Band model of conduction — in solid-state physics, an electron can move freely to another atom’s orbit (become a conduction carrier) only when it occupies which energy region?

Introduction / Context:
Semiconductor operation hinges on the band theory of solids. Electrons in different energy bands have different degrees of freedom. Understanding which band enables electrons to move through the lattice and carry current is fundamental for devices like diodes, BJTs, and MOSFETs.

Given Data / Assumptions:

• Crystalline semiconductor with distinct valence and conduction bands.
• Energy gap separates valence and conduction bands (bandgap E_g).
• Thermal or optical excitation may promote electrons across the bandgap.

Concept / Approach:
Electrons bound in the valence band are engaged in covalent bonds and cannot move freely between atoms. When sufficient energy promotes an electron to the conduction band, it becomes delocalized and free to respond to an electric field, contributing to electrical conduction. The vacancy left behind in the valence band behaves as a hole, which can also carry current.

Step-by-Step Solution:

Verification / Alternative check:
Band diagrams and density-of-states plots show mobile carriers only in the conduction band (electrons) and valence band (holes) depending on excitation and doping.

Why Other Options Are Wrong:

• Valence band: Electrons are largely localized in bonds.
• Inner orbit nearest the nucleus: Deep core states do not participate in conduction.
• “Covalent band”: Not standard terminology; conduction requires conduction-band states.

Common Pitfalls:
Confusing hole motion in the valence band (effective carrier) with electrons “moving to other atoms” as electrons; the question specifically asks about an electron moving freely — that is the conduction band.

Final Answer:
conduction band