9.1 Semiconductor materials and properties

Semiconductors are materials with unique electrical properties, bridging the gap between conductors and insulators. This topic dives into their composition, structure, and energy bands, laying the foundation for understanding their behavior in electronic devices.

We'll explore intrinsic and extrinsic semiconductors, focusing on how doping affects their properties. We'll also examine the temperature dependence of semiconductor conductivity, crucial for various applications in modern electronics.

Semiconductor Materials

Composition and Structure

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• Semiconductors are materials with electrical conductivity between conductors (metals) and insulators (ceramics)
• Silicon is the most commonly used semiconductor material in electronics
  • Abundant, inexpensive, and has a stable oxide layer
• Germanium was used in early semiconductor devices but has been largely replaced by silicon
  • Has a smaller band gap and higher electron mobility compared to silicon
• Semiconductor materials have a crystal structure, typically arranged in a diamond cubic lattice
  • Each atom is covalently bonded to four neighboring atoms, forming a regular and repeating pattern

Energy Bands and Band Gap

Electronic Structure

• Energy bands represent the allowed energy levels for electrons in a solid material
  • Determined by the periodic potential of the atomic lattice
• Valence band is the highest occupied energy band at absolute zero temperature
  • Contains the valence electrons involved in chemical bonding
• Conduction band is the lowest unoccupied energy band
  • Electrons in the conduction band are free to move and contribute to electrical conduction
• Band gap is the energy difference between the top of the valence band and the bottom of the conduction band
  • Determines the electrical properties of the material (conductor, insulator, or semiconductor)
  • For semiconductors, the band gap is typically in the range of 0.5 to 3 eV (electron volts)

Electron Excitation and Conduction

• At absolute zero, the valence band is completely filled, and the conduction band is empty
  • No electrical conduction occurs
• As temperature increases, electrons can gain enough thermal energy to be excited from the valence band to the conduction band
  • Leaves behind positively charged holes in the valence band
  • Both electrons and holes contribute to electrical conduction
• The probability of electron excitation depends on the band gap and temperature
  • Smaller band gap and higher temperature lead to more electrons in the conduction band

Semiconductor Types and Properties

Intrinsic Semiconductors

• Intrinsic semiconductors are pure semiconductor materials without any intentional impurities (dopants)
  • Examples include pure silicon and germanium
• Electrical conductivity in intrinsic semiconductors is due to the thermal excitation of electrons from the valence band to the conduction band
  • Equal number of electrons and holes are generated
• The concentration of electrons and holes in intrinsic semiconductors is relatively low, resulting in low electrical conductivity
  • Typically in the range of 10^-8 to 10^-6 S/cm (siemens per centimeter) at room temperature

Extrinsic Semiconductors

• Extrinsic semiconductors are intentionally doped with impurities to modify their electrical properties
  • Dopants can be either donors (n-type) or acceptors (p-type)
• N-type semiconductors are doped with donor impurities that provide extra electrons to the conduction band
  • Examples of donor dopants include phosphorus and arsenic in silicon
• P-type semiconductors are doped with acceptor impurities that create extra holes in the valence band
  • Examples of acceptor dopants include boron and gallium in silicon
• Doping increases the concentration of charge carriers (electrons or holes) and enhances electrical conductivity
  • Typically in the range of 10^-3 to 10^3 S/cm, depending on the doping level

Temperature Dependence

• The electrical conductivity of semiconductors strongly depends on temperature
• As temperature increases, more electrons are thermally excited from the valence band to the conduction band
  • Leads to an exponential increase in the concentration of charge carriers (electrons and holes)
• The temperature dependence of electrical conductivity in semiconductors can be described by the Arrhenius equation:
  • $\sigma = \sigma_0 \exp(-E_a/kT)$
  • $\sigma$ is the electrical conductivity, $\sigma_0$ is a constant, $E_a$ is the activation energy (related to the band gap), $k$ is the Boltzmann constant, and $T$ is the absolute temperature
• The strong temperature dependence of semiconductor properties is exploited in various applications
  • Examples include temperature sensors (thermistors) and thermal imaging devices