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Band Gap
Overview
1. The band gap is the minimum amount of energy required for an electron to break free of its bound state.
2. When the band gap energy is met, the electron is excited into a free state, and can therefore participate in conduction.
3. The band gap determines how much energy is needed from the sun for conduction, as well as how much energy is generated.
4. A hole is created where the electron was formerly bound. This hole also participates in conduction.

The band gap of a semiconductor is the minimum energy required to excite an electron that is stuck in its bound state into a free state where it can
participate in conduction. The band structure of a semiconductor gives the energy of the electrons on the y-axis and is called a "band diagram". The
lower energy level of a semiconductor is called the "valence band" (EV) and the energy level at which an electron can be considered free is called the
"conduction band" (EC). The band gap (EG) is the gap in energy between the bound state and the free state, between the valence band and conduction
band. Therefore, the band gap is the minimum change in energy required to excite the electron so that it can participate in conduction.

Schematic of the energy bands for electrons in a solid.

Once the electron becomes excited into the conduction band, it is free to move about the semiconductor and participate in conduction. However, the
excitation of an electron to the conduction band will also allow an additional conduction process to take place. The excitation of an electron to the
conduction band leaves behind an empty space for an electron. An electron from a neighboring atom can move into this empty space. When this
electron moves, it leaves behind another space. The continual movement of the space for an electron, called a "hole", can be illustrated as the
movement of a positively charged particle through the crystal structure. Consequently, the excitation of an electron into the conduction band results in
not only an electron in the conduction band but also a hole in the valence band. Thus, both the electron and hole can participate in conduction and are
called "carriers".

The concept of a moving "hole" is analogous to that of a bubble in a liquid. Although it is actually the liquid that moves, it is easier to describe the motion
of the bubble going in the opposite direction.

‹ Conduction in Semiconductors (/pvcdrom/conduction-in- Intrinsic Carrier Concentration › (/pvcdrom/pn-

semiconductors) junctions/intrinsic-carrier-concentration)
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(https://www.pveducation.org)

Christiana Honsberg
and Stuart Bowden
Instructions (/pvcdrom/welcome-to-pvcdrom/instructions)
1. Introduction (/pvcdrom/welcome-to-pvcdrom/introduction)
2. Properties of Sunlight (/pvcdrom/welcome-to-pvcdrom/properties-of-sunlight)
3. Semiconductors & Junctions (/pvcdrom/welcome-to-pvcdrom/pn-junctions)
Introduction (/pvcdrom/pn-junctions/introduction-to-semiconductors)
3.1. Basics (/pvcdrom/pn-junctions/basics)
Semiconductor Materials (/pvcdrom/pn-junctions/semiconductor-materials)
Semiconductor Structure (/pvcdrom/pn-junctions/semiconductor-structure)
Conduction in Semiconductors (/pvcdrom/conduction-in-semiconductors)
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Intrinsic Carrier Concentration (/pvcdrom/pn-junctions/intrinsic-carrier-concentration)
Doping (/pvcdrom/pn-junctions/doping)
Equilibrium Carrier Concentration (/pvcdrom/pn-junctions/equilibrium-carrier-concentration)
3.2. Generation (/pvcdrom/pn-junctions/generation)
Absorption of Light (/pvcdrom/pn-junctions/absorption-of-light)
Absorption Coefficient (/pvcdrom/pn-junctions/absorption-coefficient)
Absorption Depth (/pvcdrom/pn-junctions/absorption-depth)
Generation Rate (/pvcdrom/pn-junctions/generation-rate)
3.3. Recombination (/pvcdrom/recombination)
Types of Recombination (/pvcdrom/pn-junctions/types-of-recombination)
Lifetime (/pvcdrom/pn-junctions/lifetime)
Diffusion Length (/pvcdrom/pn-junctions/diffusion-length)
Surface Recombination (/pvcdrom/pn-junctions/surface-recombination)
3.4. Carrier Transport (/pvcdrom/carrier-transport)
Movement of Carriers in Semiconductors (/pvcdrom/pn-junctions/movement-of-carriers-in-semiconductors)
Diffusion (/pvcdrom/pn-junctions/diffusion)
Drift (/pvcdrom/pn-junctions/drift)
3.5. P-n Junctions (/pvcdrom/p-n-junctions)
Formation of a PN-Junction (/pvcdrom/pn-junctions/formation-of-a-pn-junction)
P-N Junction Diodes (/pvcdrom/pn-junctions/p-n-junction-diodes)
Bias of PN Junctions (/pvcdrom/pn-junctions/bias-of-pn-junctions)
Diode Equation (/pvcdrom/pn-junctions/diode-equation)
3.6. Diode Equations for PV (/pvcdrom/pn-junctions/diode-equations-for-pv)
Ideal Diode Equation Derivation (/pvcdrom/pn-junctions/ideal-diode-equation-derivation)
Basic Equations (/pvcdrom/pn-junctions/basic-equations)
Applying the Basic Equations to a PN Junction (/pvcdrom/applying-the-basic-equations-to-a-pn-junction)
Solving for Depletion Region (/pvcdrom/pn-junctions/solving-for-depletion-region)
Solving for Quasi Neutral Regions (/pvcdrom/solving-for-quasi-neutral-regions)
Finding Total Current (/pvcdrom/pn-junctions/finding-total-current)
Eg1: Wide Base Diode (/pvcdrom/pn-junctions/example-1-general-solution-for-wide-base-p-n-junction)
Eg2: Narrow Base Diode (/pvcdrom/pn-junctions/example-2-general-solution-for-narrow-base-diode)
Summary (/pvcdrom/pn-junctions/summary)

4. Solar Cell Operation (/pvcdrom/welcome-to-pvcdrom/solar-cell-operation)

5. Design of Silicon Cells (/pvcdrom/welcome-to-pvcdrom/design-of-silicon-cells)
6. Manufacturing Si Cells (/pvcdrom/welcome-to-pvcdrom/manufacturing-si-cells)
7. Modules and Arrays (/pvcdrom/welcome-to-pvcdrom/modules-and-arrays)
8. Characterization (/pvcdrom/welcome-to-pvcdrom/characterisation)
9. Material Properties (/pvcdrom/materials)
10. Batteries (/pvcdrom/welcome-to-pvcdrom/batteries)
11. Appendices (/pvcdrom/appendices)
Korean Version (/pvcdrom/korean)
Equations (/list-of-equations)
Flash Animations (/list-of-flash)
Interactive Graphs (/list-of-graphs)
Logout (https://pveducation.org/user/logout)
References (/biblio)

English

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