Chapter 14 — Semiconductor Electronics: Materials, Devices and Simple Circuits
Open PDFReads in your browser→Summary
NCERT Class 12 Physics Chapter 14 covers semiconductor electronics including intrinsic and extrinsic semiconductors, p-n junction formation, diode behaviour under forward and reverse bias, and rectifier circuits used to convert AC to DC.
Chapter 14 of NCERT Class 12 Physics Part II introduces semiconductor electronics. Semiconductors like Silicon (Si) and Germanium (Ge) have intermediate resistivity (10−5 to 10⁶ Ωm) between metals and insulators. Pure (intrinsic) semiconductors have equal electrons and holes (ne = nh = ni). Doping with pentavalent atoms creates n-type semiconductors (ne >> nh), while trivalent dopants create p-type (nh >> ne). A p-n junction forms a depletion region and acts as a diode, allowing current mainly in forward bias. Diodes are used as half-wave and full-wave rectifiers to convert AC into DC, with capacitor filters providing smoother output.
Key points & formulas
- 01Semiconductors (Si, Ge) have a small energy band gap (Eg < 3 eV), allowing thermal excitation of electrons from the valence band to the conduction band at room temperature.
- 02In intrinsic semiconductors, ne = nh = ni; the product nenh = ni² holds for all semiconductors including doped ones.
- 03n-type semiconductors are formed by doping with pentavalent impurities (As, Sb, P), making electrons the majority carriers; p-type semiconductors use trivalent impurities (B, Al, In), making holes the majority carriers.
- 04A p-n junction develops a depletion region and a built-in barrier potential; forward bias reduces the barrier (current in mA) while reverse bias increases it (current in μA).
- 05The threshold (cut-in) voltage for a silicon diode is ~0.7 V; a breakdown voltage (Vbr) in reverse bias causes a sharp current increase.
- 06A full-wave rectifier uses two diodes with a centre-tap transformer to rectify both halves of the AC cycle; a capacitor filter smooths the pulsating output to near-steady DC.
Frequently asked questions
01What is the difference between intrinsic and extrinsic semiconductors?
An intrinsic semiconductor is a pure material (like Si or Ge) where the number of free electrons equals the number of holes (ne = nh = ni) due to thermal excitation. An extrinsic semiconductor is formed by doping a pure semiconductor with impurity atoms; pentavalent dopants create n-type (electrons as majority carriers) and trivalent dopants create p-type (holes as majority carriers), greatly increasing conductivity.
02How does a p-n junction diode work under forward and reverse bias?
In forward bias, the p-side is connected to the positive terminal, which reduces the depletion layer and barrier height, allowing significant current (milliampere range) to flow. In reverse bias, the n-side is connected to the positive terminal, which widens the depletion region and increases the barrier, suppressing current to a very small reverse saturation current (microampere range) until the breakdown voltage is reached.
03What is the role of a capacitor filter in a rectifier circuit?
A capacitor connected in parallel with the load in a rectifier circuit charges to the peak voltage of the rectified output and slowly discharges through the load between pulses. This smooths the pulsating DC output, reducing the AC ripple and providing a voltage closer to the peak rectified voltage. A larger capacitance gives a larger time constant and a steadier output.
04Is the NCERT Class 12 Physics Chapter 14 PDF free to download?
Yes, the NCERT Class 12 Physics Part II Chapter 14 PDF is completely free to download on cbseprepmaster.com.
More chapters in Physics Part II
This is the complete Physics Part II Chapter 14 as published by NCERT — every diagram, solved example, and exercise included, free. Browse all CBSE Class 12 textbooks.
Read offline with notes, solutions & mock tests
CBSE Prepmaster — free on iOS & Android