Quiz: Electrical Engineering 31 May 2021

Quiz: Electrical Engineering
Exam: DFCCIL
Topic: Semiconductors & diodes

Each question carries 1 mark.
Negative marking: 1/4 mark
Time: 10 Minute

Q1. Zener diode is designed to specifically work in which region without getting damaged?
(a) Active region
(b) Breakdown region
(c) Forward bias
(d) Reverse bias

Q2. Zener Diode is mostly used as __
(a) Half-wave rectifier
(b) Full-wave rectifier
(c) Voltage Regulator
(d) LED

Q3. What is the level of doping in Zener Diode?
(a) Lightly Doped
(b) Heavily Doped
(c) Moderately Doped
(d) No doping

Q4. When the P-N junction is forward biased
(a) The width of depletion layer is decreased
(b) The width of depletion layer is increased
(c) The width of depletion layer remains constant
(d) None of the above

Q5. Fermi energy level for P-type extrinsic semiconductor lies….
(a) Close to valence band
(b) Close to conduction band
(c) At the middle of the band gap
(d) None of the above

Q6. A semiconductor is formed by ……. bonds?
(a) Ionic
(b) Metallic
(c) Covalent
(d) None of these

Q7. A semiconductor has __ temperature coefficient of resistance.
(a) Negative
(b) Positive
(c) Zero
(d) None of the above

Q8. A pentavalent impurity has________
(a) 3 Valence electrons
(b) 6 Valence electrons
(c) 4 Valence electrons
(d) 5 Valence electrons

Q9. An n-type semiconductor is
(a) Positively charged
(b) Electrically neutral
(c) Negatively charged
(d) None of the above

Q10. In a semiconductor, movement of charge carriers under influence of an electric field is called…….
(a) Diffusion
(b) Drift
(c) Mobility
(d) Conductivity

Solutions
S1. Ans.(b)
Sol. Zener diodes are called breakdown diodes since they operate in breakdown region. Because of this characteristic, it can be used as a constant-voltage device.

S2. Ans.(c)
Sol. The Zener diode is a specifically designed diode to operate in the breakdown region and keeps the voltage in the circuit to which it is connected as constant. Thus, it is widely used as a voltage regulator.

S3. Ans.(b)
Sol. The value of reverse voltage at which this occurs is controlled by the amount of doping of the diode. A heavily doped diode has a low Zener breakdown voltage, while a lightly doped diode has a high Zener breakdown voltage.
The Zener diode is made up of heavily doped semiconductor material. The heavily doped means the high-level impurities is added to the material for making it more conductive. The depletion region of the Zener diode is very thin because of the impurities.

S4. Ans.(a)
Sol. When P-N junction is forward biased then depletion layer decreases and when reverse biased then depletion region increases.

S5. Ans.(a)
Sol. Fermi energy level (E_F) for P- type extrinsic semiconductors lie close to valence band and for N-type, it is close to conduction band.

S6. Ans.(c)
Sol. A semiconductor is formed by covalent bond. Sharing of electrons of the neighbouring atoms to satisfy the need to have eight electrons on the valence shell in an atom is called covalent bonding.
Semiconductors, such as silicon are made up of individual atoms bonded together in a regular, periodic structure to form an arrangement whereby each atom is surrounded by 8 electrons. The electrons surrounding each atom in a semiconductor are a part of a covalent bond. The covalent bond consists of two atoms “sharing” a single electron. Each atom forms four covalent bonds with the four surrounding atoms.

S7. Ans.(a)
Sol. At low temperature, the valence band of a semiconductor is completely filled and the conduction band is completely empty. Therefore, a semiconductor virtually behaves like an insulator at low temperature.
As the temperature is increased more valence electrons cross over to the conduction band and conductivity increases. This shows that electrical conductivity of a semiconductor increases with the increase of temperature, i.e., a semiconductor has a negative temperature coefficient of resistance.

S8. Ans.(d)
Sol. The n-type semiconductor is formed by doping a pure silicon or germanium crystal with a material having five valence electrons. Antimony, arsenic, and phosphorous are examples of pentavalent materials.

S9. Ans.(b)
Sol. The n-type or P-type semiconductor formed remains electrically neutral, i.e., neither positively charged nor negatively charged.
n-type semiconductor has an excess of electrons but these extra electrons were supplied by the atoms of donor impurity and each atom of donor impurity is electrically neutral. The extra electrons are free electrons and increase the conductivity of the semiconductor. The situation with regard to the p-type semiconductor is also similar.

S10. Ans.(b)
Sol. Drift current is the electric current, or movement of charge carriers, which is due to the applied electric field.

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