# Microelectronics Questions and Answers – Analysis of a PN Junction as a Diode

This set of Microelectronics Multiple Choice Questions & Answers (MCQs) focuses on “Analysis of a PN Junction as a Diode”.

1. Consider the following circuit of a Diode with a threshold of about 4.7V.

If a D.C. sweep of 0 to 10V with an increment of 1V is made, what will be the impact of the diode?
a) Increase the power consumption of the Resistor
b) Increasing delay in the response
c) Voltage regulator
d) Voltage multiplier

Explanation: The diode will delay the response by approximately 4.7V in the circuit. This is because it will turn on after crossing the threshold voltage.

2. Which of the following depicts the V/I characteristics of a diode?
a) From origin towards the positive x axis
b) x=0.7
c) x=0.5
d) y=0

Explanation: An ideal diode allows current for V >= 0. V is plotted on the y axis while I is plotted on the x axis. The voltage drop across an ideal diode is 0 hence the V/I characteristics would start from origin and extend towards the positive x axis only.

3. Which elements are used to create the small signal model of a diode?
a) 3 resistors and 1 capacitor
b) 2 resistors and 2 capacitors
c) 1 resistor and 3 capacitors
d) No capacitors

Explanation: Two resistances appear within the PN junction which are due to the properties of the pn junction-one is rd expressed by the I/V relationship of the diode current and the other is the series resistances offered by the p and n material. The two capacitances involved are diffusion and depletion capacitances.

4. Choose the correct relation justifying the condition of Fermi-level of conduction and valence band in a forward biased PN junction diode.
a) EF-n < EF-p
b) EF-n > EF-p
c) EF-n >= EF-p
d) EF-n = 10*EF-p

Explanation: When a PN junction diode is forward biased, the depletion region becomes short and the electrons are able to travel from n-side travel towards the p-side. This is because the Fermi-level in the n-side increases due to the increase the negative potential of the n-side and this means that there are certain energy states in the p-side which are less negative than the n-side. This is implying that holes have flown into the p-side. Thus EF-n > EF-p suggests that the diode is forward biased.

5. How does the depletion region in the PN junction maintain a maximum Electric field at the junction?
a) The junction is a point of discontinuity for electric field
b) The junction is a point of discontinuity for the potential
c) The forces on the charge carriers balance each other
d) The charge concentration is discontinuous at the junction

Explanation: The charge carriers face the consequences of superposition due to two forces prevalent within the junction. The Diffusion force rises due to the gradient of the carriers while travelling in to another region. This leads to an Electric field in the opposite direction while keeps the carriers from entering the region but only upto the space charge width, thus the maximum is maintained at the junction.

6. A circuit contains 8 diodes, requiring 5.6V across them to turn on but a breakdown of 70V, in series with an active element. The active element turns on if the voltage drop across any of one of it’s terminals is more than 3V but will get destroyed if it is any more than 4V. The active element has two terminals named A, and B. We can apply a higher voltage to A while the other terminal should be at a lower potential. If 110V is applied to the entire configuration, what will happen first?
a) The first diode will break
b) The first few diodes will break
c) The last few diodes will break
d) Everything will get destroyed

Explanation: The voltage drop happens so fast that each diode will overcome the breakdown voltage and get destroyed. The element would burn out after 4V only.

7. The PN Junction introduces a ___ in the D.C. output of the circuit. It is a ___ system.
a) Delay, Time variant
b) No delay, Time invariant
c) Phase, time delaying

Explanation: The PN Junction exhibits a threshold voltage before turning on. This threshold voltage prevents the diode from turning on for Vin=0+. Plotting Vin vs Vout vs t, we find that Vout will remain 0 at t=0=. It starts conducting at t=tVth. This causes a delay in the output implying it is a time variant system.

8. What happens to the electron and hole concentration in the p and n regions during forward bias?
a) They decrease exponentially away from the junction
b) They are directly proportional to the Electric field
c) They fall down to EF-N
d) They fall up to EF-P

Explanation: The minority carriers in the space charge region are decreasing exponentially as we go deeper into the n or p region. The fermi level in the n side is greater than the p side during forward bias.

9. The reverse saturation current of a diode is 2*10-16 A. If a voltage of .7V is found across the diode while it sustains about 4A current, find the dynamic A.C. resistance.
a) .185 Ohms
b) .200 Ohms
c) 0
d) Only static resistance

Explanation: The static resistance of the diode will be .185V. The A.C. resistance is calculated over a range of values and in absence of data, we cannot conclude about the dynamic resistance.

10. If the diode is made to work as a voltage-controlled capacitor, what happens to the capacitance with increase in the reverse bias?
a) Depletion capacitance decreases
b) Diffusion capacitance increases
c) Capacitance decreases
d) Capacitance increases

Explanation: As the width of the space charge region increases, the capacitance decreases since the two plane of charge start shifting away from each other.

11. What is the resistance offered by the diode in the dynamic region termed as?
a) A.C. resistance
b) D.C. resistance
c) Transitional impedance
d) No such resistance is prevalent

Explanation: The dynamic region is the region of operation of a diode which includes a maximum and minimum value of current that passes through a diode. This current divided by the voltage change represents the A.C. resistance of the diode in the dynamic region. It is alternating and hence called so.

12. What is the relation for the time taken by the three diodes if they have threshold voltage increasing from bottom to top?

a) Same
b) Only first diode turns on
c) Only second diode turns on
d) All the diodes turn on

Explanation: As the first diode turns on, it will maintain a certain voltage drop across the other diodes that follow; This will in turn keep the other diodes from ever turning on.

13. In the following circuit, each diode can withstand I current and the cross-sectional area increases by a factor of 2 from left to right. If the voltage source sends a total current of 10I, what is the total current flowing through the third row?

a) 3.33I
b) 1.66i
c) 2.50I
d) .34I

Explanation: Since each diode is identical, each row offers equal amount of impedance. Hence, each row will draw equal amount of current and the current flowing through the third row is 3.33I.

14. Where, inside the space charge region, does the Electric field becomes maximum?
a) At the junction
b) At the center of the p-side
c) At the center of the n-die
d) Nowhere

Explanation: The ions extend up to the space charge width. The Electric field decreases as we go away from the center of the space charge region since the density of ions decrease. Hence it has to be maximum at the junction while it decreases as we travel into the n or p side.

15. At the knee region, the operating point offers a low resistance. When the diode current is sufficiently high, which resistance would be higher?
a) A.C. resistance
b) D.C. resistance
c) A.C. or D.C. resistance
d) Cannot be determined

Explanation: If the current is sufficiently high, the q-point is in the upper part of the I/V curve and thus we have a lower resistance. But we cannot conclude which resistance would be higher eventually since the range for A.C. resistance is not provided.

Sanfoundry Global Education & Learning Series – Microelectronics.

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