6.4 Electronic Properties of Solids

Solid state chemistry explores how atoms arrange in materials, affecting their properties. This section dives into the electronic structure of solids, explaining how energy bands form and influence conductivity. It's key to understanding why some materials conduct electricity while others don't.

We'll look at how energy levels in atoms merge into bands in solids, creating conductors, semiconductors, and insulators. We'll also see how adding impurities (doping) changes a material's properties, leading to cool applications like diodes and transistors in electronics.

Electronic structure of solids

Energy bands and bandgaps

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• In solids, the discrete energy levels of individual atoms merge into continuous energy bands due to the close proximity and interaction of the atoms
• The valence band is the highest occupied energy band, while the conduction band is the lowest unoccupied energy band
• The energy difference between the valence and conduction bands is called the bandgap
• The width of the energy bands and the size of the bandgap depend on the atomic structure and the strength of the interatomic interactions in the solid (covalent bonds lead to larger bandgaps than metallic bonds)

Fermi level and electron density of states

• The Fermi level represents the highest occupied energy state at absolute zero temperature
• In conductors, the Fermi level lies within a partially filled energy band, while in semiconductors and insulators, it lies within the bandgap
• The electron density of states describes the number of electronic states available per unit energy interval in each energy band
• The occupation of energy bands by electrons determines the electronic properties of solids, such as electrical conductivity and optical absorption (more electrons in the conduction band lead to higher conductivity)

Classifying solids by conductivity

Conductors

• Conductors have a partially filled valence band or overlapping valence and conduction bands, allowing electrons to move freely and conduct electricity
• Examples of conductors include metals like copper, silver, and gold
• The electrical conductivity of conductors decreases with increasing temperature due to increased electron scattering by lattice vibrations (phonons)

Semiconductors

• Semiconductors have a small bandgap (typically less than 3 eV) between the valence and conduction bands, allowing electrons to be excited into the conduction band at room temperature
• Examples of semiconductors include silicon, germanium, and gallium arsenide
• The electrical conductivity of semiconductors increases with increasing temperature as more electrons are thermally excited across the bandgap into the conduction band

Insulators

• Insulators have a large bandgap (typically greater than 3 eV) between the valence and conduction bands, making it difficult for electrons to be excited into the conduction band
• Examples of insulators include diamond, rubber, and most ceramics
• The electrical conductivity of insulators is very low and does not change significantly with temperature due to the large bandgap

Factors affecting electrical conductivity

• The electrical conductivity of a solid depends on the number of charge carriers (electrons in the conduction band and holes in the valence band) and their mobility
• The temperature dependence of electrical conductivity differs for conductors, semiconductors, and insulators due to the differences in their electronic structure (conductors decrease, semiconductors increase, insulators remain low)

Doping's effect on semiconductors

N-type and P-type doping

• Doping is the process of intentionally introducing impurities into a semiconductor to modify its electronic properties
• N-type doping involves adding impurities with extra valence electrons (phosphorus in silicon), creating donor states near the conduction band and increasing the concentration of electrons as majority charge carriers
• P-type doping involves adding impurities with fewer valence electrons (boron in silicon), creating acceptor states near the valence band and increasing the concentration of holes as majority charge carriers

Effects of doping on semiconductor properties

• The concentration of dopants determines the carrier concentration and the position of the Fermi level in the bandgap of the doped semiconductor
• Doping increases the electrical conductivity of semiconductors by several orders of magnitude compared to intrinsic (undoped) semiconductors
• The mobility of charge carriers in doped semiconductors is lower than in intrinsic semiconductors due to increased scattering by impurities
• The temperature dependence of electrical conductivity in doped semiconductors is different from intrinsic semiconductors due to the presence of additional charge carriers from dopants (less sensitive to temperature changes)

Solid state physics in electronic devices

P-N junction diodes

• A p-n junction diode is formed by joining a p-type and an n-type semiconductor, creating a depletion region with a built-in electric field at the interface
• Under forward bias, the diode conducts current, while under reverse bias, it blocks current, allowing the diode to function as a rectifier (converting AC to DC)
• The current-voltage (I-V) characteristics of a diode can be explained using the principles of solid-state physics, considering the movement of charge carriers across the p-n junction

Transistors (BJTs and FETs)

• Bipolar junction transistors (BJTs) consist of three doped semiconductor regions (emitter, base, and collector) forming two p-n junctions, allowing current amplification and switching
• The current flow through a BJT is controlled by the base-emitter voltage and the base current, enabling the transistor to function as an amplifier or switch (used in analog and digital circuits)
• Field-effect transistors (FETs) use an electric field to control the conductivity of a semiconductor channel between the source and drain electrodes
• The gate voltage in a FET controls the channel conductivity by modulating the charge carrier concentration, allowing the FET to function as a voltage-controlled current source (used in integrated circuits and high-frequency applications)

Optimizing electronic device performance

• The performance characteristics of electronic devices, such as diodes and transistors, depend on factors such as the doping concentrations, device geometry, and operating conditions
• These factors can be optimized using solid-state physics principles to improve device efficiency, speed, and reliability (minimizing leakage currents, reducing parasitic capacitances, and enhancing heat dissipation)