7.2 Extrinsic semiconductors

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 (phosphorus, arsenic)
• Donor atoms contribute extra electrons to the conduction band increases electron concentration
• Majority charge carriers electrons while minority carriers holes
• Fermi level shifts closer to the conduction band enhances conductivity

P-type semiconductors

• Formed by introducing acceptor impurities (boron, gallium) into intrinsic semiconductors
• Acceptor atoms create holes in the valence band increases hole concentration
• 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
• Band gap 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 ionization 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
• Donor-acceptor pair recombination produces characteristic emission lines
• Band-to-impurity transitions result in broader emission peaks
• Non-radiative recombination through defects can quench luminescence

Applications of extrinsic semiconductors

• Extrinsic semiconductors form the basis of numerous electronic and optoelectronic devices
• Understanding the physics of these materials crucial for advancing technology
• Condensed matter physics principles guide the development of new semiconductor applications

Transistors and diodes

• Bipolar junction transistors (BJTs) use both n-type and p-type regions for amplification
• Field-effect transistors (FETs) control current flow through doped semiconductor channels
• P-n junction diodes fundamental building blocks for rectification and switching
• Zener diodes utilize heavily doped p-n junctions for voltage regulation

Solar cells and LEDs

• Solar cells employ p-n junctions to convert light into electrical energy
• Doping optimization crucial for maximizing solar cell efficiency
• Light-emitting diodes (LEDs) use radiative recombination in doped semiconductors
• Quantum well structures in LEDs enhance emission efficiency and allow color tuning

Characterization techniques

• Various methods used to analyze the properties of extrinsic semiconductors
• Characterization techniques provide insights into doping effects and material quality
• Condensed matter physics principles underlie these measurement techniques

Hall measurements

• Determine carrier type, concentration, and mobility in extrinsic semiconductors
• Van der Pauw technique allows measurements on arbitrary sample shapes
• Temperature-dependent Hall measurements reveal activation energies of dopants
• Magnetoresistance effects can complicate Hall measurements in certain materials

DLTS and photoconductivity

• Deep Level Transient Spectroscopy (DLTS) identifies deep-level defects in semiconductors
• DLTS reveals activation energies and capture cross-sections of traps
• Photoconductivity measurements assess carrier generation and recombination processes
• Spectral dependence of photoconductivity provides information on impurity levels

Extrinsic vs intrinsic semiconductors

• Extrinsic semiconductors exhibit distinct properties compared to their intrinsic counterparts
• Understanding these differences crucial for selecting appropriate materials for specific applications
• Condensed matter physics theories explain the observed variations in behavior

Carrier concentration comparison

• Extrinsic semiconductors have much higher carrier concentrations than intrinsic materials
• Carrier concentration in extrinsics controlled by doping level rather than band gap
• Intrinsic carrier concentration depends exponentially on temperature and band gap
• Extrinsic semiconductors maintain stable carrier concentrations over wider temperature ranges

Temperature sensitivity differences

• Intrinsic semiconductors highly sensitive to temperature changes
• Extrinsic semiconductors show more stable electrical properties with temperature
• Intrinsic conduction dominates at high temperatures in both types
• Low-temperature behavior significantly different due to dopant freeze-out in extrinsics

Compensation in semiconductors

• Compensation occurs when both donors and acceptors present in a semiconductor
• Understanding compensation effects crucial for controlling semiconductor properties
• Condensed matter physics models describe the interplay between different dopant types

Donor-acceptor compensation

• Donors and acceptors can partially neutralize each other's effects
• Net carrier concentration determined by the difference between donor and acceptor concentrations
• Compensation can occur intentionally or unintentionally during growth or processing
• Partially compensated semiconductors can exhibit unique properties (semi-insulating GaAs)

Effects on carrier concentration

• Compensation reduces the effective carrier concentration below the total dopant concentration
• Can lead to high-resistivity materials even with significant doping levels
• Temperature dependence of carrier concentration more complex in compensated semiconductors
• Compensation affects the Fermi level position and the onset of degeneracy