Extrinsic semiconductors are materials with deliberately added impurities that alter their electrical properties. By introducing donor or acceptor atoms, these semiconductors can be tailored for specific electronic applications, forming the basis of modern devices.
This topic explores the types of extrinsic semiconductors, doping processes, and their impact on carrier concentrations and energy levels. We'll examine how doping affects electrical and optical properties, and discuss key applications and characterization techniques for these materials.
Types of extrinsic semiconductors
Extrinsic semiconductors form a crucial component of modern electronic devices by altering the electrical properties of intrinsic semiconductors
Doping process introduces impurities into the crystal lattice modifies the band structure and carrier concentrations
Condensed matter physics principles explain the behavior of charge carriers in these modified semiconductor materials
N-type semiconductors
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Created by doping intrinsic semiconductors with donor impurities (, arsenic)
Donor atoms contribute extra electrons to the conduction band increases
Majority charge carriers electrons while minority carriers holes
shifts closer to the conduction band enhances
P-type semiconductors
Formed by introducing acceptor impurities (, gallium) into intrinsic semiconductors
Acceptor atoms create holes in the valence band increases
Majority charge carriers holes while minority carriers electrons
Fermi level moves closer to the valence band improves hole-based conduction
Doping process
Doping alters the electronic structure of semiconductors enables tailoring of electrical properties
Precise control of dopant concentration and distribution critical for device performance
Condensed matter physics principles guide the selection of appropriate dopants and doping methods
Substitutional doping
Dopant atoms replace host atoms in the crystal lattice maintains overall structure
Commonly used for silicon doping with elements from groups III and V
Dopant size and electronic configuration must be compatible with host lattice
Activation energy required to ionize dopants depends on the energy level difference
Interstitial doping
Dopant atoms occupy spaces between host atoms in the crystal lattice
Often used for doping compound semiconductors (GaAs, InP)
Can introduce strain in the lattice affects electronic and optical properties
Diffusion coefficients of interstitial dopants typically higher than substitutional dopants
Donor and acceptor levels
Impurity energy levels introduced by dopants modify the band structure of semiconductors
Understanding these levels crucial for predicting and controlling semiconductor behavior
Condensed matter physics theories explain the formation and effects of these energy levels
Energy band diagrams
Donor levels appear just below the conduction band in n-type semiconductors
Acceptor levels form slightly above the valence band in p-type semiconductors
remains largely unchanged but carrier concentrations significantly altered
Impurity bands can form at high doping concentrations modify electronic properties
Fermi level shifts
Doping causes the Fermi level to move from mid-gap position in intrinsic semiconductors
N-type doping shifts Fermi level closer to conduction band increases electron concentration
P-type doping moves Fermi level towards valence band enhances hole concentration
Fermi-Dirac statistics describe the occupation probability of energy states
Carrier concentration
Doping dramatically increases the concentration of majority carriers in semiconductors
Carrier concentration directly impacts electrical conductivity and device performance
Condensed matter physics models predict carrier behavior under various conditions
Temperature dependence
Carrier concentration varies with temperature due to thermal of dopants
Low temperatures freeze-out region where dopants are not fully ionized
Intermediate temperatures saturation region with stable carrier concentration
High temperatures intrinsic region where thermally generated carriers dominate
Doping concentration effects
Increasing dopant concentration raises carrier concentration up to a limit
Heavy doping can lead to impurity band formation alters electronic properties
Degenerate doping occurs when Fermi level enters conduction or valence band
Doping compensation can reduce effective carrier concentration in unintentional doping
Electrical properties
Extrinsic semiconductors exhibit distinct electrical characteristics compared to intrinsic materials
Understanding these properties essential for designing and optimizing electronic devices
Condensed matter physics principles explain the observed electrical behaviors
Conductivity vs temperature
Conductivity generally increases with temperature in extrinsic semiconductors
Low temperature region dominated by ionization of dopants
Intermediate temperature region shows relatively stable conductivity
High temperature region intrinsic conduction becomes significant
Mobility decreases with temperature due to increased phonon scattering
Hall effect in extrinsics
Hall effect measurements determine carrier type, concentration, and mobility
Hall coefficient inversely proportional to carrier concentration
Sign of Hall coefficient indicates majority carrier type (negative for n-type, positive for p-type)
Hall mobility provides insight into scattering mechanisms and crystal quality
Optical properties
Doping influences the optical characteristics of semiconductors
Understanding these changes crucial for optoelectronic device design
Condensed matter physics theories explain the observed optical phenomena
Absorption spectrum changes
Doping introduces new absorption features related to impurity energy levels
Free carrier absorption increases in heavily doped semiconductors
Band gap narrowing occurs at high doping concentrations
Urbach tail formation in the absorption spectrum due to band edge fluctuations
Photoluminescence in extrinsics
Doping introduces new radiative recombination pathways