Physics · Class 12 Science
Ch 15Semiconductor Electronics: Materials, Devices and Simple Circuits — Class 12 Physics, concept-first.
The story of modern electronics begins with a simple question: how do we control the flow of electrons? Every electronic circuit, from a pocket radio to a supercomputer, relies on devices that can regulate this flow with precision.
Key concepts
Hover a concept to preview it and jump to its most relevant Q&A.
Band Gap Energy
Imagine you have a single atom. Its electrons live in specific, fixed energy levels — like rungs on a ladder. You can't put an electron halfway between two rungs; it's either on one rung or another.
Most relevant Q&A
- C, Si and Ge have same lattice structure. Why is C insulator while Si and Ge intrinsic semiconductors?Preview
- Carbon, silicon and germanium have four valence electrons each. These are characterised by valence and conduction bands separated by energy…Free
- Sn, C, and Si, Ge are all group XIV elements. Yet, Sn is a conductor, C is an insulator while Si and Ge are semiconductors. Why?Preview
In previous exams
How often this chapter’s concepts have been examined — real appearance data, never estimated.
Chapter contents
The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.
Introduction
The story of modern electronics begins with a simple question: how do we control the flow of electrons? Every electronic circuit, from a pocket radio to a supercomputer, relies on devices that can reg…
Classification of Metals, Conductors and Semiconductors
Solids are broadly divided into three categories based on their electrical conductivity or resistivity .
Intrinsic Semiconductor
A pure semiconductor, with no deliberate impurities added, is called an intrinsic semiconductor. The most common examples are silicon (Si) and germanium (Ge).
Extrinsic Semiconductor
Pure intrinsic semiconductors — silicon and germanium — have very low conductivity at room temperature.
p-n Junction
A p-n junction is the fundamental building block of almost all semiconductor devices — diodes, transistors, solar cells, LEDs, and integrated circuits.
p-n Junction Formation
A p-n junction is not made by gluing two separate pieces of semiconductor together. Instead, we begin with a thin wafer of p-type silicon (p-Si).
Semiconductor Diode
A semiconductor diode is simply a p-n junction equipped with metallic contacts on both ends, allowing an external voltage to be applied across it.
p-n Junction Diode under Forward Bias
When we connect a battery to a p-n junction diode so that the p-side is at a higher potential than the n-side, we say the diode is forward biased.
p-n Junction Diode under Reverse Bias
When the external voltage is connected so that the n-side is positive and the p-side is negative, the diode is said to be in reverse bias.
Application of Junction Diode as a Rectifier
A junction diode's V-I characteristic reveals a one-way street: current flows freely only when the diode is forward biased.
Summary
- Energy bands: In solids, discrete atomic levels broaden into bands — the valence band (filled with valence electrons) and the conduction band (empty or partially filled).
Points to Ponder
1. The conduction band minimum and valence band maximum are delocalised across the entire crystal — they are not pinned to any single atom or location.
Exercises
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- 14.1In an n-type silicon, which of the following statement is true: (a) Electrons are majority carriers and trivalent atoms are the dopants. (b)…Free
- 14.2Which of the statements given in Exercise 14.1 is true for p-type semiconductors: (a) Electrons are majority carriers and trivalent atoms ar…Free
- 14.3Carbon, silicon and germanium have four valence electrons each. These are characterised by valence and conduction bands separated by energy…Free
- 14.4In an unbiased p-n junction, holes diffuse from the p-region to n-region because (a) free electrons in the n-region attract them. (b) they m…Preview
- 14.5When a forward bias is applied to a p-n junction, it (a) raises the potential barrier. (b) reduces the majority carrier current to zero. (c)…Preview
- 14.6In half-wave rectification, what is the output frequency if the input frequency is $50\ \text{Hz}$? What is the output frequency of a full-w…Preview
- 14.7A p-n photodiode is fabricated from a semiconductor with band gap of 2.8 eV. Can it detect a wavelength of 6000 nm?Preview
Additional Exercises
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- 14.8The number of silicon atoms per m³ is $5 \times 10^{28}$. This is doped simultaneously with $5 \times 10^{22}$ atoms per m³ of Arsenic and $…Free
- 14.9In an intrinsic semiconductor the energy gap $E_g$ is $1.2\ \text{eV}$. Its hole mobility is much smaller than electron mobility and indepen…Preview
- 14.10In a p-n junction diode, the current $I$ can be expressed as $$I = I_0 \left[\exp\left(\dfrac{eV}{k_B T}\right) - 1\right]$$ where $I_0$ is…Preview
Exemplar Problems
Higher-order thinking / exemplar-style practice problems.
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- Q1The conductivity of a semiconductor increases with increase in temperature because (a) number density of free current carriers increases. (b…Free
- Q2The potential barrier of a p-n junction is plotted on the vertical axis for three different biasing conditions, giving three curves that eac…Free
- Q3Point A is held at $-10\,\text{V}$ and point B is earthed (at $0\,\text{V}$). Starting from A, a resistor R is in series with an ideal diode…Free
- Q4A $220\,\text{V}$ (rms) A.C. supply is connected between points A and B. From A, an ideal diode (arrow pointing from A into the circuit) cha…Preview
- Q5Hole is (a) an anti-particle of electron. (b) a vacancy created when an electron leaves a covalent bond. (c) absence of free electrons. (d)…Preview
- Q6An a.c. source $v_m\sin\omega t$ is connected in series with a resistor to a pair of output terminals. An ideal diode is connected directly…Preview
- Q7A steady current of $0.2\,\text{mA}$ flows from A down through the following series path to B: a $5\,\text{k}\Omega$ resistor, then a p-n ju…Preview
- Q8When an electric field is applied across a semiconductor (a) electrons move from lower energy level to higher energy level in the conduction…Preview
- Q9In the depletion region of a diode (a) there are no mobile charges. (b) equal number of holes and electrons exist, making the region neutral…Preview
- Q10To reduce the ripples in a rectifier circuit with capacitor filter (a) $R_L$ should be increased. (b) input frequency should be decreased. (…Preview
- Q11The breakdown in a reverse biased p-n junction diode is more likely to occur due to (a) large velocity of the minority charge carriers if th…Preview
- Q12Why are elemental dopants for Silicon or Germanium usually chosen from group XIII or group XV?Preview
- Q13Sn, C, and Si, Ge are all group XIV elements. Yet, Sn is a conductor, C is an insulator while Si and Ge are semiconductors. Why?Preview
- Q14Can the potential barrier across a p-n junction be measured by simply connecting a voltmeter across the junction?Preview
- Q15The input applied at A is a square wave that alternates between $+1\,\text{V}$ and $-1\,\text{V}$ (equal duration in each state). A is conne…Preview
- Q16In a common-emitter transistor circuit, a resistor $R_1$ is in the base circuit, a resistor $R_2$ is in the collector circuit, an ammeter re…Preview
- Q17A $5\,\text{V}$ battery in series with a $25\,\Omega$ resistor is connected between a left node-rail and a right node-rail, driving three pa…Preview
- Q18An a.c. source $20\sin\omega t$ (volts) is connected through a series resistor to a pair of output terminals. Across the output is a branch…Preview
- Q19Suppose a 'n'-type wafer is created by doping Si crystal having $5 \times 10^{28}$ atoms/m$^3$ with $1\ \text{ppm}$ concentration of As. On…Preview
- Q20A sealed box has three terminals A, B and C. Inside, two identical germanium diodes (each with a forward threshold of about $0.7\,\text{V}$)…Preview
Sample & Board Papers
Sample papers and previous-year board questions for this subject.
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- Q1(a) Write the truth table for the combination of the gates shown in the figure. (b) Explain briefly how a photo diode operates.Preview
- Q2Draw a labelled circuit diagram of n-p-n germanium transistor in common emitter configuration. Explain briefly, how this transistor is used…Preview
- Q3(a) Explain with the help of suitable diagram, the two processes which occur during the formations of a p-n junction diode. Hence define the…Preview