Solids: NEB Class 11 Physics Guide

Curriculum boundary

What “Solids” means in NEB Class 11 Physics

The official CDC Physics Grade 11 page and the CDC Physics curriculum place qualitative energy bands, band-based comparison of metals, insulators and semiconductors, and intrinsic versus extrinsic semiconductors in this unit. This is a quantum description of electrical properties, not a repeat of elasticity.

Useful prerequisites are electric potential and energy, current and resistance, and the modern-physics habit of reading energy scales. The unit is mainly conceptual: draw energy vertically, label filled and available states, and explain how carriers respond to an electric field.

From atoms to solids

Why discrete levels broaden into energy bands

An isolated atom has discrete allowed electron energies. When a very large number of atoms form a crystal, their electron wavefunctions interact and the Pauli exclusion principle prevents every electron from occupying the same quantum state. Each atomic level splits into many extremely close levels; on the scale of a diagram these appear as an allowed band. Regions with no allowed states are energy gaps.

The highest band occupied at zero-temperature in the simple picture is associated with valence electrons and is called the valence band. The next set of mobile states is the conduction band. Electrical conduction needs both charge carriers and nearby unoccupied allowed states into which carriers can respond under an applied field.

The institutional OpenStax band-theory explanation connects the occupancy and gap structure to conductivity. Use it to check the qualitative picture, while keeping the depth appropriate for the CDC scope.

Three electrical classes

Compare available states, gap and carrier response

These are models, not absolute labels independent of temperature, purity and field strength. A semiconductor is not merely a “poor conductor”; its carrier concentration can be controlled strongly by temperature, light and doping. An insulator can break down at a sufficiently large electric field, so “never conducts” is too strong.

Pure semiconductors

Intrinsic conduction creates an electron and a hole together

An intrinsic semiconductor is sufficiently pure that its carriers arise mainly from thermal excitation across the band gap. When an electron gains enough energy to enter the conduction band, it leaves an unoccupied state in the valence band called a hole. Nearby valence electrons can move into that state, so the vacancy behaves like a positive mobile carrier.

In the simple intrinsic model, each promoted electron leaves one hole, so electron and hole concentrations are equal. This does not mean positive protons move through the crystal. The hole is a convenient description of collective electron motion in the valence band.

Extrinsic semiconductors

Doping controls majority carriers without giving the crystal net charge

Doping deliberately substitutes a small, controlled concentration of impurity atoms into a semiconductor lattice. In a silicon-like four-valence-electron picture, a five-valence-electron donor contributes an electron that is relatively easy to excite into conduction. The result is n-type material: electrons are majority carriers and holes are minority carriers.

A three-valence-electron acceptor creates an easily available hole state. The result is p-type material: holes are majority carriers and electrons are minority carriers. The letters n and p identify dominant carrier type, not a permanently negative or positive bulk charge; fixed ionised impurities balance mobile charge overall.

The OpenStax semiconductor and doping section shows donor and acceptor levels near the relevant band edges. Do not draw a donor level as a whole new conduction band unless the question explicitly introduces impurity bands.

Temperature and light

Explain semiconductor response through carrier concentration

Heating a semiconductor can promote more electrons across the band gap, increasing electron–hole pairs and usually reducing resistance over an appropriate range. This differs from the common metallic trend, where lattice vibrations increase scattering and resistance generally rises. State the material and mechanism rather than memorising “temperature increases resistance.”

Light with suitable photon energy can also promote electrons across the gap, producing photoconductivity. This idea supports sensors and solar-cell physics, but detailed p–n junction devices belong to later study. The current unit’s task is to connect energy input, allowed states and carriers.

Do not heat or dismantle electronic components without a supervised low-voltage laboratory plan. Real devices include contacts, junctions and packaging that the simple bulk-band picture omits.

Exam readiness

Common mistakes, worked method and practice tasks

• Energy diagrams use a vertical energy axis, not physical thickness.
• A filled band alone does not conduct if no nearby states are available.
• A semiconductor has a smaller gap than an ordinary insulator in the simple model.
• Intrinsic excitation creates an electron–hole pair.
• n-type majority carriers are electrons; p-type majority carriers are holes.
• Doped material remains approximately electrically neutral overall.
• A hole is not a proton.
• Temperature trends differ between metals and semiconductors.

1. Draw and label metal, semiconductor and insulator band diagrams.
2. Explain conduction using occupancy and available states.
3. Compare intrinsic and extrinsic materials in a table.
4. Classify donor and acceptor doping.
5. Explain carrier changes with temperature and light.
6. Correct the claim “p-type silicon is positively charged.”

Use the paired Solids Study Guide for spaced revision. For online or physical NEB tuition, call 9846662070.

References and next steps

Sources and related study guides

• CDC Nepal: Physics Grade 11
• CDC Nepal: Physics Grades 11–12 curriculum
• OpenStax: Introduction to Solids
• OpenStax: Band Theory of Solids
• OpenStax: Semiconductors and Doping
• OpenStax: Semiconductor Devices

Continue with the focused study guide and connect band ideas to DC circuit measurements. Curriculum scope and time-sensitive institutional pages were checked on 2 August 2026; follow current CDC, NEB and school instructions if requirements change.