Basic Electronics Engineering Questions and Answers – Junction Diode – Theory of PN-Junction

This set of Basic Electronics Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Junction Diode – Theory of PN-Junction”.

1. Which of the following is an impurity added in a semiconductor to make it a P-Type semiconductor?
a) Boron
b) Antimony
c) Indium
d) Gallium
View Answer

Answer: b
Explanation: Boron, Indium and Gallium are trivalent in nature and are added as impurities to form N-Type of extrinsic semiconductors. Antimony is pentavalent in nature and is used to form P-Type of semiconductors. Other examples for impurities added in extrinsic semiconductor to form P-Type are Phosphorous and Arsenic.

2. Majority carriers in N-type semiconductors are ________
a) Electrons
b) Holes
c) Neutrons
d) Molecules
View Answer

Answer: a
Explanation: To form N-Type semiconductors we use trivalent impurities. Trivalent impurities are also called as donor impurities because they generate excess electrons when semiconductors are doped. Hence the majority carriers in N-Type semiconductor are electrons.

3. A diode is a device in which, we combine _________ of semiconductors.
a) N-N Type
b) P-P Type
c) P-N Type
d) Intrinsic
View Answer

Answer: c
Explanation: A diode is a basic device which is made by combining two different types of extrinsic semiconductors. A heterojunction of P-Type and N-Type semiconductors wrapped in a silicon layer.

4. Name the process, where the holes in a diode move from P-Type side to N-Type side and electrons do the vice-versa.
a) Fusion
b) Diffusion
c) Conduction
d) Induction
View Answer

Answer: a
Explanation: Diffusion is the process, when the majority carriers from each part i.e. P and N part of diode move to the other side, to reach the equilibrium state. Fusion is just combining of two parts. Induction is generating current.

5. What is the layer called, when majority carriers of both the parts of the diode accumulate at the junction the layer?
a) Diffusion Layer
b) Combination Layer
c) Heterojunction Layer
d) Depletion Layer
View Answer

Answer: d
Explanation: When holes leave the P-side, they leave behind a negative charge and when electron leaves the N-part, they leave behind a positive charge. The carriers accumulate in the junction. This accumulation on the junction is forms a layer termed as depletion layer.

6. When the holes from P-type region and electrons from N-type region move towards the junction, in a diode there is a charge generated, before the equilibrium is achieved, that is called as _________
a) Static current
b) Alternating current
c) Diffusion current
d) Direct current
View Answer

Answer: c
Explanation: When the holes and electron move towards the junction, where there is diffusion between them. They try to penetrate into the other part of diode and hence this causes a generation of charge unless this process reaches to its equilibrium. This is diffusion current. Static current is generated when there is a potential between two opposite charges.

7. What will be the effective density in conduction band for a strip of germanium at 300K?
a) 1.025 x 1025
b) 2.25 x 1025
c) 0.5 x 1025
d) 1.9 x 1025
View Answer

Answer: a
Explanation: The effective density in conduction band is given as,
Nc = 2*[(2*π*me*KT)3/2/h2] Where me denotes the mass of electron, K denotes Boltzmann’s Constant, T is the Temperature, h denotes Planck’s constant. Therefore, substituting the value, we get 1.025 x1025.

8. In a given semiconductor at T = 300K, the electron concentration varies linearly from n = 1012 cm−3 to n = 1016 cm−3 over the distance from x = 0 to x = 3 μm and Dn = 35 cm2/s. What is the diffusion current density?
a) 170 A/m3
b) 145 A/m3
c) 187 A/m3
d) 211 A/m3
View Answer

Answer: c
Explanation: We have, Jn=eDn \(\frac{dn}{dx}\) = (1.6×10-19) *(35) *(1012 – 1016/0-3 x 10-4)
Hence, Jn = 187 A/m3.
Where e is charge on electron.

9. The mobility of a carrier is 1000 cm2/Volt-seconds at 300K. What is if the diffusion coefficient of the carrier?
a) 21 cm2/s
b) 18.24 cm2/s
c) 27.56 cm2/s
d) 25.90 cm2/s
View Answer

Answer: d
Explanation: Using the Relation we have,
D=\((\frac{kT}{e})\)μ ,
where k denotes Boltzmann Constant,
T is temperature and e denote the charge of electron which is 1.6 x10-19. µ is provided as 1000 cm2/Volt-seconds and T=300K. Substituting values, we get answer as D = 25.90 cm2/s.

10. Name the potential difference, which is generated when an electric field is applied at the depletion layer.
a) Contact potential
b) Applied potential
c) External potential
d) Induced potential
View Answer

Answer: a
Explanation: An electric field is generated in the depletion region of a p-n junction that is, from the n-type side towards the p-type side, so there is a difference in potential or voltage across the junction. This voltage is called as built-in potential or the contact potential.

11. At the point equilibrium, after diffusion, the net number of electrons and holes passing through the junction are _______
a) Infinite
b) Indefinite
c) Zero
d) Very few
View Answer

Answer: c
Explanation: The electric field directed from the n-type side to the p-type side, exerts a force on the free electron and causes it to move to the n-type side. Similarly, the hole is mobilized to the p-type side. The directions in which the charges move are opposite to those due to the diffusion process. When the point of equilibrium is reached, the net number of both electrons and holes crossing the junction is zero.

Sanfoundry Global Education & Learning Series – Basic Electronics Engineering.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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