter is the carrier, the greater is its mobility. Thus on the application of an electric field it’s easier for carriers to drift.

4. Why does a gradient occur in a semi-conductor?
a) Because of current flow
b) Because of diffusion current
c) Because of drift current
d) Because of difference in concentrations

Answer: d
Explanation: As there is a different level of doping in the p and n regions of a semi-conductor, the carriers (either holes or electrons) move from a region of high concentration to a region of low concentration giving rise to diffusion current.

5. How does diffusion current produce the depletion region?
a) The diffusion causes the holes and electrons to collect at the junction
b) The diffusion is because of the depletion region
c) The depletion region aids diffusion
d) The statement is not true

Answer: a
Explanation: The diffusion of carriers from one side to another makes the holes and electrons to collect on either side of the junction creating the depletion region. This is further widened or shortened depending on the biasing.

6. Which of the following is not known as transition capacitance?
a) Junction capacitance
b) Space-Charge capacitance
c) Diffusion capacitance
d) Barrier capacitance

Answer: c
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. It occurs in a reverse biased diode. The other name for this is Junction capacitance, Space-Charge capacitance, Barrier capacitance, Depletion region capacitance. Diffusion capacitance is due to the transport of charge carriers between the two terminals of the device. It occurs in a forward biased diode.

7. Which of the following is not known as transition capacitance?
a) Depletion region capacitance
b) Space-Charge capacitance
c) Nominal capacitance
d) Junction capacitance

Answer: c
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. It occurs in a reverse biased diode. The other name for this is Junction capacitance, Space-Charge capacitance, Barrier capacitance, Depletion region capacitance. Diffusion capacitance is due to the transport of charge carriers between the two terminals of the device. It occurs in a forward biased diode.

8. Transition capacitance of a diode is directly proportional to ____________
a) Area of cross section
b) Width of depletion region
c) Reverse voltage applied across the terminals
d) Drift current

Answer: a
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.

9. Transition capacitance of a diode is inversely proportional to __________
a) Area of cross section
b) Width of depletion region
c) Relative permittivity
d) Drift current

Answer: b
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.

10. As reverse bias voltage increases transition capacitance __________
a) Increases
b) Decreases
c) Doesn’t depend upon voltage
d) Constant

Answer: b
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
As reverse bias increases depletion width also increases hence transition capacitance decreases.