Semiconductor Physics

🧗‍♀️Semiconductor Physics Unit 3 – Equilibrium Carrier Statistics in Semiconductors

Equilibrium carrier statistics in semiconductors form the foundation for understanding how these materials behave electrically. This unit covers key concepts like the Fermi-Dirac distribution, density of states, and carrier concentration equations, which are crucial for analyzing semiconductor properties. Temperature effects, doping impacts, and the distinction between equilibrium and non-equilibrium states are explored. These principles are essential for designing and optimizing various semiconductor devices, from solar cells and LEDs to transistors and lasers.

Key Concepts and Definitions

  • Semiconductors materials with electrical conductivity between insulators and conductors (silicon, germanium)
  • Carriers charge carriers responsible for electrical conduction in semiconductors
    • Electrons negative charge carriers in the conduction band
    • Holes positive charge carriers in the valence band
  • Intrinsic semiconductors pure semiconductors without any added impurities
  • Extrinsic semiconductors semiconductors with added impurities (dopants) to modify electrical properties
  • Fermi level energy level with a 50% probability of being occupied by an electron at thermal equilibrium
  • Band gap energy difference between the top of the valence band and the bottom of the conduction band
  • Density of states (DOS) number of available energy states per unit volume and energy interval in a semiconductor

Fermi-Dirac Distribution

  • Describes the probability of an electron occupying an energy state at thermal equilibrium
  • Represented by the equation: f(E)=11+e(EEF)/kTf(E) = \frac{1}{1 + e^{(E - E_F)/kT}}
    • EE energy of the state
    • EFE_F Fermi level
    • kk Boltzmann constant
    • TT absolute temperature
  • At T=0KT = 0K, the distribution is a step function, with all states below EFE_F occupied and all states above EFE_F empty
  • As temperature increases, the distribution becomes smoother, with some states above EFE_F being occupied and some below EFE_F being empty
  • The Fermi level determines the carrier concentrations in semiconductors
  • The position of the Fermi level relative to the band edges affects the electrical properties of the semiconductor

Density of States

  • Quantifies the number of available energy states per unit volume and energy interval in a semiconductor
  • Depends on the band structure and the effective mass of carriers
  • For a parabolic band, the density of states is proportional to the square root of energy: DOS(E)EDOS(E) \propto \sqrt{E}
  • The conduction band density of states is given by: NC=2(2πmekTh2)3/2N_C = 2\left(\frac{2\pi m_e^* kT}{h^2}\right)^{3/2}
    • mem_e^* effective mass of electrons
    • hh Planck's constant
  • The valence band density of states is given by: NV=2(2πmhkTh2)3/2N_V = 2\left(\frac{2\pi m_h^* kT}{h^2}\right)^{3/2}
    • mhm_h^* effective mass of holes
  • The density of states plays a crucial role in determining the carrier concentrations in semiconductors

Carrier Concentration Equations

  • Carrier concentrations in semiconductors are determined by the Fermi-Dirac distribution and the density of states
  • For intrinsic semiconductors, the electron and hole concentrations are equal: ni=pin_i = p_i
    • nin_i intrinsic electron concentration
    • pip_i intrinsic hole concentration
  • The intrinsic carrier concentration is given by: ni=NCNVeEg/2kTn_i = \sqrt{N_C N_V e^{-E_g/2kT}}
    • EgE_g band gap energy
  • For extrinsic semiconductors, the majority carrier concentration is determined by the doping level
    • n-type semiconductors: nNDn \approx N_D, where NDN_D is the donor concentration
    • p-type semiconductors: pNAp \approx N_A, where NAN_A is the acceptor concentration
  • The minority carrier concentration is calculated using the mass action law: np=ni2np = n_i^2

Temperature Effects on Carrier Concentration

  • Temperature significantly influences the carrier concentrations in semiconductors
  • As temperature increases, more electrons are excited from the valence band to the conduction band
  • The intrinsic carrier concentration increases exponentially with temperature: nieEg/2kTn_i \propto e^{-E_g/2kT}
  • In extrinsic semiconductors, the majority carrier concentration is relatively insensitive to temperature changes
    • Determined by the doping level, which is fixed
  • The minority carrier concentration increases with temperature, following the intrinsic carrier concentration
  • At high temperatures, the semiconductor may become intrinsic, with the intrinsic carrier concentration dominating over the doping-induced carriers
  • Temperature effects on carrier concentration have important implications for semiconductor device performance and reliability

Doping and Its Impact

  • Doping intentional introduction of impurities into a semiconductor to modify its electrical properties
  • n-type doping introduces donor impurities (phosphorus, arsenic) that provide extra electrons to the conduction band
  • p-type doping introduces acceptor impurities (boron, gallium) that create holes in the valence band
  • Doping shifts the Fermi level towards the corresponding band edge
    • n-type doping: Fermi level moves closer to the conduction band
    • p-type doping: Fermi level moves closer to the valence band
  • The doping concentration determines the majority carrier concentration in extrinsic semiconductors
  • Heavily doped semiconductors have a higher conductivity and lower resistivity compared to lightly doped or intrinsic semiconductors
  • Doping enables the creation of p-n junctions, which are the building blocks of many semiconductor devices (diodes, transistors, solar cells)

Equilibrium vs. Non-Equilibrium States

  • Equilibrium state: no external forces acting on the semiconductor, carrier concentrations determined by the Fermi-Dirac distribution
    • Characterized by a constant Fermi level throughout the semiconductor
    • Carrier generation and recombination rates are balanced
  • Non-equilibrium state: external forces (electric field, light, temperature gradients) disturb the equilibrium carrier concentrations
    • Characterized by a non-constant Fermi level or quasi-Fermi levels for electrons and holes
    • Carrier generation and recombination rates are not balanced
  • In non-equilibrium states, the carrier concentrations deviate from their equilibrium values
    • Excess carriers are generated or injected into the semiconductor
    • Carrier lifetime and diffusion play a role in the transport and recombination of excess carriers
  • Understanding the difference between equilibrium and non-equilibrium states is crucial for analyzing semiconductor devices under various operating conditions

Applications and Real-World Examples

  • Solar cells: p-n junctions that convert light into electrical energy
    • Carrier generation by photon absorption creates a non-equilibrium state
    • Separation of photogenerated carriers by the built-in electric field produces a photocurrent
  • Light-emitting diodes (LEDs): p-n junctions that emit light when forward-biased
    • Injection of minority carriers leads to radiative recombination and photon emission
    • The wavelength of the emitted light depends on the band gap of the semiconductor material
  • Transistors: three-terminal devices used for amplification and switching
    • Bipolar junction transistors (BJTs) rely on minority carrier injection and transport
    • Field-effect transistors (FETs) control the conductivity of a channel by applying an electric field
  • Semiconductor lasers: p-n junctions that produce coherent light through stimulated emission
    • Population inversion achieved by injecting high current densities
    • Used in fiber-optic communication, barcode scanners, and laser pointers
  • Photodetectors: devices that convert light into electrical signals
    • p-n junctions or PIN structures that generate a photocurrent when illuminated
    • Used in digital cameras, optical receivers, and light sensors


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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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