NCERT Class 12 Physics Physics Part-II: Chapter 6 — 1 INTRODUCTION
This chapter introduces semiconductor electronics, the foundation of modern electronic circuits. It contrasts semiconductor devices with older vacuum tubes, highlighting the advantages of semiconductors like smaller size, low power consumption, and high reliability. The text explains that semiconductor devices control electron flow within the solid material itself, unlike vacuum tubes which require heated cathodes and evacuated spaces. Early uses of semiconductors, like galena crystals for radio wave detection, are mentioned. The chapter classifies solids into metals, semiconductors, and insulators based on their electrical conductivity and resistivity, providing typical ranges for each. It also lists examples of elemental (Si, Ge) and compound semiconductors (inorganic and organic). This foundational knowledge is crucial for understanding the operation and application of various electronic components in the CBSE curriculum.
Quick info
| Board | CBSE / NCERT |
|---|---|
| Class | Class 12 |
| Subject | Physics |
| Book | Physics Part-II |
| Chapter | Chapter 6 — 1 INTRODUCTION |
| Language | English |
| PDF type | NCERT Textbook |
| Session | CBSE 2026 |
| Reading time | 4 minutes |
| Word count | 673 |
Learning outcomes
- Understand the basic building blocks of electronic circuits and the historical context of vacuum tubes.
- Appreciate the advantages of semiconductor devices over vacuum tubes.
- Classify solids (metals, semiconductors, insulators) based on conductivity and resistivity.
- Identify examples of elemental and compound semiconductors.
- Grasp the fundamental principles of charge carrier control in semiconductors.
Vocabulary
| Word | Meaning |
|---|---|
| Semiconductor | A solid material with electrical conductivity between that of a conductor and an insulator. |
| Vacuum tube | An electronic device that controls electric current through a vacuum in a sealed glass or metal envelope. |
| Conductivity | A measure of a material's ability to conduct electric current. |
| Resistivity | A measure of a material's opposition to the flow of electric current. |
| Valves | Another term for vacuum tubes, referring to their ability to allow current flow in only one direction. |
| Charge carriers | Mobile particles (like electrons or holes) that carry electric charge. |
| Elemental semiconductor | A semiconductor made from a single element, such as Silicon (Si) or Germanium (Ge). |
| Compound semiconductor | A semiconductor made from two or more elements, such as Gallium Arsenide (GaAs). |
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Practice questions
- What were the basic building blocks of electronic circuits before the discovery of the transistor? Answer: Before the transistor, vacuum tubes like diodes and triodes were the basic building blocks.
- List two advantages of semiconductor devices over vacuum tubes. Answer: Semiconductor devices are smaller, consume less power, operate at lower voltages, and have longer life and higher reliability compared to vacuum tubes.
- How are solids broadly classified on the basis of electrical conductivity? Answer: Solids are classified as metals, semiconductors, and insulators based on their electrical conductivity.
- Give examples of elemental semiconductors. Answer: Silicon (Si) and Germanium (Ge) are examples of elemental semiconductors.
Practice MCQs
Q1. Which of the following was a common vacuum tube device with three electrodes?
Explanation: A triode is a vacuum tube device that has three electrodes: cathode, plate, and grid.
Q2. What is the approximate resistivity range for semiconductors?
Explanation: Semiconductors have resistivity values intermediate to metals and insulators, typically in the range of 10^-5 to 10^6 Ω m.
Q3. Which of these is an example of an elemental semiconductor?
Explanation: Silicon (Si) is an elemental semiconductor. GaAs is a compound semiconductor, and Polypyrrole is an organic semiconductor.
Q4. In semiconductor devices, charge carriers flow:
Explanation: Unlike vacuum tubes, semiconductor devices control the flow of charge carriers within the solid material itself.
Q5. Galena (Lead sulphide, PbS) was historically used as a:
Explanation: A naturally occurring crystal of galena with a metal point contact was used as a detector of radio waves.
Frequently asked questions
What is the main difference between vacuum tubes and semiconductor devices?
Vacuum tubes control electron flow in a vacuum using heated cathodes, while semiconductor devices control charge carrier flow within the solid material itself, requiring no external heating or vacuum.
What are the key advantages of semiconductor devices?
Semiconductor devices are smaller, consume less power, operate at lower voltages, have longer life, and offer higher reliability compared to vacuum tubes.
How are solids classified based on their electrical properties?
Solids are classified into metals (high conductivity, low resistivity), semiconductors (intermediate conductivity/resistivity), and insulators (low conductivity, high resistivity).
What are the two main types of semiconductors mentioned?
The two main types are elemental semiconductors (like Si and Ge) and compound semiconductors (like GaAs and CdS).
When was the transistor discovered, marking a shift towards semiconductor electronics?
The transistor was discovered in 1948.
Can you give an example of an organic semiconductor?
Examples of organic semiconductors include anthracene, doped pthalocyanines, polypyrrole, polyaniline, and polythiophene.
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NCERT Class 12 Physics — Physics Part-II — Chapter 6 — 1 INTRODUCTION. Verified by NCERT Help Editorial Team. Reviewed on 29 Jul 2026. Last updated 10 Aug 2026.