Semiconductor Electronics: Materials, Devices and Simple Circuits

1. Check this before you revise anything

This chapter's PDF is Reprint 2025-26, not 2026-27. Every other chapter of Class 12 Physics is published as Reprint 2026-27. We re-fetched leph206.pdf from ncert.nic.in to check, and NCERT is still serving the 2025-26 file — so this is simply the current published version for this chapter, not an outdated download.

The "Additional Exercises" section has been removed, as from all 14 chapters. Only 6 questions remain, five of them multiple choice and one a two-line numerical.

The transistor is promised and never delivered. The Introduction states that the chapter will "discuss some semiconductor devices like junction diodes (a 2-electrode device) and bipolar junction transistor (a 3-electrode device). A few circuits illustrating their applications will also be described."

There is no transistor section. The chapter ends at 14.7, the rectifier. Summary point 1 also still lists "diode, transistor, ICs" among solid state devices. The transistor is mentioned four times in passing and taught nowhere.

Logic gates have been removed entirely — zero hits for "logic gate", "OR gate", "NAND" and "truth table".

Special-purpose diodes have been removed — zero hits for "Zener", "photodiode", "solar cell" and, on a word-boundary search, "LED". Amplifiers and oscillators are likewise absent.

The old stub page advertised the removed material. Its summary described the chapter as covering "p-n junctions, diodes, rectifiers, transistors, and logic gates", and its metadata claimed a weightage of "6-8 marks" — a figure CBSE does not publish, since the board gives no chapter-wise split. Both have been corrected in this rebuild.

2. Energy Bands and the Classification of Solids (Textbook 14.2)

In an isolated atom electrons occupy sharp energy levels. Bring $10^{23}$ atoms together into a crystal and those levels spread into bands of closely spaced levels, separated by forbidden gaps.

Two bands matter: the valence band, holding the outermost bound electrons, and the conduction band above it, where an electron is free to move through the crystal. The band gap $E_g$ separates them.

The whole classification follows from the size of that gap:

Why the group-14 elements differ so much. Carbon, silicon and germanium all have four valence electrons, yet their gaps are about 5.4 eV, 1.1 eV and 0.7 eV respectively. Moving down the group the atoms grow larger, the valence electrons sit further from the nucleus and are held less tightly, so less energy is needed to promote one. Diamond ends up an insulator while silicon and germanium are semiconductors — which is Exercise 14.3.

3. Intrinsic and Extrinsic Semiconductors (Textbook 14.3 to 14.4)

An intrinsic semiconductor is chemically pure. At absolute zero it is a perfect insulator, since every valence electron is locked in a covalent bond. Warming it breaks some bonds, and each broken bond releases an electron and leaves behind a vacancy, a hole, which behaves as a mobile positive charge.

Electrons and holes are therefore created in pairs:

$n_e=n_h=n_i$

Conductivity rises with temperature, the opposite of a metal, because heating creates more carriers rather than merely increasing collisions.

Doping adds a controlled trace of impurity, typically about one part in $10^6$, and raises the conductivity enormously.

Doped material is still electrically neutral. Each dopant atom is itself neutral, so doping changes which carriers are available to move, not the total charge. This is a standard misconception and a frequent one-mark question.

The mass action law holds in both cases:

$n_e{n}_h=n_i^2$

so increasing one carrier concentration suppresses the other.

4. The p-n Junction (Textbook 14.5)

Joining p-type and n-type material in a single crystal produces the device on which almost all electronics rests.

Two processes at the junction:

• Diffusion. Holes are abundant on the p-side and scarce on the n-side, so they diffuse across; electrons diffuse the other way. This is driven purely by the concentration gradient — the point of Exercise 14.4.
• Drift. The departing carriers leave behind immobile charged ions, which set up an electric field. That field pushes minority carriers back, opposing further diffusion.

The depletion region is the resulting zone near the junction, stripped of mobile carriers and containing only fixed ionised dopant atoms. Across it stands the barrier potential, typically 0.3 V for germanium and 0.7 V for silicon.

Note the direction of cause and effect. The barrier potential is created by diffusion; it does not cause it. Before any diffusion occurs there is no potential difference at all, which is why option (b) in Exercise 14.4 is wrong.

Equilibrium is reached when the diffusion and drift currents exactly cancel, so no net current flows in an unbiased junction.

5. The Diode Under Bias, and Rectification (Textbook 14.6 to 14.7)

Forward bias opposes the junction's own field, so the barrier drops and majority carriers cross freely. Current rises sharply once the applied voltage passes the knee or cut-in voltage, about 0.7 V for silicon. That is Exercise 14.5.

Reverse bias reinforces the barrier field. Only the few thermally generated minority carriers cross, giving a tiny reverse saturation current that is almost independent of voltage. Beyond a critical reverse voltage the junction breaks down.

This asymmetry is the whole point: a diode conducts well in one direction and barely at all in the other.

Rectification (14.7) uses that one-way behaviour to turn alternating current into direct current.

• Half-wave. A single diode conducts during one half of each cycle and blocks the other. One output pulse per input cycle, so $f_{\text{out}}=f_{\text{in}}$.
• Full-wave. Two diodes with a centre-tapped transformer use both halves, inverting the negative one rather than discarding it. Two output pulses per input cycle, so $f_{\text{out}}=2f_{\text{in}}$.

For a 50 Hz input this gives 50 Hz and 100 Hz respectively — Exercise 14.6. The full-wave output is also easier to smooth, since its ripple is smaller and its pulses arrive twice as often. A capacitor filter across the load does the smoothing.

Summary

• Energy levels of isolated atoms spread into bands in a crystal, separated by forbidden gaps.
• Metals have overlapping or partly filled bands; semiconductors a small gap; insulators a large one.
• Band gap decreases down group 14 as atoms grow larger: C about 5.4 eV, Si about 1.1 eV, Ge about 0.7 eV.
• Diamond is an insulator while silicon and germanium are semiconductors, purely because of that gap.
• An intrinsic semiconductor is pure, is an insulator at absolute zero, and has $n_e=n_h=n_i$.
• Breaking one bond creates an electron and a hole, always in pairs.
• Semiconductor conductivity rises with temperature — the opposite of a metal.
• n-type: pentavalent donor dopant, electrons majority, holes minority.
• p-type: trivalent acceptor dopant, holes majority, electrons minority.
• Doped material remains electrically neutral; doping changes the carriers available, not the charge.
• Mass action law: $n_e{n}_h=n_i^2$.
• At a junction, holes and electrons diffuse because of the concentration gradient, not because of any pre-existing field.
• The depletion region holds only immobile ions; the barrier potential is about 0.3 V for Ge and 0.7 V for Si.
• The barrier potential is created by diffusion, not the cause of it.
• Forward bias lowers the barrier, narrows the depletion region and gives a large current in milliamperes.
• Reverse bias raises the barrier, widens the depletion region and gives a tiny saturation current from minority carriers.
• Half-wave rectifier: $f_{\text{out}}=f_{\text{in}}$, so 50 Hz gives 50 Hz.
• Full-wave rectifier: $f_{\text{out}}=2f_{\text{in}}$, so 50 Hz gives 100 Hz, with smaller ripple that is easier to filter.
• Transistors, logic gates, Zener diodes, photodiodes, LEDs and solar cells have all been removed, although the Introduction and Summary still refer to the transistor.